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third_party/sqlite/src/ext/fts2/fts2.c

+/* fts2 has a design flaw which can lead to database corruption (see
+** below).  It is recommended not to use it any longer, instead use
+** fts3 (or higher).  If you believe that your use of fts2 is safe,
+** add -DSQLITE_ENABLE_BROKEN_FTS2=1 to your CFLAGS.
+*/
+#if (!defined(SQLITE_CORE) || defined(SQLITE_ENABLE_FTS2)) \
+        && !defined(SQLITE_ENABLE_BROKEN_FTS2)
+#error fts2 has a design flaw and has been deprecated.
+#endif
+/* The flaw is that fts2 uses the content table's unaliased rowid as
+** the unique docid.  fts2 embeds the rowid in the index it builds,
+** and expects the rowid to not change.  The SQLite VACUUM operation
+** will renumber such rowids, thereby breaking fts2.  If you are using
+** fts2 in a system which has disabled VACUUM, then you can continue
+** to use it safely.  Note that PRAGMA auto_vacuum does NOT disable
+** VACUUM, though systems using auto_vacuum are unlikely to invoke
+** VACUUM.
+**
+** Unlike fts1, which is safe across VACUUM if you never delete
+** documents, fts2 has a second exposure to this flaw, in the segments
+** table.  So fts2 should be considered unsafe across VACUUM in all
+** cases.
+*/
+
+/*
+** 2006 Oct 10
+**
+** The author disclaims copyright to this source code.  In place of
+** a legal notice, here is a blessing:
+**
+**    May you do good and not evil.
+**    May you find forgiveness for yourself and forgive others.
+**    May you share freely, never taking more than you give.
+**
+******************************************************************************
+**
+** This is an SQLite module implementing full-text search.
+*/
+
+/* TODO(shess): To make it easier to spot changes without groveling
+** through changelogs, I've defined GEARS_FTS2_CHANGES to call them
+** out, and I will document them here.  On imports, these changes
+** should be reviewed to make sure they are still present, or are
+** dropped as appropriate.
+**
+** SQLite core adds the custom function fts2_tokenizer() to be used
+** for defining new tokenizers.  The second parameter is a vtable
+** pointer encoded as a blob.  Obviously this cannot be exposed to
+** Gears callers for security reasons.  It could be suppressed in the
+** authorizer, but for now I have simply commented the definition out.
+*/
+#define GEARS_FTS2_CHANGES 1
+
+/*
+** The code in this file is only compiled if:
+**
+**     * The FTS2 module is being built as an extension
+**       (in which case SQLITE_CORE is not defined), or
+**
+**     * The FTS2 module is being built into the core of
+**       SQLite (in which case SQLITE_ENABLE_FTS2 is defined).
+*/
+
+/* TODO(shess) Consider exporting this comment to an HTML file or the
+** wiki.
+*/
+/* The full-text index is stored in a series of b+tree (-like)
+** structures called segments which map terms to doclists.  The
+** structures are like b+trees in layout, but are constructed from the
+** bottom up in optimal fashion and are not updatable.  Since trees
+** are built from the bottom up, things will be described from the
+** bottom up.
+**
+**
+**** Varints ****
+** The basic unit of encoding is a variable-length integer called a
+** varint.  We encode variable-length integers in little-endian order
+** using seven bits * per byte as follows:
+**
+** KEY:
+**         A = 0xxxxxxx    7 bits of data and one flag bit
+**         B = 1xxxxxxx    7 bits of data and one flag bit
+**
+**  7 bits - A
+** 14 bits - BA
+** 21 bits - BBA
+** and so on.
+**
+** This is identical to how sqlite encodes varints (see util.c).
+**
+**
+**** Document lists ****
+** A doclist (document list) holds a docid-sorted list of hits for a
+** given term.  Doclists hold docids, and can optionally associate
+** token positions and offsets with docids.
+**
+** A DL_POSITIONS_OFFSETS doclist is stored like this:
+**
+** array {
+**   varint docid;
+**   array {                (position list for column 0)
+**     varint position;     (delta from previous position plus POS_BASE)
+**     varint startOffset;  (delta from previous startOffset)
+**     varint endOffset;    (delta from startOffset)
+**   }
+**   array {
+**     varint POS_COLUMN;   (marks start of position list for new column)
+**     varint column;       (index of new column)
+**     array {
+**       varint position;   (delta from previous position plus POS_BASE)
+**       varint startOffset;(delta from previous startOffset)
+**       varint endOffset;  (delta from startOffset)
+**     }
+**   }
+**   varint POS_END;        (marks end of positions for this document.
+** }
+**
+** Here, array { X } means zero or more occurrences of X, adjacent in
+** memory.  A "position" is an index of a token in the token stream
+** generated by the tokenizer, while an "offset" is a byte offset,
+** both based at 0.  Note that POS_END and POS_COLUMN occur in the
+** same logical place as the position element, and act as sentinals
+** ending a position list array.
+**
+** A DL_POSITIONS doclist omits the startOffset and endOffset
+** information.  A DL_DOCIDS doclist omits both the position and
+** offset information, becoming an array of varint-encoded docids.
+**
+** On-disk data is stored as type DL_DEFAULT, so we don't serialize
+** the type.  Due to how deletion is implemented in the segmentation
+** system, on-disk doclists MUST store at least positions.
+**
+**
+**** Segment leaf nodes ****
+** Segment leaf nodes store terms and doclists, ordered by term.  Leaf
+** nodes are written using LeafWriter, and read using LeafReader (to
+** iterate through a single leaf node's data) and LeavesReader (to
+** iterate through a segment's entire leaf layer).  Leaf nodes have
+** the format:
+**
+** varint iHeight;             (height from leaf level, always 0)
+** varint nTerm;               (length of first term)
+** char pTerm[nTerm];          (content of first term)
+** varint nDoclist;            (length of term's associated doclist)
+** char pDoclist[nDoclist];    (content of doclist)
+** array {
+**                             (further terms are delta-encoded)
+**   varint nPrefix;           (length of prefix shared with previous term)
+**   varint nSuffix;           (length of unshared suffix)
+**   char pTermSuffix[nSuffix];(unshared suffix of next term)
+**   varint nDoclist;          (length of term's associated doclist)
+**   char pDoclist[nDoclist];  (content of doclist)
+** }
+**
+** Here, array { X } means zero or more occurrences of X, adjacent in
+** memory.
+**
+** Leaf nodes are broken into blocks which are stored contiguously in
+** the %_segments table in sorted order.  This means that when the end
+** of a node is reached, the next term is in the node with the next
+** greater node id.
+**
+** New data is spilled to a new leaf node when the current node
+** exceeds LEAF_MAX bytes (default 2048).  New data which itself is
+** larger than STANDALONE_MIN (default 1024) is placed in a standalone
+** node (a leaf node with a single term and doclist).  The goal of
+** these settings is to pack together groups of small doclists while
+** making it efficient to directly access large doclists.  The
+** assumption is that large doclists represent terms which are more
+** likely to be query targets.
+**
+** TODO(shess) It may be useful for blocking decisions to be more
+** dynamic.  For instance, it may make more sense to have a 2.5k leaf
+** node rather than splitting into 2k and .5k nodes.  My intuition is
+** that this might extend through 2x or 4x the pagesize.
+**
+**
+**** Segment interior nodes ****
+** Segment interior nodes store blockids for subtree nodes and terms
+** to describe what data is stored by the each subtree.  Interior
+** nodes are written using InteriorWriter, and read using
+** InteriorReader.  InteriorWriters are created as needed when
+** SegmentWriter creates new leaf nodes, or when an interior node
+** itself grows too big and must be split.  The format of interior
+** nodes:
+**
+** varint iHeight;           (height from leaf level, always >0)
+** varint iBlockid;          (block id of node's leftmost subtree)
+** optional {
+**   varint nTerm;           (length of first term)
+**   char pTerm[nTerm];      (content of first term)
+**   array {
+**                                (further terms are delta-encoded)
+**     varint nPrefix;            (length of shared prefix with previous term)
+**     varint nSuffix;            (length of unshared suffix)
+**     char pTermSuffix[nSuffix]; (unshared suffix of next term)
+**   }
+** }
+**
+** Here, optional { X } means an optional element, while array { X }
+** means zero or more occurrences of X, adjacent in memory.
+**
+** An interior node encodes n terms separating n+1 subtrees.  The
+** subtree blocks are contiguous, so only the first subtree's blockid
+** is encoded.  The subtree at iBlockid will contain all terms less
+** than the first term encoded (or all terms if no term is encoded).
+** Otherwise, for terms greater than or equal to pTerm[i] but less
+** than pTerm[i+1], the subtree for that term will be rooted at
+** iBlockid+i.  Interior nodes only store enough term data to
+** distinguish adjacent children (if the rightmost term of the left
+** child is "something", and the leftmost term of the right child is
+** "wicked", only "w" is stored).
+**
+** New data is spilled to a new interior node at the same height when
+** the current node exceeds INTERIOR_MAX bytes (default 2048).
+** INTERIOR_MIN_TERMS (default 7) keeps large terms from monopolizing
+** interior nodes and making the tree too skinny.  The interior nodes
+** at a given height are naturally tracked by interior nodes at
+** height+1, and so on.
+**
+**
+**** Segment directory ****
+** The segment directory in table %_segdir stores meta-information for
+** merging and deleting segments, and also the root node of the
+** segment's tree.
+**
+** The root node is the top node of the segment's tree after encoding
+** the entire segment, restricted to ROOT_MAX bytes (default 1024).
+** This could be either a leaf node or an interior node.  If the top
+** node requires more than ROOT_MAX bytes, it is flushed to %_segments
+** and a new root interior node is generated (which should always fit
+** within ROOT_MAX because it only needs space for 2 varints, the
+** height and the blockid of the previous root).
+**
+** The meta-information in the segment directory is:
+**   level               - segment level (see below)
+**   idx                 - index within level
+**                       - (level,idx uniquely identify a segment)
+**   start_block         - first leaf node
+**   leaves_end_block    - last leaf node
+**   end_block           - last block (including interior nodes)
+**   root                - contents of root node
+**
+** If the root node is a leaf node, then start_block,
+** leaves_end_block, and end_block are all 0.
+**
+**
+**** Segment merging ****
+** To amortize update costs, segments are groups into levels and
+** merged in matches.  Each increase in level represents exponentially
+** more documents.
+**
+** New documents (actually, document updates) are tokenized and
+** written individually (using LeafWriter) to a level 0 segment, with
+** incrementing idx.  When idx reaches MERGE_COUNT (default 16), all
+** level 0 segments are merged into a single level 1 segment.  Level 1
+** is populated like level 0, and eventually MERGE_COUNT level 1
+** segments are merged to a single level 2 segment (representing
+** MERGE_COUNT^2 updates), and so on.
+**
+** A segment merge traverses all segments at a given level in
+** parallel, performing a straightforward sorted merge.  Since segment
+** leaf nodes are written in to the %_segments table in order, this
+** merge traverses the underlying sqlite disk structures efficiently.
+** After the merge, all segment blocks from the merged level are
+** deleted.
+**
+** MERGE_COUNT controls how often we merge segments.  16 seems to be
+** somewhat of a sweet spot for insertion performance.  32 and 64 show
+** very similar performance numbers to 16 on insertion, though they're
+** a tiny bit slower (perhaps due to more overhead in merge-time
+** sorting).  8 is about 20% slower than 16, 4 about 50% slower than
+** 16, 2 about 66% slower than 16.
+**
+** At query time, high MERGE_COUNT increases the number of segments
+** which need to be scanned and merged.  For instance, with 100k docs
+** inserted:
+**
+**    MERGE_COUNT   segments
+**       16           25
+**        8           12
+**        4           10
+**        2            6
+**
+** This appears to have only a moderate impact on queries for very
+** frequent terms (which are somewhat dominated by segment merge
+** costs), and infrequent and non-existent terms still seem to be fast
+** even with many segments.
+**
+** TODO(shess) That said, it would be nice to have a better query-side
+** argument for MERGE_COUNT of 16.  Also, it is possible/likely that
+** optimizations to things like doclist merging will swing the sweet
+** spot around.
+**
+**
+**
+**** Handling of deletions and updates ****
+** Since we're using a segmented structure, with no docid-oriented
+** index into the term index, we clearly cannot simply update the term
+** index when a document is deleted or updated.  For deletions, we
+** write an empty doclist (varint(docid) varint(POS_END)), for updates
+** we simply write the new doclist.  Segment merges overwrite older
+** data for a particular docid with newer data, so deletes or updates
+** will eventually overtake the earlier data and knock it out.  The
+** query logic likewise merges doclists so that newer data knocks out
+** older data.
+**
+** TODO(shess) Provide a VACUUM type operation to clear out all
+** deletions and duplications.  This would basically be a forced merge
+** into a single segment.
+*/
+
+#if !defined(SQLITE_CORE) || defined(SQLITE_ENABLE_FTS2)
+
+#if defined(SQLITE_ENABLE_FTS2) && !defined(SQLITE_CORE)
+# define SQLITE_CORE 1
+#endif
+
+#include <assert.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include "fts2.h"
+#include "fts2_hash.h"
+#include "fts2_tokenizer.h"
+#include "sqlite3.h"
+#ifndef SQLITE_CORE 
+# include "sqlite3ext.h"
+  SQLITE_EXTENSION_INIT1
+#endif
+
+
+/* TODO(shess) MAN, this thing needs some refactoring.  At minimum, it
+** would be nice to order the file better, perhaps something along the
+** lines of:
+**
+**  - utility functions
+**  - table setup functions
+**  - table update functions
+**  - table query functions
+**
+** Put the query functions last because they're likely to reference
+** typedefs or functions from the table update section.
+*/
+
+#if 0
+# define TRACE(A)  printf A; fflush(stdout)
+#else
+# define TRACE(A)
+#endif
+
+#if 0
+/* Useful to set breakpoints.  See main.c sqlite3Corrupt(). */
+static int fts2Corrupt(void){
+  return SQLITE_CORRUPT;
+}
+# define SQLITE_CORRUPT_BKPT fts2Corrupt()
+#else
+# define SQLITE_CORRUPT_BKPT SQLITE_CORRUPT
+#endif
+
+/* It is not safe to call isspace(), tolower(), or isalnum() on
+** hi-bit-set characters.  This is the same solution used in the
+** tokenizer.
+*/
+/* TODO(shess) The snippet-generation code should be using the
+** tokenizer-generated tokens rather than doing its own local
+** tokenization.
+*/
+/* TODO(shess) Is __isascii() a portable version of (c&0x80)==0? */
+static int safe_isspace(char c){
+  return c==' ' || c=='\t' || c=='\n' || c=='\r' || c=='\v' || c=='\f';
+}
+static int safe_tolower(char c){
+  return (c>='A' && c<='Z') ? (c - 'A' + 'a') : c;
+}
+static int safe_isalnum(char c){
+  return (c>='0' && c<='9') || (c>='A' && c<='Z') || (c>='a' && c<='z');
+}
+
+typedef enum DocListType {
+  DL_DOCIDS,              /* docids only */
+  DL_POSITIONS,           /* docids + positions */
+  DL_POSITIONS_OFFSETS    /* docids + positions + offsets */
+} DocListType;
+
+/*
+** By default, only positions and not offsets are stored in the doclists.
+** To change this so that offsets are stored too, compile with
+**
+**          -DDL_DEFAULT=DL_POSITIONS_OFFSETS
+**
+** If DL_DEFAULT is set to DL_DOCIDS, your table can only be inserted
+** into (no deletes or updates).
+*/
+#ifndef DL_DEFAULT
+# define DL_DEFAULT DL_POSITIONS
+#endif
+
+enum {
+  POS_END = 0,        /* end of this position list */
+  POS_COLUMN,         /* followed by new column number */
+  POS_BASE
+};
+
+/* MERGE_COUNT controls how often we merge segments (see comment at
+** top of file).
+*/
+#define MERGE_COUNT 16
+
+/* utility functions */
+
+/* CLEAR() and SCRAMBLE() abstract memset() on a pointer to a single
+** record to prevent errors of the form:
+**
+** my_function(SomeType *b){
+**   memset(b, '\0', sizeof(b));  // sizeof(b)!=sizeof(*b)
+** }
+*/
+/* TODO(shess) Obvious candidates for a header file. */
+#define CLEAR(b) memset(b, '\0', sizeof(*(b)))
+
+#ifndef NDEBUG
+#  define SCRAMBLE(b) memset(b, 0x55, sizeof(*(b)))
+#else
+#  define SCRAMBLE(b)
+#endif
+
+/* We may need up to VARINT_MAX bytes to store an encoded 64-bit integer. */
+#define VARINT_MAX 10
+
+/* Write a 64-bit variable-length integer to memory starting at p[0].
+ * The length of data written will be between 1 and VARINT_MAX bytes.
+ * The number of bytes written is returned. */
+static int putVarint(char *p, sqlite_int64 v){
+  unsigned char *q = (unsigned char *) p;
+  sqlite_uint64 vu = v;
+  do{
+    *q++ = (unsigned char) ((vu & 0x7f) | 0x80);
+    vu >>= 7;
+  }while( vu!=0 );
+  q[-1] &= 0x7f;  /* turn off high bit in final byte */
+  assert( q - (unsigned char *)p <= VARINT_MAX );
+  return (int) (q - (unsigned char *)p);
+}
+
+/* Read a 64-bit variable-length integer from memory starting at p[0].
+ * Return the number of bytes read, or 0 on error.
+ * The value is stored in *v. */
+static int getVarintSafe(const char *p, sqlite_int64 *v, int max){
+  const unsigned char *q = (const unsigned char *) p;
+  sqlite_uint64 x = 0, y = 1;
+  if( max>VARINT_MAX ) max = VARINT_MAX;
+  while( max && (*q & 0x80) == 0x80 ){
+    max--;
+    x += y * (*q++ & 0x7f);
+    y <<= 7;
+  }
+  if ( !max ){
+    assert( 0 );
+    return 0;  /* tried to read too much; bad data */
+  }
+  x += y * (*q++);
+  *v = (sqlite_int64) x;
+  return (int) (q - (unsigned char *)p);
+}
+
+static int getVarint(const char *p, sqlite_int64 *v){
+  return getVarintSafe(p, v, VARINT_MAX);
+}
+
+static int getVarint32Safe(const char *p, int *pi, int max){
+ sqlite_int64 i;
+ int ret = getVarintSafe(p, &i, max);
+ if( !ret ) return ret;
+ *pi = (int) i;
+ assert( *pi==i );
+ return ret;
+}
+
+static int getVarint32(const char* p, int *pi){
+  return getVarint32Safe(p, pi, VARINT_MAX);
+}
+
+/*******************************************************************/
+/* DataBuffer is used to collect data into a buffer in piecemeal
+** fashion.  It implements the usual distinction between amount of
+** data currently stored (nData) and buffer capacity (nCapacity).
+**
+** dataBufferInit - create a buffer with given initial capacity.
+** dataBufferReset - forget buffer's data, retaining capacity.
+** dataBufferDestroy - free buffer's data.
+** dataBufferSwap - swap contents of two buffers.
+** dataBufferExpand - expand capacity without adding data.
+** dataBufferAppend - append data.
+** dataBufferAppend2 - append two pieces of data at once.
+** dataBufferReplace - replace buffer's data.
+*/
+typedef struct DataBuffer {
+  char *pData;          /* Pointer to malloc'ed buffer. */
+  int nCapacity;        /* Size of pData buffer. */
+  int nData;            /* End of data loaded into pData. */
+} DataBuffer;
+
+static void dataBufferInit(DataBuffer *pBuffer, int nCapacity){
+  assert( nCapacity>=0 );
+  pBuffer->nData = 0;
+  pBuffer->nCapacity = nCapacity;
+  pBuffer->pData = nCapacity==0 ? NULL : sqlite3_malloc(nCapacity);
+}
+static void dataBufferReset(DataBuffer *pBuffer){
+  pBuffer->nData = 0;
+}
+static void dataBufferDestroy(DataBuffer *pBuffer){
+  if( pBuffer->pData!=NULL ) sqlite3_free(pBuffer->pData);
+  SCRAMBLE(pBuffer);
+}
+static void dataBufferSwap(DataBuffer *pBuffer1, DataBuffer *pBuffer2){
+  DataBuffer tmp = *pBuffer1;
+  *pBuffer1 = *pBuffer2;
+  *pBuffer2 = tmp;
+}
+static void dataBufferExpand(DataBuffer *pBuffer, int nAddCapacity){
+  assert( nAddCapacity>0 );
+  /* TODO(shess) Consider expanding more aggressively.  Note that the
+  ** underlying malloc implementation may take care of such things for
+  ** us already.
+  */
+  if( pBuffer->nData+nAddCapacity>pBuffer->nCapacity ){
+    pBuffer->nCapacity = pBuffer->nData+nAddCapacity;
+    pBuffer->pData = sqlite3_realloc(pBuffer->pData, pBuffer->nCapacity);
+  }
+}
+static void dataBufferAppend(DataBuffer *pBuffer,
+                             const char *pSource, int nSource){
+  assert( nSource>0 && pSource!=NULL );
+  dataBufferExpand(pBuffer, nSource);
+  memcpy(pBuffer->pData+pBuffer->nData, pSource, nSource);
+  pBuffer->nData += nSource;
+}
+static void dataBufferAppend2(DataBuffer *pBuffer,
+                              const char *pSource1, int nSource1,
+                              const char *pSource2, int nSource2){
+  assert( nSource1>0 && pSource1!=NULL );
+  assert( nSource2>0 && pSource2!=NULL );
+  dataBufferExpand(pBuffer, nSource1+nSource2);
+  memcpy(pBuffer->pData+pBuffer->nData, pSource1, nSource1);
+  memcpy(pBuffer->pData+pBuffer->nData+nSource1, pSource2, nSource2);
+  pBuffer->nData += nSource1+nSource2;
+}
+static void dataBufferReplace(DataBuffer *pBuffer,
+                              const char *pSource, int nSource){
+  dataBufferReset(pBuffer);
+  dataBufferAppend(pBuffer, pSource, nSource);
+}
+
+/* StringBuffer is a null-terminated version of DataBuffer. */
+typedef struct StringBuffer {
+  DataBuffer b;            /* Includes null terminator. */
+} StringBuffer;
+
+static void initStringBuffer(StringBuffer *sb){
+  dataBufferInit(&sb->b, 100);
+  dataBufferReplace(&sb->b, "", 1);
+}
+static int stringBufferLength(StringBuffer *sb){
+  return sb->b.nData-1;
+}
+static char *stringBufferData(StringBuffer *sb){
+  return sb->b.pData;
+}
+static void stringBufferDestroy(StringBuffer *sb){
+  dataBufferDestroy(&sb->b);
+}
+
+static void nappend(StringBuffer *sb, const char *zFrom, int nFrom){
+  assert( sb->b.nData>0 );
+  if( nFrom>0 ){
+    sb->b.nData--;
+    dataBufferAppend2(&sb->b, zFrom, nFrom, "", 1);
+  }
+}
+static void append(StringBuffer *sb, const char *zFrom){
+  nappend(sb, zFrom, strlen(zFrom));
+}
+
+/* Append a list of strings separated by commas. */
+static void appendList(StringBuffer *sb, int nString, char **azString){
+  int i;
+  for(i=0; i<nString; ++i){
+    if( i>0 ) append(sb, ", ");
+    append(sb, azString[i]);
+  }
+}
+
+static int endsInWhiteSpace(StringBuffer *p){
+  return stringBufferLength(p)>0 &&
+    safe_isspace(stringBufferData(p)[stringBufferLength(p)-1]);
+}
+
+/* If the StringBuffer ends in something other than white space, add a
+** single space character to the end.
+*/
+static void appendWhiteSpace(StringBuffer *p){
+  if( stringBufferLength(p)==0 ) return;
+  if( !endsInWhiteSpace(p) ) append(p, " ");
+}
+
+/* Remove white space from the end of the StringBuffer */
+static void trimWhiteSpace(StringBuffer *p){
+  while( endsInWhiteSpace(p) ){
+    p->b.pData[--p->b.nData-1] = '\0';
+  }
+}
+
+/*******************************************************************/
+/* DLReader is used to read document elements from a doclist.  The
+** current docid is cached, so dlrDocid() is fast.  DLReader does not
+** own the doclist buffer.
+**
+** dlrAtEnd - true if there's no more data to read.
+** dlrDocid - docid of current document.
+** dlrDocData - doclist data for current document (including docid).
+** dlrDocDataBytes - length of same.
+** dlrAllDataBytes - length of all remaining data.
+** dlrPosData - position data for current document.
+** dlrPosDataLen - length of pos data for current document (incl POS_END).
+** dlrStep - step to current document.
+** dlrInit - initial for doclist of given type against given data.
+** dlrDestroy - clean up.
+**
+** Expected usage is something like:
+**
+**   DLReader reader;
+**   dlrInit(&reader, pData, nData);
+**   while( !dlrAtEnd(&reader) ){
+**     // calls to dlrDocid() and kin.
+**     dlrStep(&reader);
+**   }
+**   dlrDestroy(&reader);
+*/
+typedef struct DLReader {
+  DocListType iType;
+  const char *pData;
+  int nData;
+
+  sqlite_int64 iDocid;
+  int nElement;
+} DLReader;
+
+static int dlrAtEnd(DLReader *pReader){
+  assert( pReader->nData>=0 );
+  return pReader->nData<=0;
+}
+static sqlite_int64 dlrDocid(DLReader *pReader){
+  assert( !dlrAtEnd(pReader) );
+  return pReader->iDocid;
+}
+static const char *dlrDocData(DLReader *pReader){
+  assert( !dlrAtEnd(pReader) );
+  return pReader->pData;
+}
+static int dlrDocDataBytes(DLReader *pReader){
+  assert( !dlrAtEnd(pReader) );
+  return pReader->nElement;
+}
+static int dlrAllDataBytes(DLReader *pReader){
+  assert( !dlrAtEnd(pReader) );
+  return pReader->nData;
+}
+/* TODO(shess) Consider adding a field to track iDocid varint length
+** to make these two functions faster.  This might matter (a tiny bit)
+** for queries.
+*/
+static const char *dlrPosData(DLReader *pReader){
+  sqlite_int64 iDummy;
+  int n = getVarintSafe(pReader->pData, &iDummy, pReader->nElement);
+  if( !n ) return NULL;
+  assert( !dlrAtEnd(pReader) );
+  return pReader->pData+n;
+}
+static int dlrPosDataLen(DLReader *pReader){
+  sqlite_int64 iDummy;
+  int n = getVarint(pReader->pData, &iDummy);
+  assert( !dlrAtEnd(pReader) );
+  return pReader->nElement-n;
+}
+static int dlrStep(DLReader *pReader){
+  assert( !dlrAtEnd(pReader) );
+
+  /* Skip past current doclist element. */
+  assert( pReader->nElement<=pReader->nData );
+  pReader->pData += pReader->nElement;
+  pReader->nData -= pReader->nElement;
+
+  /* If there is more data, read the next doclist element. */
+  if( pReader->nData>0 ){
+    sqlite_int64 iDocidDelta;
+    int nTotal = 0;
+    int iDummy, n = getVarintSafe(pReader->pData, &iDocidDelta, pReader->nData);
+    if( !n ) return SQLITE_CORRUPT_BKPT;
+    nTotal += n;
+    pReader->iDocid += iDocidDelta;
+    if( pReader->iType>=DL_POSITIONS ){
+      while( 1 ){
+        n = getVarint32Safe(pReader->pData+nTotal, &iDummy,
+                            pReader->nData-nTotal);
+        if( !n ) return SQLITE_CORRUPT_BKPT;
+        nTotal += n;
+        if( iDummy==POS_END ) break;
+        if( iDummy==POS_COLUMN ){
+          n = getVarint32Safe(pReader->pData+nTotal, &iDummy,
+                              pReader->nData-nTotal);
+          if( !n ) return SQLITE_CORRUPT_BKPT;
+          nTotal += n;
+        }else if( pReader->iType==DL_POSITIONS_OFFSETS ){
+          n = getVarint32Safe(pReader->pData+nTotal, &iDummy,
+                              pReader->nData-nTotal);
+          if( !n ) return SQLITE_CORRUPT_BKPT;
+          nTotal += n;
+          n = getVarint32Safe(pReader->pData+nTotal, &iDummy,
+                              pReader->nData-nTotal);
+          if( !n ) return SQLITE_CORRUPT_BKPT;
+          nTotal += n;
+        }
+      }
+    }
+    pReader->nElement = nTotal;
+    assert( pReader->nElement<=pReader->nData );
+  }
+  return SQLITE_OK;
+}
+static void dlrDestroy(DLReader *pReader){
+  SCRAMBLE(pReader);
+}
+static int dlrInit(DLReader *pReader, DocListType iType,
+                   const char *pData, int nData){
+  int rc;
+  assert( pData!=NULL && nData!=0 );
+  pReader->iType = iType;
+  pReader->pData = pData;
+  pReader->nData = nData;
+  pReader->nElement = 0;
+  pReader->iDocid = 0;
+
+  /* Load the first element's data.  There must be a first element. */
+  rc = dlrStep(pReader);
+  if( rc!=SQLITE_OK ) dlrDestroy(pReader);
+  return rc;
+}
+
+#ifndef NDEBUG
+/* Verify that the doclist can be validly decoded.  Also returns the
+** last docid found because it is convenient in other assertions for
+** DLWriter.
+*/
+static void docListValidate(DocListType iType, const char *pData, int nData,
+                            sqlite_int64 *pLastDocid){
+  sqlite_int64 iPrevDocid = 0;
+  assert( nData>0 );
+  assert( pData!=0 );
+  assert( pData+nData>pData );
+  while( nData!=0 ){
+    sqlite_int64 iDocidDelta;
+    int n = getVarint(pData, &iDocidDelta);
+    iPrevDocid += iDocidDelta;
+    if( iType>DL_DOCIDS ){
+      int iDummy;
+      while( 1 ){
+        n += getVarint32(pData+n, &iDummy);
+        if( iDummy==POS_END ) break;
+        if( iDummy==POS_COLUMN ){
+          n += getVarint32(pData+n, &iDummy);
+        }else if( iType>DL_POSITIONS ){
+          n += getVarint32(pData+n, &iDummy);
+          n += getVarint32(pData+n, &iDummy);
+        }
+        assert( n<=nData );
+      }
+    }
+    assert( n<=nData );
+    pData += n;
+    nData -= n;
+  }
+  if( pLastDocid ) *pLastDocid = iPrevDocid;
+}
+#define ASSERT_VALID_DOCLIST(i, p, n, o) docListValidate(i, p, n, o)
+#else
+#define ASSERT_VALID_DOCLIST(i, p, n, o) assert( 1 )
+#endif
+
+/*******************************************************************/
+/* DLWriter is used to write doclist data to a DataBuffer.  DLWriter
+** always appends to the buffer and does not own it.
+**
+** dlwInit - initialize to write a given type doclistto a buffer.
+** dlwDestroy - clear the writer's memory.  Does not free buffer.
+** dlwAppend - append raw doclist data to buffer.
+** dlwCopy - copy next doclist from reader to writer.
+** dlwAdd - construct doclist element and append to buffer.
+**    Only apply dlwAdd() to DL_DOCIDS doclists (else use PLWriter).
+*/
+typedef struct DLWriter {
+  DocListType iType;
+  DataBuffer *b;
+  sqlite_int64 iPrevDocid;
+#ifndef NDEBUG
+  int has_iPrevDocid;
+#endif
+} DLWriter;
+
+static void dlwInit(DLWriter *pWriter, DocListType iType, DataBuffer *b){
+  pWriter->b = b;
+  pWriter->iType = iType;
+  pWriter->iPrevDocid = 0;
+#ifndef NDEBUG
+  pWriter->has_iPrevDocid = 0;
+#endif
+}
+static void dlwDestroy(DLWriter *pWriter){
+  SCRAMBLE(pWriter);
+}
+/* iFirstDocid is the first docid in the doclist in pData.  It is
+** needed because pData may point within a larger doclist, in which
+** case the first item would be delta-encoded.
+**
+** iLastDocid is the final docid in the doclist in pData.  It is
+** needed to create the new iPrevDocid for future delta-encoding.  The
+** code could decode the passed doclist to recreate iLastDocid, but
+** the only current user (docListMerge) already has decoded this
+** information.
+*/
+/* TODO(shess) This has become just a helper for docListMerge.
+** Consider a refactor to make this cleaner.
+*/
+static int dlwAppend(DLWriter *pWriter,
+                     const char *pData, int nData,
+                     sqlite_int64 iFirstDocid, sqlite_int64 iLastDocid){
+  sqlite_int64 iDocid = 0;
+  char c[VARINT_MAX];
+  int nFirstOld, nFirstNew;     /* Old and new varint len of first docid. */
+#ifndef NDEBUG
+  sqlite_int64 iLastDocidDelta;
+#endif
+
+  /* Recode the initial docid as delta from iPrevDocid. */
+  nFirstOld = getVarintSafe(pData, &iDocid, nData);
+  if( !nFirstOld ) return SQLITE_CORRUPT_BKPT;
+  assert( nFirstOld<nData || (nFirstOld==nData && pWriter->iType==DL_DOCIDS) );
+  nFirstNew = putVarint(c, iFirstDocid-pWriter->iPrevDocid);
+
+  /* Verify that the incoming doclist is valid AND that it ends with
+  ** the expected docid.  This is essential because we'll trust this
+  ** docid in future delta-encoding.
+  */
+  ASSERT_VALID_DOCLIST(pWriter->iType, pData, nData, &iLastDocidDelta);
+  assert( iLastDocid==iFirstDocid-iDocid+iLastDocidDelta );
+
+  /* Append recoded initial docid and everything else.  Rest of docids
+  ** should have been delta-encoded from previous initial docid.
+  */
+  if( nFirstOld<nData ){
+    dataBufferAppend2(pWriter->b, c, nFirstNew,
+                      pData+nFirstOld, nData-nFirstOld);
+  }else{
+    dataBufferAppend(pWriter->b, c, nFirstNew);
+  }
+  pWriter->iPrevDocid = iLastDocid;
+  return SQLITE_OK;
+}
+static int dlwCopy(DLWriter *pWriter, DLReader *pReader){
+  return dlwAppend(pWriter, dlrDocData(pReader), dlrDocDataBytes(pReader),
+                   dlrDocid(pReader), dlrDocid(pReader));
+}
+static void dlwAdd(DLWriter *pWriter, sqlite_int64 iDocid){
+  char c[VARINT_MAX];
+  int n = putVarint(c, iDocid-pWriter->iPrevDocid);
+
+  /* Docids must ascend. */
+  assert( !pWriter->has_iPrevDocid || iDocid>pWriter->iPrevDocid );
+  assert( pWriter->iType==DL_DOCIDS );
+
+  dataBufferAppend(pWriter->b, c, n);
+  pWriter->iPrevDocid = iDocid;
+#ifndef NDEBUG
+  pWriter->has_iPrevDocid = 1;
+#endif
+}
+
+/*******************************************************************/
+/* PLReader is used to read data from a document's position list.  As
+** the caller steps through the list, data is cached so that varints
+** only need to be decoded once.
+**
+** plrInit, plrDestroy - create/destroy a reader.
+** plrColumn, plrPosition, plrStartOffset, plrEndOffset - accessors
+** plrAtEnd - at end of stream, only call plrDestroy once true.
+** plrStep - step to the next element.
+*/
+typedef struct PLReader {
+  /* These refer to the next position's data.  nData will reach 0 when
+  ** reading the last position, so plrStep() signals EOF by setting
+  ** pData to NULL.
+  */
+  const char *pData;
+  int nData;
+
+  DocListType iType;
+  int iColumn;         /* the last column read */
+  int iPosition;       /* the last position read */
+  int iStartOffset;    /* the last start offset read */
+  int iEndOffset;      /* the last end offset read */
+} PLReader;
+
+static int plrAtEnd(PLReader *pReader){
+  return pReader->pData==NULL;
+}
+static int plrColumn(PLReader *pReader){
+  assert( !plrAtEnd(pReader) );
+  return pReader->iColumn;
+}
+static int plrPosition(PLReader *pReader){
+  assert( !plrAtEnd(pReader) );
+  return pReader->iPosition;
+}
+static int plrStartOffset(PLReader *pReader){
+  assert( !plrAtEnd(pReader) );
+  return pReader->iStartOffset;
+}
+static int plrEndOffset(PLReader *pReader){
+  assert( !plrAtEnd(pReader) );
+  return pReader->iEndOffset;
+}
+static int plrStep(PLReader *pReader){
+  int i, n, nTotal = 0;
+
+  assert( !plrAtEnd(pReader) );
+
+  if( pReader->nData<=0 ){
+    pReader->pData = NULL;
+    return SQLITE_OK;
+  }
+
+  n = getVarint32Safe(pReader->pData, &i, pReader->nData);
+  if( !n ) return SQLITE_CORRUPT_BKPT;
+  nTotal += n;
+  if( i==POS_COLUMN ){
+    n = getVarint32Safe(pReader->pData+nTotal, &pReader->iColumn,
+                        pReader->nData-nTotal);
+    if( !n ) return SQLITE_CORRUPT_BKPT;
+    nTotal += n;
+    pReader->iPosition = 0;
+    pReader->iStartOffset = 0;
+    n = getVarint32Safe(pReader->pData+nTotal, &i, pReader->nData-nTotal);
+    if( !n ) return SQLITE_CORRUPT_BKPT;
+    nTotal += n;
+  }
+  /* Should never see adjacent column changes. */
+  assert( i!=POS_COLUMN );
+
+  if( i==POS_END ){
+    assert( nTotal<=pReader->nData );
+    pReader->nData = 0;
+    pReader->pData = NULL;
+    return SQLITE_OK;
+  }
+
+  pReader->iPosition += i-POS_BASE;
+  if( pReader->iType==DL_POSITIONS_OFFSETS ){
+    n = getVarint32Safe(pReader->pData+nTotal, &i, pReader->nData-nTotal);
+    if( !n ) return SQLITE_CORRUPT_BKPT;
+    nTotal += n;
+    pReader->iStartOffset += i;
+    n = getVarint32Safe(pReader->pData+nTotal, &i, pReader->nData-nTotal);
+    if( !n ) return SQLITE_CORRUPT_BKPT;
+    nTotal += n;
+    pReader->iEndOffset = pReader->iStartOffset+i;
+  }
+  assert( nTotal<=pReader->nData );
+  pReader->pData += nTotal;
+  pReader->nData -= nTotal;
+  return SQLITE_OK;
+}
+
+static void plrDestroy(PLReader *pReader){
+  SCRAMBLE(pReader);
+}
+
+static int plrInit(PLReader *pReader, DLReader *pDLReader){
+  int rc;
+  pReader->pData = dlrPosData(pDLReader);
+  pReader->nData = dlrPosDataLen(pDLReader);
+  pReader->iType = pDLReader->iType;
+  pReader->iColumn = 0;
+  pReader->iPosition = 0;
+  pReader->iStartOffset = 0;
+  pReader->iEndOffset = 0;
+  rc = plrStep(pReader);
+  if( rc!=SQLITE_OK ) plrDestroy(pReader);
+  return rc;
+}
+
+/*******************************************************************/
+/* PLWriter is used in constructing a document's position list.  As a
+** convenience, if iType is DL_DOCIDS, PLWriter becomes a no-op.
+** PLWriter writes to the associated DLWriter's buffer.
+**
+** plwInit - init for writing a document's poslist.
+** plwDestroy - clear a writer.
+** plwAdd - append position and offset information.
+** plwCopy - copy next position's data from reader to writer.
+** plwTerminate - add any necessary doclist terminator.
+**
+** Calling plwAdd() after plwTerminate() may result in a corrupt
+** doclist.
+*/
+/* TODO(shess) Until we've written the second item, we can cache the
+** first item's information.  Then we'd have three states:
+**
+** - initialized with docid, no positions.
+** - docid and one position.
+** - docid and multiple positions.
+**
+** Only the last state needs to actually write to dlw->b, which would
+** be an improvement in the DLCollector case.
+*/
+typedef struct PLWriter {
+  DLWriter *dlw;
+
+  int iColumn;    /* the last column written */
+  int iPos;       /* the last position written */
+  int iOffset;    /* the last start offset written */
+} PLWriter;
+
+/* TODO(shess) In the case where the parent is reading these values
+** from a PLReader, we could optimize to a copy if that PLReader has
+** the same type as pWriter.
+*/
+static void plwAdd(PLWriter *pWriter, int iColumn, int iPos,
+                   int iStartOffset, int iEndOffset){
+  /* Worst-case space for POS_COLUMN, iColumn, iPosDelta,
+  ** iStartOffsetDelta, and iEndOffsetDelta.
+  */
+  char c[5*VARINT_MAX];
+  int n = 0;
+
+  /* Ban plwAdd() after plwTerminate(). */
+  assert( pWriter->iPos!=-1 );
+
+  if( pWriter->dlw->iType==DL_DOCIDS ) return;
+
+  if( iColumn!=pWriter->iColumn ){
+    n += putVarint(c+n, POS_COLUMN);
+    n += putVarint(c+n, iColumn);
+    pWriter->iColumn = iColumn;
+    pWriter->iPos = 0;
+    pWriter->iOffset = 0;
+  }
+  assert( iPos>=pWriter->iPos );
+  n += putVarint(c+n, POS_BASE+(iPos-pWriter->iPos));
+  pWriter->iPos = iPos;
+  if( pWriter->dlw->iType==DL_POSITIONS_OFFSETS ){
+    assert( iStartOffset>=pWriter->iOffset );
+    n += putVarint(c+n, iStartOffset-pWriter->iOffset);
+    pWriter->iOffset = iStartOffset;
+    assert( iEndOffset>=iStartOffset );
+    n += putVarint(c+n, iEndOffset-iStartOffset);
+  }
+  dataBufferAppend(pWriter->dlw->b, c, n);
+}
+static void plwCopy(PLWriter *pWriter, PLReader *pReader){
+  plwAdd(pWriter, plrColumn(pReader), plrPosition(pReader),
+         plrStartOffset(pReader), plrEndOffset(pReader));
+}
+static void plwInit(PLWriter *pWriter, DLWriter *dlw, sqlite_int64 iDocid){
+  char c[VARINT_MAX];
+  int n;
+
+  pWriter->dlw = dlw;
+
+  /* Docids must ascend. */
+  assert( !pWriter->dlw->has_iPrevDocid || iDocid>pWriter->dlw->iPrevDocid );
+  n = putVarint(c, iDocid-pWriter->dlw->iPrevDocid);
+  dataBufferAppend(pWriter->dlw->b, c, n);
+  pWriter->dlw->iPrevDocid = iDocid;
+#ifndef NDEBUG
+  pWriter->dlw->has_iPrevDocid = 1;
+#endif
+
+  pWriter->iColumn = 0;
+  pWriter->iPos = 0;
+  pWriter->iOffset = 0;
+}
+/* TODO(shess) Should plwDestroy() also terminate the doclist?  But
+** then plwDestroy() would no longer be just a destructor, it would
+** also be doing work, which isn't consistent with the overall idiom.
+** Another option would be for plwAdd() to always append any necessary
+** terminator, so that the output is always correct.  But that would
+** add incremental work to the common case with the only benefit being
+** API elegance.  Punt for now.
+*/
+static void plwTerminate(PLWriter *pWriter){
+  if( pWriter->dlw->iType>DL_DOCIDS ){
+    char c[VARINT_MAX];
+    int n = putVarint(c, POS_END);
+    dataBufferAppend(pWriter->dlw->b, c, n);
+  }
+#ifndef NDEBUG
+  /* Mark as terminated for assert in plwAdd(). */
+  pWriter->iPos = -1;
+#endif
+}
+static void plwDestroy(PLWriter *pWriter){
+  SCRAMBLE(pWriter);
+}
+
+/*******************************************************************/
+/* DLCollector wraps PLWriter and DLWriter to provide a
+** dynamically-allocated doclist area to use during tokenization.
+**
+** dlcNew - malloc up and initialize a collector.
+** dlcDelete - destroy a collector and all contained items.
+** dlcAddPos - append position and offset information.
+** dlcAddDoclist - add the collected doclist to the given buffer.
+** dlcNext - terminate the current document and open another.
+*/
+typedef struct DLCollector {
+  DataBuffer b;
+  DLWriter dlw;
+  PLWriter plw;
+} DLCollector;
+
+/* TODO(shess) This could also be done by calling plwTerminate() and
+** dataBufferAppend().  I tried that, expecting nominal performance
+** differences, but it seemed to pretty reliably be worth 1% to code
+** it this way.  I suspect it is the incremental malloc overhead (some
+** percentage of the plwTerminate() calls will cause a realloc), so
+** this might be worth revisiting if the DataBuffer implementation
+** changes.
+*/
+static void dlcAddDoclist(DLCollector *pCollector, DataBuffer *b){
+  if( pCollector->dlw.iType>DL_DOCIDS ){
+    char c[VARINT_MAX];
+    int n = putVarint(c, POS_END);
+    dataBufferAppend2(b, pCollector->b.pData, pCollector->b.nData, c, n);
+  }else{
+    dataBufferAppend(b, pCollector->b.pData, pCollector->b.nData);
+  }
+}
+static void dlcNext(DLCollector *pCollector, sqlite_int64 iDocid){
+  plwTerminate(&pCollector->plw);
+  plwDestroy(&pCollector->plw);
+  plwInit(&pCollector->plw, &pCollector->dlw, iDocid);
+}
+static void dlcAddPos(DLCollector *pCollector, int iColumn, int iPos,
+                      int iStartOffset, int iEndOffset){
+  plwAdd(&pCollector->plw, iColumn, iPos, iStartOffset, iEndOffset);
+}
+
+static DLCollector *dlcNew(sqlite_int64 iDocid, DocListType iType){
+  DLCollector *pCollector = sqlite3_malloc(sizeof(DLCollector));
+  dataBufferInit(&pCollector->b, 0);
+  dlwInit(&pCollector->dlw, iType, &pCollector->b);
+  plwInit(&pCollector->plw, &pCollector->dlw, iDocid);
+  return pCollector;
+}
+static void dlcDelete(DLCollector *pCollector){
+  plwDestroy(&pCollector->plw);
+  dlwDestroy(&pCollector->dlw);
+  dataBufferDestroy(&pCollector->b);
+  SCRAMBLE(pCollector);
+  sqlite3_free(pCollector);
+}
+
+
+/* Copy the doclist data of iType in pData/nData into *out, trimming
+** unnecessary data as we go.  Only columns matching iColumn are
+** copied, all columns copied if iColumn is -1.  Elements with no
+** matching columns are dropped.  The output is an iOutType doclist.
+*/
+/* NOTE(shess) This code is only valid after all doclists are merged.
+** If this is run before merges, then doclist items which represent
+** deletion will be trimmed, and will thus not effect a deletion
+** during the merge.
+*/
+static int docListTrim(DocListType iType, const char *pData, int nData,
+                       int iColumn, DocListType iOutType, DataBuffer *out){
+  DLReader dlReader;
+  DLWriter dlWriter;
+  int rc;
+
+  assert( iOutType<=iType );
+
+  rc = dlrInit(&dlReader, iType, pData, nData);
+  if( rc!=SQLITE_OK ) return rc;
+  dlwInit(&dlWriter, iOutType, out);
+
+  while( !dlrAtEnd(&dlReader) ){
+    PLReader plReader;
+    PLWriter plWriter;
+    int match = 0;
+
+    rc = plrInit(&plReader, &dlReader);
+    if( rc!=SQLITE_OK ) break;
+
+    while( !plrAtEnd(&plReader) ){
+      if( iColumn==-1 || plrColumn(&plReader)==iColumn ){
+        if( !match ){
+          plwInit(&plWriter, &dlWriter, dlrDocid(&dlReader));
+          match = 1;
+        }
+        plwAdd(&plWriter, plrColumn(&plReader), plrPosition(&plReader),
+               plrStartOffset(&plReader), plrEndOffset(&plReader));
+      }
+      rc = plrStep(&plReader);
+      if( rc!=SQLITE_OK ){
+        plrDestroy(&plReader);
+        goto err;
+      }
+    }
+    if( match ){
+      plwTerminate(&plWriter);
+      plwDestroy(&plWriter);
+    }
+
+    plrDestroy(&plReader);
+    rc = dlrStep(&dlReader);
+    if( rc!=SQLITE_OK ) break;
+  }
+err:
+  dlwDestroy(&dlWriter);
+  dlrDestroy(&dlReader);
+  return rc;
+}
+
+/* Used by docListMerge() to keep doclists in the ascending order by
+** docid, then ascending order by age (so the newest comes first).
+*/
+typedef struct OrderedDLReader {
+  DLReader *pReader;
+
+  /* TODO(shess) If we assume that docListMerge pReaders is ordered by
+  ** age (which we do), then we could use pReader comparisons to break
+  ** ties.
+  */
+  int idx;
+} OrderedDLReader;
+
+/* Order eof to end, then by docid asc, idx desc. */
+static int orderedDLReaderCmp(OrderedDLReader *r1, OrderedDLReader *r2){
+  if( dlrAtEnd(r1->pReader) ){
+    if( dlrAtEnd(r2->pReader) ) return 0;  /* Both atEnd(). */
+    return 1;                              /* Only r1 atEnd(). */
+  }
+  if( dlrAtEnd(r2->pReader) ) return -1;   /* Only r2 atEnd(). */
+
+  if( dlrDocid(r1->pReader)<dlrDocid(r2->pReader) ) return -1;
+  if( dlrDocid(r1->pReader)>dlrDocid(r2->pReader) ) return 1;
+
+  /* Descending on idx. */
+  return r2->idx-r1->idx;
+}
+
+/* Bubble p[0] to appropriate place in p[1..n-1].  Assumes that
+** p[1..n-1] is already sorted.
+*/
+/* TODO(shess) Is this frequent enough to warrant a binary search?
+** Before implementing that, instrument the code to check.  In most
+** current usage, I expect that p[0] will be less than p[1] a very
+** high proportion of the time.
+*/
+static void orderedDLReaderReorder(OrderedDLReader *p, int n){
+  while( n>1 && orderedDLReaderCmp(p, p+1)>0 ){
+    OrderedDLReader tmp = p[0];
+    p[0] = p[1];
+    p[1] = tmp;
+    n--;
+    p++;
+  }
+}
+
+/* Given an array of doclist readers, merge their doclist elements
+** into out in sorted order (by docid), dropping elements from older
+** readers when there is a duplicate docid.  pReaders is assumed to be
+** ordered by age, oldest first.
+*/
+/* TODO(shess) nReaders must be <= MERGE_COUNT.  This should probably
+** be fixed.
+*/
+static int docListMerge(DataBuffer *out,
+                        DLReader *pReaders, int nReaders){
+  OrderedDLReader readers[MERGE_COUNT];
+  DLWriter writer;
+  int i, n;
+  const char *pStart = 0;
+  int nStart = 0;
+  sqlite_int64 iFirstDocid = 0, iLastDocid = 0;
+  int rc = SQLITE_OK;
+
+  assert( nReaders>0 );
+  if( nReaders==1 ){
+    dataBufferAppend(out, dlrDocData(pReaders), dlrAllDataBytes(pReaders));
+    return SQLITE_OK;
+  }
+
+  assert( nReaders<=MERGE_COUNT );
+  n = 0;
+  for(i=0; i<nReaders; i++){
+    assert( pReaders[i].iType==pReaders[0].iType );
+    readers[i].pReader = pReaders+i;
+    readers[i].idx = i;
+    n += dlrAllDataBytes(&pReaders[i]);
+  }
+  /* Conservatively size output to sum of inputs.  Output should end
+  ** up strictly smaller than input.
+  */
+  dataBufferExpand(out, n);
+
+  /* Get the readers into sorted order. */
+  while( i-->0 ){
+    orderedDLReaderReorder(readers+i, nReaders-i);
+  }
+
+  dlwInit(&writer, pReaders[0].iType, out);
+  while( !dlrAtEnd(readers[0].pReader) ){
+    sqlite_int64 iDocid = dlrDocid(readers[0].pReader);
+
+    /* If this is a continuation of the current buffer to copy, extend
+    ** that buffer.  memcpy() seems to be more efficient if it has a
+    ** lots of data to copy.
+    */
+    if( dlrDocData(readers[0].pReader)==pStart+nStart ){
+      nStart += dlrDocDataBytes(readers[0].pReader);
+    }else{
+      if( pStart!=0 ){
+        rc = dlwAppend(&writer, pStart, nStart, iFirstDocid, iLastDocid);
+        if( rc!=SQLITE_OK ) goto err;
+      }
+      pStart = dlrDocData(readers[0].pReader);
+      nStart = dlrDocDataBytes(readers[0].pReader);
+      iFirstDocid = iDocid;
+    }
+    iLastDocid = iDocid;
+    rc = dlrStep(readers[0].pReader);
+    if( rc!=SQLITE_OK ) goto err;
+
+    /* Drop all of the older elements with the same docid. */
+    for(i=1; i<nReaders &&
+             !dlrAtEnd(readers[i].pReader) &&
+             dlrDocid(readers[i].pReader)==iDocid; i++){
+      rc = dlrStep(readers[i].pReader);
+      if( rc!=SQLITE_OK ) goto err;
+    }
+
+    /* Get the readers back into order. */
+    while( i-->0 ){
+      orderedDLReaderReorder(readers+i, nReaders-i);
+    }
+  }
+
+  /* Copy over any remaining elements. */
+  if( nStart>0 )
+    rc = dlwAppend(&writer, pStart, nStart, iFirstDocid, iLastDocid);
+err:
+  dlwDestroy(&writer);
+  return rc;
+}
+
+/* Helper function for posListUnion().  Compares the current position
+** between left and right, returning as standard C idiom of <0 if
+** left<right, >0 if left>right, and 0 if left==right.  "End" always
+** compares greater.
+*/
+static int posListCmp(PLReader *pLeft, PLReader *pRight){
+  assert( pLeft->iType==pRight->iType );
+  if( pLeft->iType==DL_DOCIDS ) return 0;
+
+  if( plrAtEnd(pLeft) ) return plrAtEnd(pRight) ? 0 : 1;
+  if( plrAtEnd(pRight) ) return -1;
+
+  if( plrColumn(pLeft)<plrColumn(pRight) ) return -1;
+  if( plrColumn(pLeft)>plrColumn(pRight) ) return 1;
+
+  if( plrPosition(pLeft)<plrPosition(pRight) ) return -1;
+  if( plrPosition(pLeft)>plrPosition(pRight) ) return 1;
+  if( pLeft->iType==DL_POSITIONS ) return 0;
+
+  if( plrStartOffset(pLeft)<plrStartOffset(pRight) ) return -1;
+  if( plrStartOffset(pLeft)>plrStartOffset(pRight) ) return 1;
+
+  if( plrEndOffset(pLeft)<plrEndOffset(pRight) ) return -1;
+  if( plrEndOffset(pLeft)>plrEndOffset(pRight) ) return 1;
+
+  return 0;
+}
+
+/* Write the union of position lists in pLeft and pRight to pOut.
+** "Union" in this case meaning "All unique position tuples".  Should
+** work with any doclist type, though both inputs and the output
+** should be the same type.
+*/
+static int posListUnion(DLReader *pLeft, DLReader *pRight, DLWriter *pOut){
+  PLReader left, right;
+  PLWriter writer;
+  int rc;
+
+  assert( dlrDocid(pLeft)==dlrDocid(pRight) );
+  assert( pLeft->iType==pRight->iType );
+  assert( pLeft->iType==pOut->iType );
+
+  rc = plrInit(&left, pLeft);
+  if( rc != SQLITE_OK ) return rc;
+  rc = plrInit(&right, pRight);
+  if( rc != SQLITE_OK ){
+    plrDestroy(&left);
+    return rc;
+  }
+  plwInit(&writer, pOut, dlrDocid(pLeft));
+
+  while( !plrAtEnd(&left) || !plrAtEnd(&right) ){
+    int c = posListCmp(&left, &right);
+    if( c<0 ){
+      plwCopy(&writer, &left);
+      rc = plrStep(&left);
+      if( rc != SQLITE_OK ) break;
+    }else if( c>0 ){
+      plwCopy(&writer, &right);
+      rc = plrStep(&right);
+      if( rc != SQLITE_OK ) break;
+    }else{
+      plwCopy(&writer, &left);
+      rc = plrStep(&left);
+      if( rc != SQLITE_OK ) break;
+      rc = plrStep(&right);
+      if( rc != SQLITE_OK ) break;
+    }
+  }
+
+  plwTerminate(&writer);
+  plwDestroy(&writer);
+  plrDestroy(&left);
+  plrDestroy(&right);
+  return rc;
+}
+
+/* Write the union of doclists in pLeft and pRight to pOut.  For
+** docids in common between the inputs, the union of the position
+** lists is written.  Inputs and outputs are always type DL_DEFAULT.
+*/
+static int docListUnion(
+  const char *pLeft, int nLeft,
+  const char *pRight, int nRight,
+  DataBuffer *pOut      /* Write the combined doclist here */
+){
+  DLReader left, right;
+  DLWriter writer;
+  int rc;
+
+  if( nLeft==0 ){
+    if( nRight!=0) dataBufferAppend(pOut, pRight, nRight);
+    return SQLITE_OK;
+  }
+  if( nRight==0 ){
+    dataBufferAppend(pOut, pLeft, nLeft);
+    return SQLITE_OK;
+  }
+
+  rc = dlrInit(&left, DL_DEFAULT, pLeft, nLeft);
+  if( rc!=SQLITE_OK ) return rc;
+  rc = dlrInit(&right, DL_DEFAULT, pRight, nRight);
+  if( rc!=SQLITE_OK ){
+    dlrDestroy(&left);
+    return rc;
+  }
+  dlwInit(&writer, DL_DEFAULT, pOut);
+
+  while( !dlrAtEnd(&left) || !dlrAtEnd(&right) ){
+    if( dlrAtEnd(&right) ){
+      rc = dlwCopy(&writer, &left);
+      if( rc!=SQLITE_OK ) break;
+      rc = dlrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+    }else if( dlrAtEnd(&left) ){
+      rc = dlwCopy(&writer, &right);
+      if( rc!=SQLITE_OK ) break;
+      rc = dlrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }else if( dlrDocid(&left)<dlrDocid(&right) ){
+      rc = dlwCopy(&writer, &left);
+      if( rc!=SQLITE_OK ) break;
+      rc = dlrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+    }else if( dlrDocid(&left)>dlrDocid(&right) ){
+      rc = dlwCopy(&writer, &right);
+      if( rc!=SQLITE_OK ) break;
+      rc = dlrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }else{
+      rc = posListUnion(&left, &right, &writer);
+      if( rc!=SQLITE_OK ) break;
+      rc = dlrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+      rc = dlrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }
+  }
+
+  dlrDestroy(&left);
+  dlrDestroy(&right);
+  dlwDestroy(&writer);
+  return rc;
+}
+
+/* pLeft and pRight are DLReaders positioned to the same docid.
+**
+** If there are no instances in pLeft or pRight where the position
+** of pLeft is one less than the position of pRight, then this
+** routine adds nothing to pOut.
+**
+** If there are one or more instances where positions from pLeft
+** are exactly one less than positions from pRight, then add a new
+** document record to pOut.  If pOut wants to hold positions, then
+** include the positions from pRight that are one more than a
+** position in pLeft.  In other words:  pRight.iPos==pLeft.iPos+1.
+*/
+static int posListPhraseMerge(DLReader *pLeft, DLReader *pRight,
+                              DLWriter *pOut){
+  PLReader left, right;
+  PLWriter writer;
+  int match = 0;
+  int rc;
+
+  assert( dlrDocid(pLeft)==dlrDocid(pRight) );
+  assert( pOut->iType!=DL_POSITIONS_OFFSETS );
+
+  rc = plrInit(&left, pLeft);
+  if( rc!=SQLITE_OK ) return rc;
+  rc = plrInit(&right, pRight);
+  if( rc!=SQLITE_OK ){
+    plrDestroy(&left);
+    return rc;
+  }
+
+  while( !plrAtEnd(&left) && !plrAtEnd(&right) ){
+    if( plrColumn(&left)<plrColumn(&right) ){
+      rc = plrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+    }else if( plrColumn(&left)>plrColumn(&right) ){
+      rc = plrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }else if( plrPosition(&left)+1<plrPosition(&right) ){
+      rc = plrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+    }else if( plrPosition(&left)+1>plrPosition(&right) ){
+      rc = plrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }else{
+      if( !match ){
+        plwInit(&writer, pOut, dlrDocid(pLeft));
+        match = 1;
+      }
+      plwAdd(&writer, plrColumn(&right), plrPosition(&right), 0, 0);
+      rc = plrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+      rc = plrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }
+  }
+
+  if( match ){
+    plwTerminate(&writer);
+    plwDestroy(&writer);
+  }
+
+  plrDestroy(&left);
+  plrDestroy(&right);
+  return rc;
+}
+
+/* We have two doclists with positions:  pLeft and pRight.
+** Write the phrase intersection of these two doclists into pOut.
+**
+** A phrase intersection means that two documents only match
+** if pLeft.iPos+1==pRight.iPos.
+**
+** iType controls the type of data written to pOut.  If iType is
+** DL_POSITIONS, the positions are those from pRight.
+*/
+static int docListPhraseMerge(
+  const char *pLeft, int nLeft,
+  const char *pRight, int nRight,
+  DocListType iType,
+  DataBuffer *pOut      /* Write the combined doclist here */
+){
+  DLReader left, right;
+  DLWriter writer;
+  int rc;
+
+  if( nLeft==0 || nRight==0 ) return SQLITE_OK;
+
+  assert( iType!=DL_POSITIONS_OFFSETS );
+
+  rc = dlrInit(&left, DL_POSITIONS, pLeft, nLeft);
+  if( rc!=SQLITE_OK ) return rc;
+  rc = dlrInit(&right, DL_POSITIONS, pRight, nRight);
+  if( rc!=SQLITE_OK ){
+    dlrDestroy(&left);
+    return rc;
+  }
+  dlwInit(&writer, iType, pOut);
+
+  while( !dlrAtEnd(&left) && !dlrAtEnd(&right) ){
+    if( dlrDocid(&left)<dlrDocid(&right) ){
+      rc = dlrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+    }else if( dlrDocid(&right)<dlrDocid(&left) ){
+      rc = dlrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }else{
+      rc = posListPhraseMerge(&left, &right, &writer);
+      if( rc!=SQLITE_OK ) break;
+      rc = dlrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+      rc = dlrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }
+  }
+
+  dlrDestroy(&left);
+  dlrDestroy(&right);
+  dlwDestroy(&writer);
+  return rc;
+}
+
+/* We have two DL_DOCIDS doclists:  pLeft and pRight.
+** Write the intersection of these two doclists into pOut as a
+** DL_DOCIDS doclist.
+*/
+static int docListAndMerge(
+  const char *pLeft, int nLeft,
+  const char *pRight, int nRight,
+  DataBuffer *pOut      /* Write the combined doclist here */
+){
+  DLReader left, right;
+  DLWriter writer;
+  int rc;
+
+  if( nLeft==0 || nRight==0 ) return SQLITE_OK;
+
+  rc = dlrInit(&left, DL_DOCIDS, pLeft, nLeft);
+  if( rc!=SQLITE_OK ) return rc;
+  rc = dlrInit(&right, DL_DOCIDS, pRight, nRight);
+  if( rc!=SQLITE_OK ){
+    dlrDestroy(&left);
+    return rc;
+  }
+  dlwInit(&writer, DL_DOCIDS, pOut);
+
+  while( !dlrAtEnd(&left) && !dlrAtEnd(&right) ){
+    if( dlrDocid(&left)<dlrDocid(&right) ){
+      rc = dlrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+    }else if( dlrDocid(&right)<dlrDocid(&left) ){
+      rc = dlrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }else{
+      dlwAdd(&writer, dlrDocid(&left));
+      rc = dlrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+      rc = dlrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }
+  }
+
+  dlrDestroy(&left);
+  dlrDestroy(&right);
+  dlwDestroy(&writer);
+  return rc;
+}
+
+/* We have two DL_DOCIDS doclists:  pLeft and pRight.
+** Write the union of these two doclists into pOut as a
+** DL_DOCIDS doclist.
+*/
+static int docListOrMerge(
+  const char *pLeft, int nLeft,
+  const char *pRight, int nRight,
+  DataBuffer *pOut      /* Write the combined doclist here */
+){
+  DLReader left, right;
+  DLWriter writer;
+  int rc;
+
+  if( nLeft==0 ){
+    if( nRight!=0 ) dataBufferAppend(pOut, pRight, nRight);
+    return SQLITE_OK;
+  }
+  if( nRight==0 ){
+    dataBufferAppend(pOut, pLeft, nLeft);
+    return SQLITE_OK;
+  }
+
+  rc = dlrInit(&left, DL_DOCIDS, pLeft, nLeft);
+  if( rc!=SQLITE_OK ) return rc;
+  rc = dlrInit(&right, DL_DOCIDS, pRight, nRight);
+  if( rc!=SQLITE_OK ){
+    dlrDestroy(&left);
+    return rc;
+  }
+  dlwInit(&writer, DL_DOCIDS, pOut);
+
+  while( !dlrAtEnd(&left) || !dlrAtEnd(&right) ){
+    if( dlrAtEnd(&right) ){
+      dlwAdd(&writer, dlrDocid(&left));
+      rc = dlrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+    }else if( dlrAtEnd(&left) ){
+      dlwAdd(&writer, dlrDocid(&right));
+      rc = dlrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }else if( dlrDocid(&left)<dlrDocid(&right) ){
+      dlwAdd(&writer, dlrDocid(&left));
+      rc = dlrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+    }else if( dlrDocid(&right)<dlrDocid(&left) ){
+      dlwAdd(&writer, dlrDocid(&right));
+      rc = dlrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }else{
+      dlwAdd(&writer, dlrDocid(&left));
+      rc = dlrStep(&left);
+      if( rc!=SQLITE_OK ) break;
+      rc = dlrStep(&right);
+      if( rc!=SQLITE_OK ) break;
+    }
+  }
+
+  dlrDestroy(&left);
+  dlrDestroy(&right);
+  dlwDestroy(&writer);
+  return rc;
+}
+
+/* We have two DL_DOCIDS doclists:  pLeft and pRight.
+** Write into pOut as DL_DOCIDS doclist containing all documents that
+** occur in pLeft but not in pRight.
+*/
+static int docListExceptMerge(
+  const char *pLeft, int nLeft,
+  const char *pRight, int nRight,
+  DataBuffer *pOut      /* Write the combined doclist here */
+){
+  DLReader left, right;
+  DLWriter writer;
+  int rc;
+
+  if( nLeft==0 ) return SQLITE_OK;
+  if( nRight==0 ){
+    dataBufferAppend(pOut, pLeft, nLeft);
+    return SQLITE_OK;
+  }
+
+  rc = dlrInit(&left, DL_DOCIDS, pLeft, nLeft);
+  if( rc!=SQLITE_OK ) return rc;
+  rc = dlrInit(&right, DL_DOCIDS, pRight, nRight);
+  if( rc!=SQLITE_OK ){
+    dlrDestroy(&left);
+    return rc;
+  }
+  dlwInit(&writer, DL_DOCIDS, pOut);
+
+  while( !dlrAtEnd(&left) ){
+    while( !dlrAtEnd(&right) && dlrDocid(&right)<dlrDocid(&left) ){
+      rc = dlrStep(&right);
+      if( rc!=SQLITE_OK ) goto err;
+    }
+    if( dlrAtEnd(&right) || dlrDocid(&left)<dlrDocid(&right) ){
+      dlwAdd(&writer, dlrDocid(&left));
+    }
+    rc = dlrStep(&left);
+    if( rc!=SQLITE_OK ) break;
+  }
+
+err:
+  dlrDestroy(&left);
+  dlrDestroy(&right);
+  dlwDestroy(&writer);
+  return rc;
+}
+
+static char *string_dup_n(const char *s, int n){
+  char *str = sqlite3_malloc(n + 1);
+  memcpy(str, s, n);
+  str[n] = '\0';
+  return str;
+}
+
+/* Duplicate a string; the caller must free() the returned string.
+ * (We don't use strdup() since it is not part of the standard C library and
+ * may not be available everywhere.) */
+static char *string_dup(const char *s){
+  return string_dup_n(s, strlen(s));
+}
+
+/* Format a string, replacing each occurrence of the % character with
+ * zDb.zName.  This may be more convenient than sqlite_mprintf()
+ * when one string is used repeatedly in a format string.
+ * The caller must free() the returned string. */
+static char *string_format(const char *zFormat,
+                           const char *zDb, const char *zName){
+  const char *p;
+  size_t len = 0;
+  size_t nDb = strlen(zDb);
+  size_t nName = strlen(zName);
+  size_t nFullTableName = nDb+1+nName;
+  char *result;
+  char *r;
+
+  /* first compute length needed */
+  for(p = zFormat ; *p ; ++p){
+    len += (*p=='%' ? nFullTableName : 1);
+  }
+  len += 1;  /* for null terminator */
+
+  r = result = sqlite3_malloc(len);
+  for(p = zFormat; *p; ++p){
+    if( *p=='%' ){
+      memcpy(r, zDb, nDb);
+      r += nDb;
+      *r++ = '.';
+      memcpy(r, zName, nName);
+      r += nName;
+    } else {
+      *r++ = *p;
+    }
+  }
+  *r++ = '\0';
+  assert( r == result + len );
+  return result;
+}
+
+static int sql_exec(sqlite3 *db, const char *zDb, const char *zName,
+                    const char *zFormat){
+  char *zCommand = string_format(zFormat, zDb, zName);
+  int rc;
+  TRACE(("FTS2 sql: %s\n", zCommand));
+  rc = sqlite3_exec(db, zCommand, NULL, 0, NULL);
+  sqlite3_free(zCommand);
+  return rc;
+}
+
+static int sql_prepare(sqlite3 *db, const char *zDb, const char *zName,
+                       sqlite3_stmt **ppStmt, const char *zFormat){
+  char *zCommand = string_format(zFormat, zDb, zName);
+  int rc;
+  TRACE(("FTS2 prepare: %s\n", zCommand));
+  rc = sqlite3_prepare_v2(db, zCommand, -1, ppStmt, NULL);
+  sqlite3_free(zCommand);
+  return rc;
+}
+
+/* end utility functions */
+
+/* Forward reference */
+typedef struct fulltext_vtab fulltext_vtab;
+
+/* A single term in a query is represented by an instances of
+** the following structure.
+*/
+typedef struct QueryTerm {
+  short int nPhrase; /* How many following terms are part of the same phrase */
+  short int iPhrase; /* This is the i-th term of a phrase. */
+  short int iColumn; /* Column of the index that must match this term */
+  signed char isOr;  /* this term is preceded by "OR" */
+  signed char isNot; /* this term is preceded by "-" */
+  signed char isPrefix; /* this term is followed by "*" */
+  char *pTerm;       /* text of the term.  '\000' terminated.  malloced */
+  int nTerm;         /* Number of bytes in pTerm[] */
+} QueryTerm;
+
+
+/* A query string is parsed into a Query structure.
+ *
+ * We could, in theory, allow query strings to be complicated
+ * nested expressions with precedence determined by parentheses.
+ * But none of the major search engines do this.  (Perhaps the
+ * feeling is that an parenthesized expression is two complex of
+ * an idea for the average user to grasp.)  Taking our lead from
+ * the major search engines, we will allow queries to be a list
+ * of terms (with an implied AND operator) or phrases in double-quotes,
+ * with a single optional "-" before each non-phrase term to designate
+ * negation and an optional OR connector.
+ *
+ * OR binds more tightly than the implied AND, which is what the
+ * major search engines seem to do.  So, for example:
+ * 
+ *    [one two OR three]     ==>    one AND (two OR three)
+ *    [one OR two three]     ==>    (one OR two) AND three
+ *
+ * A "-" before a term matches all entries that lack that term.
+ * The "-" must occur immediately before the term with in intervening
+ * space.  This is how the search engines do it.
+ *
+ * A NOT term cannot be the right-hand operand of an OR.  If this
+ * occurs in the query string, the NOT is ignored:
+ *
+ *    [one OR -two]          ==>    one OR two
+ *
+ */
+typedef struct Query {
+  fulltext_vtab *pFts;  /* The full text index */
+  int nTerms;           /* Number of terms in the query */
+  QueryTerm *pTerms;    /* Array of terms.  Space obtained from malloc() */
+  int nextIsOr;         /* Set the isOr flag on the next inserted term */
+  int nextColumn;       /* Next word parsed must be in this column */
+  int dfltColumn;       /* The default column */
+} Query;
+
+
+/*
+** An instance of the following structure keeps track of generated
+** matching-word offset information and snippets.
+*/
+typedef struct Snippet {
+  int nMatch;     /* Total number of matches */
+  int nAlloc;     /* Space allocated for aMatch[] */
+  struct snippetMatch { /* One entry for each matching term */
+    char snStatus;       /* Status flag for use while constructing snippets */
+    short int iCol;      /* The column that contains the match */
+    short int iTerm;     /* The index in Query.pTerms[] of the matching term */
+    short int nByte;     /* Number of bytes in the term */
+    int iStart;          /* The offset to the first character of the term */
+  } *aMatch;      /* Points to space obtained from malloc */
+  char *zOffset;  /* Text rendering of aMatch[] */
+  int nOffset;    /* strlen(zOffset) */
+  char *zSnippet; /* Snippet text */
+  int nSnippet;   /* strlen(zSnippet) */
+} Snippet;
+
+
+typedef enum QueryType {
+  QUERY_GENERIC,   /* table scan */
+  QUERY_ROWID,     /* lookup by rowid */
+  QUERY_FULLTEXT   /* QUERY_FULLTEXT + [i] is a full-text search for column i*/
+} QueryType;
+
+typedef enum fulltext_statement {
+  CONTENT_INSERT_STMT,
+  CONTENT_SELECT_STMT,
+  CONTENT_UPDATE_STMT,
+  CONTENT_DELETE_STMT,
+  CONTENT_EXISTS_STMT,
+
+  BLOCK_INSERT_STMT,
+  BLOCK_SELECT_STMT,
+  BLOCK_DELETE_STMT,
+  BLOCK_DELETE_ALL_STMT,
+
+  SEGDIR_MAX_INDEX_STMT,
+  SEGDIR_SET_STMT,
+  SEGDIR_SELECT_LEVEL_STMT,
+  SEGDIR_SPAN_STMT,
+  SEGDIR_DELETE_STMT,
+  SEGDIR_SELECT_SEGMENT_STMT,
+  SEGDIR_SELECT_ALL_STMT,
+  SEGDIR_DELETE_ALL_STMT,
+  SEGDIR_COUNT_STMT,
+
+  MAX_STMT                     /* Always at end! */
+} fulltext_statement;
+
+/* These must exactly match the enum above. */
+/* TODO(shess): Is there some risk that a statement will be used in two
+** cursors at once, e.g.  if a query joins a virtual table to itself?
+** If so perhaps we should move some of these to the cursor object.
+*/
+static const char *const fulltext_zStatement[MAX_STMT] = {
+  /* CONTENT_INSERT */ NULL,  /* generated in contentInsertStatement() */
+  /* CONTENT_SELECT */ "select * from %_content where rowid = ?",
+  /* CONTENT_UPDATE */ NULL,  /* generated in contentUpdateStatement() */
+  /* CONTENT_DELETE */ "delete from %_content where rowid = ?",
+  /* CONTENT_EXISTS */ "select rowid from %_content limit 1",
+
+  /* BLOCK_INSERT */ "insert into %_segments values (?)",
+  /* BLOCK_SELECT */ "select block from %_segments where rowid = ?",
+  /* BLOCK_DELETE */ "delete from %_segments where rowid between ? and ?",
+  /* BLOCK_DELETE_ALL */ "delete from %_segments",
+
+  /* SEGDIR_MAX_INDEX */ "select max(idx) from %_segdir where level = ?",
+  /* SEGDIR_SET */ "insert into %_segdir values (?, ?, ?, ?, ?, ?)",
+  /* SEGDIR_SELECT_LEVEL */
+  "select start_block, leaves_end_block, root, idx from %_segdir "
+  " where level = ? order by idx",
+  /* SEGDIR_SPAN */
+  "select min(start_block), max(end_block) from %_segdir "
+  " where level = ? and start_block <> 0",
+  /* SEGDIR_DELETE */ "delete from %_segdir where level = ?",
+
+  /* NOTE(shess): The first three results of the following two
+  ** statements must match.
+  */
+  /* SEGDIR_SELECT_SEGMENT */
+  "select start_block, leaves_end_block, root from %_segdir "
+  " where level = ? and idx = ?",
+  /* SEGDIR_SELECT_ALL */
+  "select start_block, leaves_end_block, root from %_segdir "
+  " order by level desc, idx asc",
+  /* SEGDIR_DELETE_ALL */ "delete from %_segdir",
+  /* SEGDIR_COUNT */ "select count(*), ifnull(max(level),0) from %_segdir",
+};
+
+/*
+** A connection to a fulltext index is an instance of the following
+** structure.  The xCreate and xConnect methods create an instance
+** of this structure and xDestroy and xDisconnect free that instance.
+** All other methods receive a pointer to the structure as one of their
+** arguments.
+*/
+struct fulltext_vtab {
+  sqlite3_vtab base;               /* Base class used by SQLite core */
+  sqlite3 *db;                     /* The database connection */
+  const char *zDb;                 /* logical database name */
+  const char *zName;               /* virtual table name */
+  int nColumn;                     /* number of columns in virtual table */
+  char **azColumn;                 /* column names.  malloced */
+  char **azContentColumn;          /* column names in content table; malloced */
+  sqlite3_tokenizer *pTokenizer;   /* tokenizer for inserts and queries */
+
+  /* Precompiled statements which we keep as long as the table is
+  ** open.
+  */
+  sqlite3_stmt *pFulltextStatements[MAX_STMT];
+
+  /* Precompiled statements used for segment merges.  We run a
+  ** separate select across the leaf level of each tree being merged.
+  */
+  sqlite3_stmt *pLeafSelectStmts[MERGE_COUNT];
+  /* The statement used to prepare pLeafSelectStmts. */
+#define LEAF_SELECT \
+  "select block from %_segments where rowid between ? and ? order by rowid"
+
+  /* These buffer pending index updates during transactions.
+  ** nPendingData estimates the memory size of the pending data.  It
+  ** doesn't include the hash-bucket overhead, nor any malloc
+  ** overhead.  When nPendingData exceeds kPendingThreshold, the
+  ** buffer is flushed even before the transaction closes.
+  ** pendingTerms stores the data, and is only valid when nPendingData
+  ** is >=0 (nPendingData<0 means pendingTerms has not been
+  ** initialized).  iPrevDocid is the last docid written, used to make
+  ** certain we're inserting in sorted order.
+  */
+  int nPendingData;
+#define kPendingThreshold (1*1024*1024)
+  sqlite_int64 iPrevDocid;
+  fts2Hash pendingTerms;
+};
+
+/*
+** When the core wants to do a query, it create a cursor using a
+** call to xOpen.  This structure is an instance of a cursor.  It
+** is destroyed by xClose.
+*/
+typedef struct fulltext_cursor {
+  sqlite3_vtab_cursor base;        /* Base class used by SQLite core */
+  QueryType iCursorType;           /* Copy of sqlite3_index_info.idxNum */
+  sqlite3_stmt *pStmt;             /* Prepared statement in use by the cursor */
+  int eof;                         /* True if at End Of Results */
+  Query q;                         /* Parsed query string */
+  Snippet snippet;                 /* Cached snippet for the current row */
+  int iColumn;                     /* Column being searched */
+  DataBuffer result;               /* Doclist results from fulltextQuery */
+  DLReader reader;                 /* Result reader if result not empty */
+} fulltext_cursor;
+
+static struct fulltext_vtab *cursor_vtab(fulltext_cursor *c){
+  return (fulltext_vtab *) c->base.pVtab;
+}
+
+static const sqlite3_module fts2Module;   /* forward declaration */
+
+/* Return a dynamically generated statement of the form
+ *   insert into %_content (rowid, ...) values (?, ...)
+ */
+static const char *contentInsertStatement(fulltext_vtab *v){
+  StringBuffer sb;
+  int i;
+
+  initStringBuffer(&sb);
+  append(&sb, "insert into %_content (rowid, ");
+  appendList(&sb, v->nColumn, v->azContentColumn);
+  append(&sb, ") values (?");
+  for(i=0; i<v->nColumn; ++i)
+    append(&sb, ", ?");
+  append(&sb, ")");
+  return stringBufferData(&sb);
+}
+
+/* Return a dynamically generated statement of the form
+ *   update %_content set [col_0] = ?, [col_1] = ?, ...
+ *                    where rowid = ?
+ */
+static const char *contentUpdateStatement(fulltext_vtab *v){
+  StringBuffer sb;
+  int i;
+
+  initStringBuffer(&sb);
+  append(&sb, "update %_content set ");
+  for(i=0; i<v->nColumn; ++i) {
+    if( i>0 ){
+      append(&sb, ", ");
+    }
+    append(&sb, v->azContentColumn[i]);
+    append(&sb, " = ?");
+  }
+  append(&sb, " where rowid = ?");
+  return stringBufferData(&sb);
+}
+
+/* Puts a freshly-prepared statement determined by iStmt in *ppStmt.
+** If the indicated statement has never been prepared, it is prepared
+** and cached, otherwise the cached version is reset.
+*/
+static int sql_get_statement(fulltext_vtab *v, fulltext_statement iStmt,
+                             sqlite3_stmt **ppStmt){
+  assert( iStmt<MAX_STMT );
+  if( v->pFulltextStatements[iStmt]==NULL ){
+    const char *zStmt;
+    int rc;
+    switch( iStmt ){
+      case CONTENT_INSERT_STMT:
+        zStmt = contentInsertStatement(v); break;
+      case CONTENT_UPDATE_STMT:
+        zStmt = contentUpdateStatement(v); break;
+      default:
+        zStmt = fulltext_zStatement[iStmt];
+    }
+    rc = sql_prepare(v->db, v->zDb, v->zName, &v->pFulltextStatements[iStmt],
+                         zStmt);
+    if( zStmt != fulltext_zStatement[iStmt]) sqlite3_free((void *) zStmt);
+    if( rc!=SQLITE_OK ) return rc;
+  } else {
+    int rc = sqlite3_reset(v->pFulltextStatements[iStmt]);
+    if( rc!=SQLITE_OK ) return rc;
+  }
+
+  *ppStmt = v->pFulltextStatements[iStmt];
+  return SQLITE_OK;
+}
+
+/* Like sqlite3_step(), but convert SQLITE_DONE to SQLITE_OK and
+** SQLITE_ROW to SQLITE_ERROR.  Useful for statements like UPDATE,
+** where we expect no results.
+*/
+static int sql_single_step(sqlite3_stmt *s){
+  int rc = sqlite3_step(s);
+  return (rc==SQLITE_DONE) ? SQLITE_OK : rc;
+}
+
+/* Like sql_get_statement(), but for special replicated LEAF_SELECT
+** statements.  idx -1 is a special case for an uncached version of
+** the statement (used in the optimize implementation).
+*/
+/* TODO(shess) Write version for generic statements and then share
+** that between the cached-statement functions.
+*/
+static int sql_get_leaf_statement(fulltext_vtab *v, int idx,
+                                  sqlite3_stmt **ppStmt){
+  assert( idx>=-1 && idx<MERGE_COUNT );
+  if( idx==-1 ){
+    return sql_prepare(v->db, v->zDb, v->zName, ppStmt, LEAF_SELECT);
+  }else if( v->pLeafSelectStmts[idx]==NULL ){
+    int rc = sql_prepare(v->db, v->zDb, v->zName, &v->pLeafSelectStmts[idx],
+                         LEAF_SELECT);
+    if( rc!=SQLITE_OK ) return rc;
+  }else{
+    int rc = sqlite3_reset(v->pLeafSelectStmts[idx]);
+    if( rc!=SQLITE_OK ) return rc;
+  }
+
+  *ppStmt = v->pLeafSelectStmts[idx];
+  return SQLITE_OK;
+}
+
+/* insert into %_content (rowid, ...) values ([rowid], [pValues]) */
+static int content_insert(fulltext_vtab *v, sqlite3_value *rowid,
+                          sqlite3_value **pValues){
+  sqlite3_stmt *s;
+  int i;
+  int rc = sql_get_statement(v, CONTENT_INSERT_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_value(s, 1, rowid);
+  if( rc!=SQLITE_OK ) return rc;
+
+  for(i=0; i<v->nColumn; ++i){
+    rc = sqlite3_bind_value(s, 2+i, pValues[i]);
+    if( rc!=SQLITE_OK ) return rc;
+  }
+
+  return sql_single_step(s);
+}
+
+/* update %_content set col0 = pValues[0], col1 = pValues[1], ...
+ *                  where rowid = [iRowid] */
+static int content_update(fulltext_vtab *v, sqlite3_value **pValues,
+                          sqlite_int64 iRowid){
+  sqlite3_stmt *s;
+  int i;
+  int rc = sql_get_statement(v, CONTENT_UPDATE_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  for(i=0; i<v->nColumn; ++i){
+    rc = sqlite3_bind_value(s, 1+i, pValues[i]);
+    if( rc!=SQLITE_OK ) return rc;
+  }
+
+  rc = sqlite3_bind_int64(s, 1+v->nColumn, iRowid);
+  if( rc!=SQLITE_OK ) return rc;
+
+  return sql_single_step(s);
+}
+
+static void freeStringArray(int nString, const char **pString){
+  int i;
+
+  for (i=0 ; i < nString ; ++i) {
+    if( pString[i]!=NULL ) sqlite3_free((void *) pString[i]);
+  }
+  sqlite3_free((void *) pString);
+}
+
+/* select * from %_content where rowid = [iRow]
+ * The caller must delete the returned array and all strings in it.
+ * null fields will be NULL in the returned array.
+ *
+ * TODO: Perhaps we should return pointer/length strings here for consistency
+ * with other code which uses pointer/length. */
+static int content_select(fulltext_vtab *v, sqlite_int64 iRow,
+                          const char ***pValues){
+  sqlite3_stmt *s;
+  const char **values;
+  int i;
+  int rc;
+
+  *pValues = NULL;
+
+  rc = sql_get_statement(v, CONTENT_SELECT_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int64(s, 1, iRow);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_step(s);
+  if( rc!=SQLITE_ROW ) return rc;
+
+  values = (const char **) sqlite3_malloc(v->nColumn * sizeof(const char *));
+  for(i=0; i<v->nColumn; ++i){
+    if( sqlite3_column_type(s, i)==SQLITE_NULL ){
+      values[i] = NULL;
+    }else{
+      values[i] = string_dup((char*)sqlite3_column_text(s, i));
+    }
+  }
+
+  /* We expect only one row.  We must execute another sqlite3_step()
+   * to complete the iteration; otherwise the table will remain locked. */
+  rc = sqlite3_step(s);
+  if( rc==SQLITE_DONE ){
+    *pValues = values;
+    return SQLITE_OK;
+  }
+
+  freeStringArray(v->nColumn, values);
+  return rc;
+}
+
+/* delete from %_content where rowid = [iRow ] */
+static int content_delete(fulltext_vtab *v, sqlite_int64 iRow){
+  sqlite3_stmt *s;
+  int rc = sql_get_statement(v, CONTENT_DELETE_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int64(s, 1, iRow);
+  if( rc!=SQLITE_OK ) return rc;
+
+  return sql_single_step(s);
+}
+
+/* Returns SQLITE_ROW if any rows exist in %_content, SQLITE_DONE if
+** no rows exist, and any error in case of failure.
+*/
+static int content_exists(fulltext_vtab *v){
+  sqlite3_stmt *s;
+  int rc = sql_get_statement(v, CONTENT_EXISTS_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_step(s);
+  if( rc!=SQLITE_ROW ) return rc;
+
+  /* We expect only one row.  We must execute another sqlite3_step()
+   * to complete the iteration; otherwise the table will remain locked. */
+  rc = sqlite3_step(s);
+  if( rc==SQLITE_DONE ) return SQLITE_ROW;
+  if( rc==SQLITE_ROW ) return SQLITE_ERROR;
+  return rc;
+}
+
+/* insert into %_segments values ([pData])
+**   returns assigned rowid in *piBlockid
+*/
+static int block_insert(fulltext_vtab *v, const char *pData, int nData,
+                        sqlite_int64 *piBlockid){
+  sqlite3_stmt *s;
+  int rc = sql_get_statement(v, BLOCK_INSERT_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_blob(s, 1, pData, nData, SQLITE_STATIC);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_step(s);
+  if( rc==SQLITE_ROW ) return SQLITE_ERROR;
+  if( rc!=SQLITE_DONE ) return rc;
+
+  *piBlockid = sqlite3_last_insert_rowid(v->db);
+  return SQLITE_OK;
+}
+
+/* delete from %_segments
+**   where rowid between [iStartBlockid] and [iEndBlockid]
+**
+** Deletes the range of blocks, inclusive, used to delete the blocks
+** which form a segment.
+*/
+static int block_delete(fulltext_vtab *v,
+                        sqlite_int64 iStartBlockid, sqlite_int64 iEndBlockid){
+  sqlite3_stmt *s;
+  int rc = sql_get_statement(v, BLOCK_DELETE_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int64(s, 1, iStartBlockid);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int64(s, 2, iEndBlockid);
+  if( rc!=SQLITE_OK ) return rc;
+
+  return sql_single_step(s);
+}
+
+/* Returns SQLITE_ROW with *pidx set to the maximum segment idx found
+** at iLevel.  Returns SQLITE_DONE if there are no segments at
+** iLevel.  Otherwise returns an error.
+*/
+static int segdir_max_index(fulltext_vtab *v, int iLevel, int *pidx){
+  sqlite3_stmt *s;
+  int rc = sql_get_statement(v, SEGDIR_MAX_INDEX_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int(s, 1, iLevel);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_step(s);
+  /* Should always get at least one row due to how max() works. */
+  if( rc==SQLITE_DONE ) return SQLITE_DONE;
+  if( rc!=SQLITE_ROW ) return rc;
+
+  /* NULL means that there were no inputs to max(). */
+  if( SQLITE_NULL==sqlite3_column_type(s, 0) ){
+    rc = sqlite3_step(s);
+    if( rc==SQLITE_ROW ) return SQLITE_ERROR;
+    return rc;
+  }
+
+  *pidx = sqlite3_column_int(s, 0);
+
+  /* We expect only one row.  We must execute another sqlite3_step()
+   * to complete the iteration; otherwise the table will remain locked. */
+  rc = sqlite3_step(s);
+  if( rc==SQLITE_ROW ) return SQLITE_ERROR;
+  if( rc!=SQLITE_DONE ) return rc;
+  return SQLITE_ROW;
+}
+
+/* insert into %_segdir values (
+**   [iLevel], [idx],
+**   [iStartBlockid], [iLeavesEndBlockid], [iEndBlockid],
+**   [pRootData]
+** )
+*/
+static int segdir_set(fulltext_vtab *v, int iLevel, int idx,
+                      sqlite_int64 iStartBlockid,
+                      sqlite_int64 iLeavesEndBlockid,
+                      sqlite_int64 iEndBlockid,
+                      const char *pRootData, int nRootData){
+  sqlite3_stmt *s;
+  int rc = sql_get_statement(v, SEGDIR_SET_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int(s, 1, iLevel);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int(s, 2, idx);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int64(s, 3, iStartBlockid);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int64(s, 4, iLeavesEndBlockid);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int64(s, 5, iEndBlockid);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_blob(s, 6, pRootData, nRootData, SQLITE_STATIC);
+  if( rc!=SQLITE_OK ) return rc;
+
+  return sql_single_step(s);
+}
+
+/* Queries %_segdir for the block span of the segments in level
+** iLevel.  Returns SQLITE_DONE if there are no blocks for iLevel,
+** SQLITE_ROW if there are blocks, else an error.
+*/
+static int segdir_span(fulltext_vtab *v, int iLevel,
+                       sqlite_int64 *piStartBlockid,
+                       sqlite_int64 *piEndBlockid){
+  sqlite3_stmt *s;
+  int rc = sql_get_statement(v, SEGDIR_SPAN_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int(s, 1, iLevel);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_step(s);
+  if( rc==SQLITE_DONE ) return SQLITE_DONE;  /* Should never happen */
+  if( rc!=SQLITE_ROW ) return rc;
+
+  /* This happens if all segments at this level are entirely inline. */
+  if( SQLITE_NULL==sqlite3_column_type(s, 0) ){
+    /* We expect only one row.  We must execute another sqlite3_step()
+     * to complete the iteration; otherwise the table will remain locked. */
+    int rc2 = sqlite3_step(s);
+    if( rc2==SQLITE_ROW ) return SQLITE_ERROR;
+    return rc2;
+  }
+
+  *piStartBlockid = sqlite3_column_int64(s, 0);
+  *piEndBlockid = sqlite3_column_int64(s, 1);
+
+  /* We expect only one row.  We must execute another sqlite3_step()
+   * to complete the iteration; otherwise the table will remain locked. */
+  rc = sqlite3_step(s);
+  if( rc==SQLITE_ROW ) return SQLITE_ERROR;
+  if( rc!=SQLITE_DONE ) return rc;
+  return SQLITE_ROW;
+}
+
+/* Delete the segment blocks and segment directory records for all
+** segments at iLevel.
+*/
+static int segdir_delete(fulltext_vtab *v, int iLevel){
+  sqlite3_stmt *s;
+  sqlite_int64 iStartBlockid, iEndBlockid;
+  int rc = segdir_span(v, iLevel, &iStartBlockid, &iEndBlockid);
+  if( rc!=SQLITE_ROW && rc!=SQLITE_DONE ) return rc;
+
+  if( rc==SQLITE_ROW ){
+    rc = block_delete(v, iStartBlockid, iEndBlockid);
+    if( rc!=SQLITE_OK ) return rc;
+  }
+
+  /* Delete the segment directory itself. */
+  rc = sql_get_statement(v, SEGDIR_DELETE_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int64(s, 1, iLevel);
+  if( rc!=SQLITE_OK ) return rc;
+
+  return sql_single_step(s);
+}
+
+/* Delete entire fts index, SQLITE_OK on success, relevant error on
+** failure.
+*/
+static int segdir_delete_all(fulltext_vtab *v){
+  sqlite3_stmt *s;
+  int rc = sql_get_statement(v, SEGDIR_DELETE_ALL_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sql_single_step(s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sql_get_statement(v, BLOCK_DELETE_ALL_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  return sql_single_step(s);
+}
+
+/* Returns SQLITE_OK with *pnSegments set to the number of entries in
+** %_segdir and *piMaxLevel set to the highest level which has a
+** segment.  Otherwise returns the SQLite error which caused failure.
+*/
+static int segdir_count(fulltext_vtab *v, int *pnSegments, int *piMaxLevel){
+  sqlite3_stmt *s;
+  int rc = sql_get_statement(v, SEGDIR_COUNT_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_step(s);
+  /* TODO(shess): This case should not be possible?  Should stronger
+  ** measures be taken if it happens?
+  */
+  if( rc==SQLITE_DONE ){
+    *pnSegments = 0;
+    *piMaxLevel = 0;
+    return SQLITE_OK;
+  }
+  if( rc!=SQLITE_ROW ) return rc;
+
+  *pnSegments = sqlite3_column_int(s, 0);
+  *piMaxLevel = sqlite3_column_int(s, 1);
+
+  /* We expect only one row.  We must execute another sqlite3_step()
+   * to complete the iteration; otherwise the table will remain locked. */
+  rc = sqlite3_step(s);
+  if( rc==SQLITE_DONE ) return SQLITE_OK;
+  if( rc==SQLITE_ROW ) return SQLITE_ERROR;
+  return rc;
+}
+
+/* TODO(shess) clearPendingTerms() is far down the file because
+** writeZeroSegment() is far down the file because LeafWriter is far
+** down the file.  Consider refactoring the code to move the non-vtab
+** code above the vtab code so that we don't need this forward
+** reference.
+*/
+static int clearPendingTerms(fulltext_vtab *v);
+
+/*
+** Free the memory used to contain a fulltext_vtab structure.
+*/
+static void fulltext_vtab_destroy(fulltext_vtab *v){
+  int iStmt, i;
+
+  TRACE(("FTS2 Destroy %p\n", v));
+  for( iStmt=0; iStmt<MAX_STMT; iStmt++ ){
+    if( v->pFulltextStatements[iStmt]!=NULL ){
+      sqlite3_finalize(v->pFulltextStatements[iStmt]);
+      v->pFulltextStatements[iStmt] = NULL;
+    }
+  }
+
+  for( i=0; i<MERGE_COUNT; i++ ){
+    if( v->pLeafSelectStmts[i]!=NULL ){
+      sqlite3_finalize(v->pLeafSelectStmts[i]);
+      v->pLeafSelectStmts[i] = NULL;
+    }
+  }
+
+  if( v->pTokenizer!=NULL ){
+    v->pTokenizer->pModule->xDestroy(v->pTokenizer);
+    v->pTokenizer = NULL;
+  }
+
+  clearPendingTerms(v);
+
+  sqlite3_free(v->azColumn);
+  for(i = 0; i < v->nColumn; ++i) {
+    sqlite3_free(v->azContentColumn[i]);
+  }
+  sqlite3_free(v->azContentColumn);
+  sqlite3_free(v);
+}
+
+/*
+** Token types for parsing the arguments to xConnect or xCreate.
+*/
+#define TOKEN_EOF         0    /* End of file */
+#define TOKEN_SPACE       1    /* Any kind of whitespace */
+#define TOKEN_ID          2    /* An identifier */
+#define TOKEN_STRING      3    /* A string literal */
+#define TOKEN_PUNCT       4    /* A single punctuation character */
+
+/*
+** If X is a character that can be used in an identifier then
+** IdChar(X) will be true.  Otherwise it is false.
+**
+** For ASCII, any character with the high-order bit set is
+** allowed in an identifier.  For 7-bit characters, 
+** sqlite3IsIdChar[X] must be 1.
+**
+** Ticket #1066.  the SQL standard does not allow '$' in the
+** middle of identfiers.  But many SQL implementations do. 
+** SQLite will allow '$' in identifiers for compatibility.
+** But the feature is undocumented.
+*/
+static const char isIdChar[] = {
+/* x0 x1 x2 x3 x4 x5 x6 x7 x8 x9 xA xB xC xD xE xF */
+    0, 0, 0, 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  /* 2x */
+    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0, 0,  /* 3x */
+    0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,  /* 4x */
+    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 1,  /* 5x */
+    0, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1,  /* 6x */
+    1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 1, 0, 0, 0, 0, 0,  /* 7x */
+};
+#define IdChar(C)  (((c=C)&0x80)!=0 || (c>0x1f && isIdChar[c-0x20]))
+
+
+/*
+** Return the length of the token that begins at z[0]. 
+** Store the token type in *tokenType before returning.
+*/
+static int getToken(const char *z, int *tokenType){
+  int i, c;
+  switch( *z ){
+    case 0: {
+      *tokenType = TOKEN_EOF;
+      return 0;
+    }
+    case ' ': case '\t': case '\n': case '\f': case '\r': {
+      for(i=1; safe_isspace(z[i]); i++){}
+      *tokenType = TOKEN_SPACE;
+      return i;
+    }
+    case '`':
+    case '\'':
+    case '"': {
+      int delim = z[0];
+      for(i=1; (c=z[i])!=0; i++){
+        if( c==delim ){
+          if( z[i+1]==delim ){
+            i++;
+          }else{
+            break;
+          }
+        }
+      }
+      *tokenType = TOKEN_STRING;
+      return i + (c!=0);
+    }
+    case '[': {
+      for(i=1, c=z[0]; c!=']' && (c=z[i])!=0; i++){}
+      *tokenType = TOKEN_ID;
+      return i;
+    }
+    default: {
+      if( !IdChar(*z) ){
+        break;
+      }
+      for(i=1; IdChar(z[i]); i++){}
+      *tokenType = TOKEN_ID;
+      return i;
+    }
+  }
+  *tokenType = TOKEN_PUNCT;
+  return 1;
+}
+
+/*
+** A token extracted from a string is an instance of the following
+** structure.
+*/
+typedef struct Token {
+  const char *z;       /* Pointer to token text.  Not '\000' terminated */
+  short int n;         /* Length of the token text in bytes. */
+} Token;
+
+/*
+** Given a input string (which is really one of the argv[] parameters
+** passed into xConnect or xCreate) split the string up into tokens.
+** Return an array of pointers to '\000' terminated strings, one string
+** for each non-whitespace token.
+**
+** The returned array is terminated by a single NULL pointer.
+**
+** Space to hold the returned array is obtained from a single
+** malloc and should be freed by passing the return value to free().
+** The individual strings within the token list are all a part of
+** the single memory allocation and will all be freed at once.
+*/
+static char **tokenizeString(const char *z, int *pnToken){
+  int nToken = 0;
+  Token *aToken = sqlite3_malloc( strlen(z) * sizeof(aToken[0]) );
+  int n = 1;
+  int e, i;
+  int totalSize = 0;
+  char **azToken;
+  char *zCopy;
+  while( n>0 ){
+    n = getToken(z, &e);
+    if( e!=TOKEN_SPACE ){
+      aToken[nToken].z = z;
+      aToken[nToken].n = n;
+      nToken++;
+      totalSize += n+1;
+    }
+    z += n;
+  }
+  azToken = (char**)sqlite3_malloc( nToken*sizeof(char*) + totalSize );
+  zCopy = (char*)&azToken[nToken];
+  nToken--;
+  for(i=0; i<nToken; i++){
+    azToken[i] = zCopy;
+    n = aToken[i].n;
+    memcpy(zCopy, aToken[i].z, n);
+    zCopy[n] = 0;
+    zCopy += n+1;
+  }
+  azToken[nToken] = 0;
+  sqlite3_free(aToken);
+  *pnToken = nToken;
+  return azToken;
+}
+
+/*
+** Convert an SQL-style quoted string into a normal string by removing
+** the quote characters.  The conversion is done in-place.  If the
+** input does not begin with a quote character, then this routine
+** is a no-op.
+**
+** Examples:
+**
+**     "abc"   becomes   abc
+**     'xyz'   becomes   xyz
+**     [pqr]   becomes   pqr
+**     `mno`   becomes   mno
+*/
+static void dequoteString(char *z){
+  int quote;
+  int i, j;
+  if( z==0 ) return;
+  quote = z[0];
+  switch( quote ){
+    case '\'':  break;
+    case '"':   break;
+    case '`':   break;                /* For MySQL compatibility */
+    case '[':   quote = ']';  break;  /* For MS SqlServer compatibility */
+    default:    return;
+  }
+  for(i=1, j=0; z[i]; i++){
+    if( z[i]==quote ){
+      if( z[i+1]==quote ){
+        z[j++] = quote;
+        i++;
+      }else{
+        z[j++] = 0;
+        break;
+      }
+    }else{
+      z[j++] = z[i];
+    }
+  }
+}
+
+/*
+** The input azIn is a NULL-terminated list of tokens.  Remove the first
+** token and all punctuation tokens.  Remove the quotes from
+** around string literal tokens.
+**
+** Example:
+**
+**     input:      tokenize chinese ( 'simplifed' , 'mixed' )
+**     output:     chinese simplifed mixed
+**
+** Another example:
+**
+**     input:      delimiters ( '[' , ']' , '...' )
+**     output:     [ ] ...
+*/
+static void tokenListToIdList(char **azIn){
+  int i, j;
+  if( azIn ){
+    for(i=0, j=-1; azIn[i]; i++){
+      if( safe_isalnum(azIn[i][0]) || azIn[i][1] ){
+        dequoteString(azIn[i]);
+        if( j>=0 ){
+          azIn[j] = azIn[i];
+        }
+        j++;
+      }
+    }
+    azIn[j] = 0;
+  }
+}
+
+
+/*
+** Find the first alphanumeric token in the string zIn.  Null-terminate
+** this token.  Remove any quotation marks.  And return a pointer to
+** the result.
+*/
+static char *firstToken(char *zIn, char **pzTail){
+  int n, ttype;
+  while(1){
+    n = getToken(zIn, &ttype);
+    if( ttype==TOKEN_SPACE ){
+      zIn += n;
+    }else if( ttype==TOKEN_EOF ){
+      *pzTail = zIn;
+      return 0;
+    }else{
+      zIn[n] = 0;
+      *pzTail = &zIn[1];
+      dequoteString(zIn);
+      return zIn;
+    }
+  }
+  /*NOTREACHED*/
+}
+
+/* Return true if...
+**
+**   *  s begins with the string t, ignoring case
+**   *  s is longer than t
+**   *  The first character of s beyond t is not a alphanumeric
+** 
+** Ignore leading space in *s.
+**
+** To put it another way, return true if the first token of
+** s[] is t[].
+*/
+static int startsWith(const char *s, const char *t){
+  while( safe_isspace(*s) ){ s++; }
+  while( *t ){
+    if( safe_tolower(*s++)!=safe_tolower(*t++) ) return 0;
+  }
+  return *s!='_' && !safe_isalnum(*s);
+}
+
+/*
+** An instance of this structure defines the "spec" of a
+** full text index.  This structure is populated by parseSpec
+** and use by fulltextConnect and fulltextCreate.
+*/
+typedef struct TableSpec {
+  const char *zDb;         /* Logical database name */
+  const char *zName;       /* Name of the full-text index */
+  int nColumn;             /* Number of columns to be indexed */
+  char **azColumn;         /* Original names of columns to be indexed */
+  char **azContentColumn;  /* Column names for %_content */
+  char **azTokenizer;      /* Name of tokenizer and its arguments */
+} TableSpec;
+
+/*
+** Reclaim all of the memory used by a TableSpec
+*/
+static void clearTableSpec(TableSpec *p) {
+  sqlite3_free(p->azColumn);
+  sqlite3_free(p->azContentColumn);
+  sqlite3_free(p->azTokenizer);
+}
+
+/* Parse a CREATE VIRTUAL TABLE statement, which looks like this:
+ *
+ * CREATE VIRTUAL TABLE email
+ *        USING fts2(subject, body, tokenize mytokenizer(myarg))
+ *
+ * We return parsed information in a TableSpec structure.
+ * 
+ */
+static int parseSpec(TableSpec *pSpec, int argc, const char *const*argv,
+                     char**pzErr){
+  int i, n;
+  char *z, *zDummy;
+  char **azArg;
+  const char *zTokenizer = 0;    /* argv[] entry describing the tokenizer */
+
+  assert( argc>=3 );
+  /* Current interface:
+  ** argv[0] - module name
+  ** argv[1] - database name
+  ** argv[2] - table name
+  ** argv[3..] - columns, optionally followed by tokenizer specification
+  **             and snippet delimiters specification.
+  */
+
+  /* Make a copy of the complete argv[][] array in a single allocation.
+  ** The argv[][] array is read-only and transient.  We can write to the
+  ** copy in order to modify things and the copy is persistent.
+  */
+  CLEAR(pSpec);
+  for(i=n=0; i<argc; i++){
+    n += strlen(argv[i]) + 1;
+  }
+  azArg = sqlite3_malloc( sizeof(char*)*argc + n );
+  if( azArg==0 ){
+    return SQLITE_NOMEM;
+  }
+  z = (char*)&azArg[argc];
+  for(i=0; i<argc; i++){
+    azArg[i] = z;
+    strcpy(z, argv[i]);
+    z += strlen(z)+1;
+  }
+
+  /* Identify the column names and the tokenizer and delimiter arguments
+  ** in the argv[][] array.
+  */
+  pSpec->zDb = azArg[1];
+  pSpec->zName = azArg[2];
+  pSpec->nColumn = 0;
+  pSpec->azColumn = azArg;
+  zTokenizer = "tokenize simple";
+  for(i=3; i<argc; ++i){
+    if( startsWith(azArg[i],"tokenize") ){
+      zTokenizer = azArg[i];
+    }else{
+      z = azArg[pSpec->nColumn] = firstToken(azArg[i], &zDummy);
+      pSpec->nColumn++;
+    }
+  }
+  if( pSpec->nColumn==0 ){
+    azArg[0] = "content";
+    pSpec->nColumn = 1;
+  }
+
+  /*
+  ** Construct the list of content column names.
+  **
+  ** Each content column name will be of the form cNNAAAA
+  ** where NN is the column number and AAAA is the sanitized
+  ** column name.  "sanitized" means that special characters are
+  ** converted to "_".  The cNN prefix guarantees that all column
+  ** names are unique.
+  **
+  ** The AAAA suffix is not strictly necessary.  It is included
+  ** for the convenience of people who might examine the generated
+  ** %_content table and wonder what the columns are used for.
+  */
+  pSpec->azContentColumn = sqlite3_malloc( pSpec->nColumn * sizeof(char *) );
+  if( pSpec->azContentColumn==0 ){
+    clearTableSpec(pSpec);
+    return SQLITE_NOMEM;
+  }
+  for(i=0; i<pSpec->nColumn; i++){
+    char *p;
+    pSpec->azContentColumn[i] = sqlite3_mprintf("c%d%s", i, azArg[i]);
+    for (p = pSpec->azContentColumn[i]; *p ; ++p) {
+      if( !safe_isalnum(*p) ) *p = '_';
+    }
+  }
+
+  /*
+  ** Parse the tokenizer specification string.
+  */
+  pSpec->azTokenizer = tokenizeString(zTokenizer, &n);
+  tokenListToIdList(pSpec->azTokenizer);
+
+  return SQLITE_OK;
+}
+
+/*
+** Generate a CREATE TABLE statement that describes the schema of
+** the virtual table.  Return a pointer to this schema string.
+**
+** Space is obtained from sqlite3_mprintf() and should be freed
+** using sqlite3_free().
+*/
+static char *fulltextSchema(
+  int nColumn,                  /* Number of columns */
+  const char *const* azColumn,  /* List of columns */
+  const char *zTableName        /* Name of the table */
+){
+  int i;
+  char *zSchema, *zNext;
+  const char *zSep = "(";
+  zSchema = sqlite3_mprintf("CREATE TABLE x");
+  for(i=0; i<nColumn; i++){
+    zNext = sqlite3_mprintf("%s%s%Q", zSchema, zSep, azColumn[i]);
+    sqlite3_free(zSchema);
+    zSchema = zNext;
+    zSep = ",";
+  }
+  zNext = sqlite3_mprintf("%s,%Q)", zSchema, zTableName);
+  sqlite3_free(zSchema);
+  return zNext;
+}
+
+/*
+** Build a new sqlite3_vtab structure that will describe the
+** fulltext index defined by spec.
+*/
+static int constructVtab(
+  sqlite3 *db,              /* The SQLite database connection */
+  fts2Hash *pHash,          /* Hash table containing tokenizers */
+  TableSpec *spec,          /* Parsed spec information from parseSpec() */
+  sqlite3_vtab **ppVTab,    /* Write the resulting vtab structure here */
+  char **pzErr              /* Write any error message here */
+){
+  int rc;
+  int n;
+  fulltext_vtab *v = 0;
+  const sqlite3_tokenizer_module *m = NULL;
+  char *schema;
+
+  char const *zTok;         /* Name of tokenizer to use for this fts table */
+  int nTok;                 /* Length of zTok, including nul terminator */
+
+  v = (fulltext_vtab *) sqlite3_malloc(sizeof(fulltext_vtab));
+  if( v==0 ) return SQLITE_NOMEM;
+  CLEAR(v);
+  /* sqlite will initialize v->base */
+  v->db = db;
+  v->zDb = spec->zDb;       /* Freed when azColumn is freed */
+  v->zName = spec->zName;   /* Freed when azColumn is freed */
+  v->nColumn = spec->nColumn;
+  v->azContentColumn = spec->azContentColumn;
+  spec->azContentColumn = 0;
+  v->azColumn = spec->azColumn;
+  spec->azColumn = 0;
+
+  if( spec->azTokenizer==0 ){
+    return SQLITE_NOMEM;
+  }
+
+  zTok = spec->azTokenizer[0]; 
+  if( !zTok ){
+    zTok = "simple";
+  }
+  nTok = strlen(zTok)+1;
+
+  m = (sqlite3_tokenizer_module *)sqlite3Fts2HashFind(pHash, zTok, nTok);
+  if( !m ){
+    *pzErr = sqlite3_mprintf("unknown tokenizer: %s", spec->azTokenizer[0]);
+    rc = SQLITE_ERROR;
+    goto err;
+  }
+
+  for(n=0; spec->azTokenizer[n]; n++){}
+  if( n ){
+    rc = m->xCreate(n-1, (const char*const*)&spec->azTokenizer[1],
+                    &v->pTokenizer);
+  }else{
+    rc = m->xCreate(0, 0, &v->pTokenizer);
+  }
+  if( rc!=SQLITE_OK ) goto err;
+  v->pTokenizer->pModule = m;
+
+  /* TODO: verify the existence of backing tables foo_content, foo_term */
+
+  schema = fulltextSchema(v->nColumn, (const char*const*)v->azColumn,
+                          spec->zName);
+  rc = sqlite3_declare_vtab(db, schema);
+  sqlite3_free(schema);
+  if( rc!=SQLITE_OK ) goto err;
+
+  memset(v->pFulltextStatements, 0, sizeof(v->pFulltextStatements));
+
+  /* Indicate that the buffer is not live. */
+  v->nPendingData = -1;
+
+  *ppVTab = &v->base;
+  TRACE(("FTS2 Connect %p\n", v));
+
+  return rc;
+
+err:
+  fulltext_vtab_destroy(v);
+  return rc;
+}
+
+static int fulltextConnect(
+  sqlite3 *db,
+  void *pAux,
+  int argc, const char *const*argv,
+  sqlite3_vtab **ppVTab,
+  char **pzErr
+){
+  TableSpec spec;
+  int rc = parseSpec(&spec, argc, argv, pzErr);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = constructVtab(db, (fts2Hash *)pAux, &spec, ppVTab, pzErr);
+  clearTableSpec(&spec);
+  return rc;
+}
+
+/* The %_content table holds the text of each document, with
+** the rowid used as the docid.
+*/
+/* TODO(shess) This comment needs elaboration to match the updated
+** code.  Work it into the top-of-file comment at that time.
+*/
+static int fulltextCreate(sqlite3 *db, void *pAux,
+                          int argc, const char * const *argv,
+                          sqlite3_vtab **ppVTab, char **pzErr){
+  int rc;
+  TableSpec spec;
+  StringBuffer schema;
+  TRACE(("FTS2 Create\n"));
+
+  rc = parseSpec(&spec, argc, argv, pzErr);
+  if( rc!=SQLITE_OK ) return rc;
+
+  initStringBuffer(&schema);
+  append(&schema, "CREATE TABLE %_content(");
+  appendList(&schema, spec.nColumn, spec.azContentColumn);
+  append(&schema, ")");
+  rc = sql_exec(db, spec.zDb, spec.zName, stringBufferData(&schema));
+  stringBufferDestroy(&schema);
+  if( rc!=SQLITE_OK ) goto out;
+
+  rc = sql_exec(db, spec.zDb, spec.zName,
+                "create table %_segments(block blob);");
+  if( rc!=SQLITE_OK ) goto out;
+
+  rc = sql_exec(db, spec.zDb, spec.zName,
+                "create table %_segdir("
+                "  level integer,"
+                "  idx integer,"
+                "  start_block integer,"
+                "  leaves_end_block integer,"
+                "  end_block integer,"
+                "  root blob,"
+                "  primary key(level, idx)"
+                ");");
+  if( rc!=SQLITE_OK ) goto out;
+
+  rc = constructVtab(db, (fts2Hash *)pAux, &spec, ppVTab, pzErr);
+
+out:
+  clearTableSpec(&spec);
+  return rc;
+}
+
+/* Decide how to handle an SQL query. */
+static int fulltextBestIndex(sqlite3_vtab *pVTab, sqlite3_index_info *pInfo){
+  int i;
+  TRACE(("FTS2 BestIndex\n"));
+
+  for(i=0; i<pInfo->nConstraint; ++i){
+    const struct sqlite3_index_constraint *pConstraint;
+    pConstraint = &pInfo->aConstraint[i];
+    if( pConstraint->usable ) {
+      if( pConstraint->iColumn==-1 &&
+          pConstraint->op==SQLITE_INDEX_CONSTRAINT_EQ ){
+        pInfo->idxNum = QUERY_ROWID;      /* lookup by rowid */
+        TRACE(("FTS2 QUERY_ROWID\n"));
+      } else if( pConstraint->iColumn>=0 &&
+                 pConstraint->op==SQLITE_INDEX_CONSTRAINT_MATCH ){
+        /* full-text search */
+        pInfo->idxNum = QUERY_FULLTEXT + pConstraint->iColumn;
+        TRACE(("FTS2 QUERY_FULLTEXT %d\n", pConstraint->iColumn));
+      } else continue;
+
+      pInfo->aConstraintUsage[i].argvIndex = 1;
+      pInfo->aConstraintUsage[i].omit = 1;
+
+      /* An arbitrary value for now.
+       * TODO: Perhaps rowid matches should be considered cheaper than
+       * full-text searches. */
+      pInfo->estimatedCost = 1.0;   
+
+      return SQLITE_OK;
+    }
+  }
+  pInfo->idxNum = QUERY_GENERIC;
+  return SQLITE_OK;
+}
+
+static int fulltextDisconnect(sqlite3_vtab *pVTab){
+  TRACE(("FTS2 Disconnect %p\n", pVTab));
+  fulltext_vtab_destroy((fulltext_vtab *)pVTab);
+  return SQLITE_OK;
+}
+
+static int fulltextDestroy(sqlite3_vtab *pVTab){
+  fulltext_vtab *v = (fulltext_vtab *)pVTab;
+  int rc;
+
+  TRACE(("FTS2 Destroy %p\n", pVTab));
+  rc = sql_exec(v->db, v->zDb, v->zName,
+                "drop table if exists %_content;"
+                "drop table if exists %_segments;"
+                "drop table if exists %_segdir;"
+                );
+  if( rc!=SQLITE_OK ) return rc;
+
+  fulltext_vtab_destroy((fulltext_vtab *)pVTab);
+  return SQLITE_OK;
+}
+
+static int fulltextOpen(sqlite3_vtab *pVTab, sqlite3_vtab_cursor **ppCursor){
+  fulltext_cursor *c;
+
+  c = (fulltext_cursor *) sqlite3_malloc(sizeof(fulltext_cursor));
+  if( c ){
+    memset(c, 0, sizeof(fulltext_cursor));
+    /* sqlite will initialize c->base */
+    *ppCursor = &c->base;
+    TRACE(("FTS2 Open %p: %p\n", pVTab, c));
+    return SQLITE_OK;
+  }else{
+    return SQLITE_NOMEM;
+  }
+}
+
+
+/* Free all of the dynamically allocated memory held by *q
+*/
+static void queryClear(Query *q){
+  int i;
+  for(i = 0; i < q->nTerms; ++i){
+    sqlite3_free(q->pTerms[i].pTerm);
+  }
+  sqlite3_free(q->pTerms);
+  CLEAR(q);
+}
+
+/* Free all of the dynamically allocated memory held by the
+** Snippet
+*/
+static void snippetClear(Snippet *p){
+  sqlite3_free(p->aMatch);
+  sqlite3_free(p->zOffset);
+  sqlite3_free(p->zSnippet);
+  CLEAR(p);
+}
+/*
+** Append a single entry to the p->aMatch[] log.
+*/
+static void snippetAppendMatch(
+  Snippet *p,               /* Append the entry to this snippet */
+  int iCol, int iTerm,      /* The column and query term */
+  int iStart, int nByte     /* Offset and size of the match */
+){
+  int i;
+  struct snippetMatch *pMatch;
+  if( p->nMatch+1>=p->nAlloc ){
+    p->nAlloc = p->nAlloc*2 + 10;
+    p->aMatch = sqlite3_realloc(p->aMatch, p->nAlloc*sizeof(p->aMatch[0]) );
+    if( p->aMatch==0 ){
+      p->nMatch = 0;
+      p->nAlloc = 0;
+      return;
+    }
+  }
+  i = p->nMatch++;
+  pMatch = &p->aMatch[i];
+  pMatch->iCol = iCol;
+  pMatch->iTerm = iTerm;
+  pMatch->iStart = iStart;
+  pMatch->nByte = nByte;
+}
+
+/*
+** Sizing information for the circular buffer used in snippetOffsetsOfColumn()
+*/
+#define FTS2_ROTOR_SZ   (32)
+#define FTS2_ROTOR_MASK (FTS2_ROTOR_SZ-1)
+
+/*
+** Add entries to pSnippet->aMatch[] for every match that occurs against
+** document zDoc[0..nDoc-1] which is stored in column iColumn.
+*/
+static void snippetOffsetsOfColumn(
+  Query *pQuery,
+  Snippet *pSnippet,
+  int iColumn,
+  const char *zDoc,
+  int nDoc
+){
+  const sqlite3_tokenizer_module *pTModule;  /* The tokenizer module */
+  sqlite3_tokenizer *pTokenizer;             /* The specific tokenizer */
+  sqlite3_tokenizer_cursor *pTCursor;        /* Tokenizer cursor */
+  fulltext_vtab *pVtab;                /* The full text index */
+  int nColumn;                         /* Number of columns in the index */
+  const QueryTerm *aTerm;              /* Query string terms */
+  int nTerm;                           /* Number of query string terms */  
+  int i, j;                            /* Loop counters */
+  int rc;                              /* Return code */
+  unsigned int match, prevMatch;       /* Phrase search bitmasks */
+  const char *zToken;                  /* Next token from the tokenizer */
+  int nToken;                          /* Size of zToken */
+  int iBegin, iEnd, iPos;              /* Offsets of beginning and end */
+
+  /* The following variables keep a circular buffer of the last
+  ** few tokens */
+  unsigned int iRotor = 0;             /* Index of current token */
+  int iRotorBegin[FTS2_ROTOR_SZ];      /* Beginning offset of token */
+  int iRotorLen[FTS2_ROTOR_SZ];        /* Length of token */
+
+  pVtab = pQuery->pFts;
+  nColumn = pVtab->nColumn;
+  pTokenizer = pVtab->pTokenizer;
+  pTModule = pTokenizer->pModule;
+  rc = pTModule->xOpen(pTokenizer, zDoc, nDoc, &pTCursor);
+  if( rc ) return;
+  pTCursor->pTokenizer = pTokenizer;
+  aTerm = pQuery->pTerms;
+  nTerm = pQuery->nTerms;
+  if( nTerm>=FTS2_ROTOR_SZ ){
+    nTerm = FTS2_ROTOR_SZ - 1;
+  }
+  prevMatch = 0;
+  while(1){
+    rc = pTModule->xNext(pTCursor, &zToken, &nToken, &iBegin, &iEnd, &iPos);
+    if( rc ) break;
+    iRotorBegin[iRotor&FTS2_ROTOR_MASK] = iBegin;
+    iRotorLen[iRotor&FTS2_ROTOR_MASK] = iEnd-iBegin;
+    match = 0;
+    for(i=0; i<nTerm; i++){
+      int iCol;
+      iCol = aTerm[i].iColumn;
+      if( iCol>=0 && iCol<nColumn && iCol!=iColumn ) continue;
+      if( aTerm[i].nTerm>nToken ) continue;
+      if( !aTerm[i].isPrefix && aTerm[i].nTerm<nToken ) continue;
+      assert( aTerm[i].nTerm<=nToken );
+      if( memcmp(aTerm[i].pTerm, zToken, aTerm[i].nTerm) ) continue;
+      if( aTerm[i].iPhrase>1 && (prevMatch & (1<<i))==0 ) continue;
+      match |= 1<<i;
+      if( i==nTerm-1 || aTerm[i+1].iPhrase==1 ){
+        for(j=aTerm[i].iPhrase-1; j>=0; j--){
+          int k = (iRotor-j) & FTS2_ROTOR_MASK;
+          snippetAppendMatch(pSnippet, iColumn, i-j,
+                iRotorBegin[k], iRotorLen[k]);
+        }
+      }
+    }
+    prevMatch = match<<1;
+    iRotor++;
+  }
+  pTModule->xClose(pTCursor);  
+}
+
+
+/*
+** Compute all offsets for the current row of the query.  
+** If the offsets have already been computed, this routine is a no-op.
+*/
+static void snippetAllOffsets(fulltext_cursor *p){
+  int nColumn;
+  int iColumn, i;
+  int iFirst, iLast;
+  fulltext_vtab *pFts;
+
+  if( p->snippet.nMatch ) return;
+  if( p->q.nTerms==0 ) return;
+  pFts = p->q.pFts;
+  nColumn = pFts->nColumn;
+  iColumn = (p->iCursorType - QUERY_FULLTEXT);
+  if( iColumn<0 || iColumn>=nColumn ){
+    iFirst = 0;
+    iLast = nColumn-1;
+  }else{
+    iFirst = iColumn;
+    iLast = iColumn;
+  }
+  for(i=iFirst; i<=iLast; i++){
+    const char *zDoc;
+    int nDoc;
+    zDoc = (const char*)sqlite3_column_text(p->pStmt, i+1);
+    nDoc = sqlite3_column_bytes(p->pStmt, i+1);
+    snippetOffsetsOfColumn(&p->q, &p->snippet, i, zDoc, nDoc);
+  }
+}
+
+/*
+** Convert the information in the aMatch[] array of the snippet
+** into the string zOffset[0..nOffset-1].
+*/
+static void snippetOffsetText(Snippet *p){
+  int i;
+  int cnt = 0;
+  StringBuffer sb;
+  char zBuf[200];
+  if( p->zOffset ) return;
+  initStringBuffer(&sb);
+  for(i=0; i<p->nMatch; i++){
+    struct snippetMatch *pMatch = &p->aMatch[i];
+    zBuf[0] = ' ';
+    sqlite3_snprintf(sizeof(zBuf)-1, &zBuf[cnt>0], "%d %d %d %d",
+        pMatch->iCol, pMatch->iTerm, pMatch->iStart, pMatch->nByte);
+    append(&sb, zBuf);
+    cnt++;
+  }
+  p->zOffset = stringBufferData(&sb);
+  p->nOffset = stringBufferLength(&sb);
+}
+
+/*
+** zDoc[0..nDoc-1] is phrase of text.  aMatch[0..nMatch-1] are a set
+** of matching words some of which might be in zDoc.  zDoc is column
+** number iCol.
+**
+** iBreak is suggested spot in zDoc where we could begin or end an
+** excerpt.  Return a value similar to iBreak but possibly adjusted
+** to be a little left or right so that the break point is better.
+*/
+static int wordBoundary(
+  int iBreak,                   /* The suggested break point */
+  const char *zDoc,             /* Document text */
+  int nDoc,                     /* Number of bytes in zDoc[] */
+  struct snippetMatch *aMatch,  /* Matching words */
+  int nMatch,                   /* Number of entries in aMatch[] */
+  int iCol                      /* The column number for zDoc[] */
+){
+  int i;
+  if( iBreak<=10 ){
+    return 0;
+  }
+  if( iBreak>=nDoc-10 ){
+    return nDoc;
+  }
+  for(i=0; i<nMatch && aMatch[i].iCol<iCol; i++){}
+  while( i<nMatch && aMatch[i].iStart+aMatch[i].nByte<iBreak ){ i++; }
+  if( i<nMatch ){
+    if( aMatch[i].iStart<iBreak+10 ){
+      return aMatch[i].iStart;
+    }
+    if( i>0 && aMatch[i-1].iStart+aMatch[i-1].nByte>=iBreak ){
+      return aMatch[i-1].iStart;
+    }
+  }
+  for(i=1; i<=10; i++){
+    if( safe_isspace(zDoc[iBreak-i]) ){
+      return iBreak - i + 1;
+    }
+    if( safe_isspace(zDoc[iBreak+i]) ){
+      return iBreak + i + 1;
+    }
+  }
+  return iBreak;
+}
+
+
+
+/*
+** Allowed values for Snippet.aMatch[].snStatus
+*/
+#define SNIPPET_IGNORE  0   /* It is ok to omit this match from the snippet */
+#define SNIPPET_DESIRED 1   /* We want to include this match in the snippet */
+
+/*
+** Generate the text of a snippet.
+*/
+static void snippetText(
+  fulltext_cursor *pCursor,   /* The cursor we need the snippet for */
+  const char *zStartMark,     /* Markup to appear before each match */
+  const char *zEndMark,       /* Markup to appear after each match */
+  const char *zEllipsis       /* Ellipsis mark */
+){
+  int i, j;
+  struct snippetMatch *aMatch;
+  int nMatch;
+  int nDesired;
+  StringBuffer sb;
+  int tailCol;
+  int tailOffset;
+  int iCol;
+  int nDoc;
+  const char *zDoc;
+  int iStart, iEnd;
+  int tailEllipsis = 0;
+  int iMatch;
+  
+
+  sqlite3_free(pCursor->snippet.zSnippet);
+  pCursor->snippet.zSnippet = 0;
+  aMatch = pCursor->snippet.aMatch;
+  nMatch = pCursor->snippet.nMatch;
+  initStringBuffer(&sb);
+
+  for(i=0; i<nMatch; i++){
+    aMatch[i].snStatus = SNIPPET_IGNORE;
+  }
+  nDesired = 0;
+  for(i=0; i<pCursor->q.nTerms; i++){
+    for(j=0; j<nMatch; j++){
+      if( aMatch[j].iTerm==i ){
+        aMatch[j].snStatus = SNIPPET_DESIRED;
+        nDesired++;
+        break;
+      }
+    }
+  }
+
+  iMatch = 0;
+  tailCol = -1;
+  tailOffset = 0;
+  for(i=0; i<nMatch && nDesired>0; i++){
+    if( aMatch[i].snStatus!=SNIPPET_DESIRED ) continue;
+    nDesired--;
+    iCol = aMatch[i].iCol;
+    zDoc = (const char*)sqlite3_column_text(pCursor->pStmt, iCol+1);
+    nDoc = sqlite3_column_bytes(pCursor->pStmt, iCol+1);
+    iStart = aMatch[i].iStart - 40;
+    iStart = wordBoundary(iStart, zDoc, nDoc, aMatch, nMatch, iCol);
+    if( iStart<=10 ){
+      iStart = 0;
+    }
+    if( iCol==tailCol && iStart<=tailOffset+20 ){
+      iStart = tailOffset;
+    }
+    if( (iCol!=tailCol && tailCol>=0) || iStart!=tailOffset ){
+      trimWhiteSpace(&sb);
+      appendWhiteSpace(&sb);
+      append(&sb, zEllipsis);
+      appendWhiteSpace(&sb);
+    }
+    iEnd = aMatch[i].iStart + aMatch[i].nByte + 40;
+    iEnd = wordBoundary(iEnd, zDoc, nDoc, aMatch, nMatch, iCol);
+    if( iEnd>=nDoc-10 ){
+      iEnd = nDoc;
+      tailEllipsis = 0;
+    }else{
+      tailEllipsis = 1;
+    }
+    while( iMatch<nMatch && aMatch[iMatch].iCol<iCol ){ iMatch++; }
+    while( iStart<iEnd ){
+      while( iMatch<nMatch && aMatch[iMatch].iStart<iStart
+             && aMatch[iMatch].iCol<=iCol ){
+        iMatch++;
+      }
+      if( iMatch<nMatch && aMatch[iMatch].iStart<iEnd
+             && aMatch[iMatch].iCol==iCol ){
+        nappend(&sb, &zDoc[iStart], aMatch[iMatch].iStart - iStart);
+        iStart = aMatch[iMatch].iStart;
+        append(&sb, zStartMark);
+        nappend(&sb, &zDoc[iStart], aMatch[iMatch].nByte);
+        append(&sb, zEndMark);
+        iStart += aMatch[iMatch].nByte;
+        for(j=iMatch+1; j<nMatch; j++){
+          if( aMatch[j].iTerm==aMatch[iMatch].iTerm
+              && aMatch[j].snStatus==SNIPPET_DESIRED ){
+            nDesired--;
+            aMatch[j].snStatus = SNIPPET_IGNORE;
+          }
+        }
+      }else{
+        nappend(&sb, &zDoc[iStart], iEnd - iStart);
+        iStart = iEnd;
+      }
+    }
+    tailCol = iCol;
+    tailOffset = iEnd;
+  }
+  trimWhiteSpace(&sb);
+  if( tailEllipsis ){
+    appendWhiteSpace(&sb);
+    append(&sb, zEllipsis);
+  }
+  pCursor->snippet.zSnippet = stringBufferData(&sb);
+  pCursor->snippet.nSnippet = stringBufferLength(&sb);
+}
+
+
+/*
+** Close the cursor.  For additional information see the documentation
+** on the xClose method of the virtual table interface.
+*/
+static int fulltextClose(sqlite3_vtab_cursor *pCursor){
+  fulltext_cursor *c = (fulltext_cursor *) pCursor;
+  TRACE(("FTS2 Close %p\n", c));
+  sqlite3_finalize(c->pStmt);
+  queryClear(&c->q);
+  snippetClear(&c->snippet);
+  if( c->result.nData!=0 ) dlrDestroy(&c->reader);
+  dataBufferDestroy(&c->result);
+  sqlite3_free(c);
+  return SQLITE_OK;
+}
+
+static int fulltextNext(sqlite3_vtab_cursor *pCursor){
+  fulltext_cursor *c = (fulltext_cursor *) pCursor;
+  int rc;
+
+  TRACE(("FTS2 Next %p\n", pCursor));
+  snippetClear(&c->snippet);
+  if( c->iCursorType < QUERY_FULLTEXT ){
+    /* TODO(shess) Handle SQLITE_SCHEMA AND SQLITE_BUSY. */
+    rc = sqlite3_step(c->pStmt);
+    switch( rc ){
+      case SQLITE_ROW:
+        c->eof = 0;
+        return SQLITE_OK;
+      case SQLITE_DONE:
+        c->eof = 1;
+        return SQLITE_OK;
+      default:
+        c->eof = 1;
+        return rc;
+    }
+  } else {  /* full-text query */
+    rc = sqlite3_reset(c->pStmt);
+    if( rc!=SQLITE_OK ) return rc;
+
+    if( c->result.nData==0 || dlrAtEnd(&c->reader) ){
+      c->eof = 1;
+      return SQLITE_OK;
+    }
+    rc = sqlite3_bind_int64(c->pStmt, 1, dlrDocid(&c->reader));
+    if( rc!=SQLITE_OK ) return rc;
+    rc = dlrStep(&c->reader);
+    if( rc!=SQLITE_OK ) return rc;
+    /* TODO(shess) Handle SQLITE_SCHEMA AND SQLITE_BUSY. */
+    rc = sqlite3_step(c->pStmt);
+    if( rc==SQLITE_ROW ){   /* the case we expect */
+      c->eof = 0;
+      return SQLITE_OK;
+    }
+
+    /* Corrupt if the index refers to missing document. */
+    if( rc==SQLITE_DONE ) return SQLITE_CORRUPT_BKPT;
+
+    return rc;
+  }
+}
+
+
+/* TODO(shess) If we pushed LeafReader to the top of the file, or to
+** another file, term_select() could be pushed above
+** docListOfTerm().
+*/
+static int termSelect(fulltext_vtab *v, int iColumn,
+                      const char *pTerm, int nTerm, int isPrefix,
+                      DocListType iType, DataBuffer *out);
+
+/* Return a DocList corresponding to the query term *pTerm.  If *pTerm
+** is the first term of a phrase query, go ahead and evaluate the phrase
+** query and return the doclist for the entire phrase query.
+**
+** The resulting DL_DOCIDS doclist is stored in pResult, which is
+** overwritten.
+*/
+static int docListOfTerm(
+  fulltext_vtab *v,   /* The full text index */
+  int iColumn,        /* column to restrict to.  No restriction if >=nColumn */
+  QueryTerm *pQTerm,  /* Term we are looking for, or 1st term of a phrase */
+  DataBuffer *pResult /* Write the result here */
+){
+  DataBuffer left, right, new;
+  int i, rc;
+
+  /* No phrase search if no position info. */
+  assert( pQTerm->nPhrase==0 || DL_DEFAULT!=DL_DOCIDS );
+
+  /* This code should never be called with buffered updates. */
+  assert( v->nPendingData<0 );
+
+  dataBufferInit(&left, 0);
+  rc = termSelect(v, iColumn, pQTerm->pTerm, pQTerm->nTerm, pQTerm->isPrefix,
+                  0<pQTerm->nPhrase ? DL_POSITIONS : DL_DOCIDS, &left);
+  if( rc ) return rc;
+  for(i=1; i<=pQTerm->nPhrase && left.nData>0; i++){
+    dataBufferInit(&right, 0);
+    rc = termSelect(v, iColumn, pQTerm[i].pTerm, pQTerm[i].nTerm,
+                    pQTerm[i].isPrefix, DL_POSITIONS, &right);
+    if( rc ){
+      dataBufferDestroy(&left);
+      return rc;
+    }
+    dataBufferInit(&new, 0);
+    rc = docListPhraseMerge(left.pData, left.nData, right.pData, right.nData,
+                            i<pQTerm->nPhrase ? DL_POSITIONS : DL_DOCIDS, &new);
+    dataBufferDestroy(&left);
+    dataBufferDestroy(&right);
+    if( rc!=SQLITE_OK ){
+      dataBufferDestroy(&new);
+      return rc;
+    }
+    left = new;
+  }
+  *pResult = left;
+  return rc;
+}
+
+/* Add a new term pTerm[0..nTerm-1] to the query *q.
+*/
+static void queryAdd(Query *q, const char *pTerm, int nTerm){
+  QueryTerm *t;
+  ++q->nTerms;
+  q->pTerms = sqlite3_realloc(q->pTerms, q->nTerms * sizeof(q->pTerms[0]));
+  if( q->pTerms==0 ){
+    q->nTerms = 0;
+    return;
+  }
+  t = &q->pTerms[q->nTerms - 1];
+  CLEAR(t);
+  t->pTerm = sqlite3_malloc(nTerm+1);
+  memcpy(t->pTerm, pTerm, nTerm);
+  t->pTerm[nTerm] = 0;
+  t->nTerm = nTerm;
+  t->isOr = q->nextIsOr;
+  t->isPrefix = 0;
+  q->nextIsOr = 0;
+  t->iColumn = q->nextColumn;
+  q->nextColumn = q->dfltColumn;
+}
+
+/*
+** Check to see if the string zToken[0...nToken-1] matches any
+** column name in the virtual table.   If it does,
+** return the zero-indexed column number.  If not, return -1.
+*/
+static int checkColumnSpecifier(
+  fulltext_vtab *pVtab,    /* The virtual table */
+  const char *zToken,      /* Text of the token */
+  int nToken               /* Number of characters in the token */
+){
+  int i;
+  for(i=0; i<pVtab->nColumn; i++){
+    if( memcmp(pVtab->azColumn[i], zToken, nToken)==0
+        && pVtab->azColumn[i][nToken]==0 ){
+      return i;
+    }
+  }
+  return -1;
+}
+
+/*
+** Parse the text at pSegment[0..nSegment-1].  Add additional terms
+** to the query being assemblied in pQuery.
+**
+** inPhrase is true if pSegment[0..nSegement-1] is contained within
+** double-quotes.  If inPhrase is true, then the first term
+** is marked with the number of terms in the phrase less one and
+** OR and "-" syntax is ignored.  If inPhrase is false, then every
+** term found is marked with nPhrase=0 and OR and "-" syntax is significant.
+*/
+static int tokenizeSegment(
+  sqlite3_tokenizer *pTokenizer,          /* The tokenizer to use */
+  const char *pSegment, int nSegment,     /* Query expression being parsed */
+  int inPhrase,                           /* True if within "..." */
+  Query *pQuery                           /* Append results here */
+){
+  const sqlite3_tokenizer_module *pModule = pTokenizer->pModule;
+  sqlite3_tokenizer_cursor *pCursor;
+  int firstIndex = pQuery->nTerms;
+  int iCol;
+  int nTerm = 1;
+  int iEndLast = -1;
+  
+  int rc = pModule->xOpen(pTokenizer, pSegment, nSegment, &pCursor);
+  if( rc!=SQLITE_OK ) return rc;
+  pCursor->pTokenizer = pTokenizer;
+
+  while( 1 ){
+    const char *pToken;
+    int nToken, iBegin, iEnd, iPos;
+
+    rc = pModule->xNext(pCursor,
+                        &pToken, &nToken,
+                        &iBegin, &iEnd, &iPos);
+    if( rc!=SQLITE_OK ) break;
+    if( !inPhrase &&
+        pSegment[iEnd]==':' &&
+         (iCol = checkColumnSpecifier(pQuery->pFts, pToken, nToken))>=0 ){
+      pQuery->nextColumn = iCol;
+      continue;
+    }
+    if( !inPhrase && pQuery->nTerms>0 && nToken==2
+         && pSegment[iBegin]=='O' && pSegment[iBegin+1]=='R' ){
+      pQuery->nextIsOr = 1;
+      continue;
+    }
+
+    /*
+     * The ICU tokenizer considers '*' a break character, so the code below
+     * sets isPrefix correctly, but since that code doesn't eat the '*', the
+     * ICU tokenizer returns it as the next token.  So eat it here until a
+     * better solution presents itself.
+     */
+    if( pQuery->nTerms>0 && nToken==1 && pSegment[iBegin]=='*' &&
+        iEndLast==iBegin){
+      pQuery->pTerms[pQuery->nTerms-1].isPrefix = 1;
+      continue;
+    }
+    iEndLast = iEnd;
+    
+    queryAdd(pQuery, pToken, nToken);
+    if( !inPhrase && iBegin>0 && pSegment[iBegin-1]=='-' ){
+      pQuery->pTerms[pQuery->nTerms-1].isNot = 1;
+    }
+    if( iEnd<nSegment && pSegment[iEnd]=='*' ){
+      pQuery->pTerms[pQuery->nTerms-1].isPrefix = 1;
+    }
+    pQuery->pTerms[pQuery->nTerms-1].iPhrase = nTerm;
+    if( inPhrase ){
+      nTerm++;
+    }
+  }
+
+  if( inPhrase && pQuery->nTerms>firstIndex ){
+    pQuery->pTerms[firstIndex].nPhrase = pQuery->nTerms - firstIndex - 1;
+  }
+
+  return pModule->xClose(pCursor);
+}
+
+/* Parse a query string, yielding a Query object pQuery.
+**
+** The calling function will need to queryClear() to clean up
+** the dynamically allocated memory held by pQuery.
+*/
+static int parseQuery(
+  fulltext_vtab *v,        /* The fulltext index */
+  const char *zInput,      /* Input text of the query string */
+  int nInput,              /* Size of the input text */
+  int dfltColumn,          /* Default column of the index to match against */
+  Query *pQuery            /* Write the parse results here. */
+){
+  int iInput, inPhrase = 0;
+
+  if( zInput==0 ) nInput = 0;
+  if( nInput<0 ) nInput = strlen(zInput);
+  pQuery->nTerms = 0;
+  pQuery->pTerms = NULL;
+  pQuery->nextIsOr = 0;
+  pQuery->nextColumn = dfltColumn;
+  pQuery->dfltColumn = dfltColumn;
+  pQuery->pFts = v;
+
+  for(iInput=0; iInput<nInput; ++iInput){
+    int i;
+    for(i=iInput; i<nInput && zInput[i]!='"'; ++i){}
+    if( i>iInput ){
+      tokenizeSegment(v->pTokenizer, zInput+iInput, i-iInput, inPhrase,
+                       pQuery);
+    }
+    iInput = i;
+    if( i<nInput ){
+      assert( zInput[i]=='"' );
+      inPhrase = !inPhrase;
+    }
+  }
+
+  if( inPhrase ){
+    /* unmatched quote */
+    queryClear(pQuery);
+    return SQLITE_ERROR;
+  }
+  return SQLITE_OK;
+}
+
+/* TODO(shess) Refactor the code to remove this forward decl. */
+static int flushPendingTerms(fulltext_vtab *v);
+
+/* Perform a full-text query using the search expression in
+** zInput[0..nInput-1].  Return a list of matching documents
+** in pResult.
+**
+** Queries must match column iColumn.  Or if iColumn>=nColumn
+** they are allowed to match against any column.
+*/
+static int fulltextQuery(
+  fulltext_vtab *v,      /* The full text index */
+  int iColumn,           /* Match against this column by default */
+  const char *zInput,    /* The query string */
+  int nInput,            /* Number of bytes in zInput[] */
+  DataBuffer *pResult,   /* Write the result doclist here */
+  Query *pQuery          /* Put parsed query string here */
+){
+  int i, iNext, rc;
+  DataBuffer left, right, or, new;
+  int nNot = 0;
+  QueryTerm *aTerm;
+
+  /* TODO(shess) Instead of flushing pendingTerms, we could query for
+  ** the relevant term and merge the doclist into what we receive from
+  ** the database.  Wait and see if this is a common issue, first.
+  **
+  ** A good reason not to flush is to not generate update-related
+  ** error codes from here.
+  */
+
+  /* Flush any buffered updates before executing the query. */
+  rc = flushPendingTerms(v);
+  if( rc!=SQLITE_OK ) return rc;
+
+  /* TODO(shess) I think that the queryClear() calls below are not
+  ** necessary, because fulltextClose() already clears the query.
+  */
+  rc = parseQuery(v, zInput, nInput, iColumn, pQuery);
+  if( rc!=SQLITE_OK ) return rc;
+
+  /* Empty or NULL queries return no results. */
+  if( pQuery->nTerms==0 ){
+    dataBufferInit(pResult, 0);
+    return SQLITE_OK;
+  }
+
+  /* Merge AND terms. */
+  /* TODO(shess) I think we can early-exit if( i>nNot && left.nData==0 ). */
+  aTerm = pQuery->pTerms;
+  for(i = 0; i<pQuery->nTerms; i=iNext){
+    if( aTerm[i].isNot ){
+      /* Handle all NOT terms in a separate pass */
+      nNot++;
+      iNext = i + aTerm[i].nPhrase+1;
+      continue;
+    }
+    iNext = i + aTerm[i].nPhrase + 1;
+    rc = docListOfTerm(v, aTerm[i].iColumn, &aTerm[i], &right);
+    if( rc ){
+      if( i!=nNot ) dataBufferDestroy(&left);
+      queryClear(pQuery);
+      return rc;
+    }
+    while( iNext<pQuery->nTerms && aTerm[iNext].isOr ){
+      rc = docListOfTerm(v, aTerm[iNext].iColumn, &aTerm[iNext], &or);
+      iNext += aTerm[iNext].nPhrase + 1;
+      if( rc ){
+        if( i!=nNot ) dataBufferDestroy(&left);
+        dataBufferDestroy(&right);
+        queryClear(pQuery);
+        return rc;
+      }
+      dataBufferInit(&new, 0);
+      rc = docListOrMerge(right.pData, right.nData, or.pData, or.nData, &new);
+      dataBufferDestroy(&right);
+      dataBufferDestroy(&or);
+      if( rc!=SQLITE_OK ){
+        if( i!=nNot ) dataBufferDestroy(&left);
+        queryClear(pQuery);
+        dataBufferDestroy(&new);
+        return rc;
+      }
+      right = new;
+    }
+    if( i==nNot ){           /* first term processed. */
+      left = right;
+    }else{
+      dataBufferInit(&new, 0);
+      rc = docListAndMerge(left.pData, left.nData,
+                           right.pData, right.nData, &new);
+      dataBufferDestroy(&right);
+      dataBufferDestroy(&left);
+      if( rc!=SQLITE_OK ){
+        queryClear(pQuery);
+        dataBufferDestroy(&new);
+        return rc;
+      }
+      left = new;
+    }
+  }
+
+  if( nNot==pQuery->nTerms ){
+    /* We do not yet know how to handle a query of only NOT terms */
+    return SQLITE_ERROR;
+  }
+
+  /* Do the EXCEPT terms */
+  for(i=0; i<pQuery->nTerms;  i += aTerm[i].nPhrase + 1){
+    if( !aTerm[i].isNot ) continue;
+    rc = docListOfTerm(v, aTerm[i].iColumn, &aTerm[i], &right);
+    if( rc ){
+      queryClear(pQuery);
+      dataBufferDestroy(&left);
+      return rc;
+    }
+    dataBufferInit(&new, 0);
+    rc = docListExceptMerge(left.pData, left.nData,
+                            right.pData, right.nData, &new);
+    dataBufferDestroy(&right);
+    dataBufferDestroy(&left);
+    if( rc!=SQLITE_OK ){
+      queryClear(pQuery);
+      dataBufferDestroy(&new);
+      return rc;
+    }
+    left = new;
+  }
+
+  *pResult = left;
+  return rc;
+}
+
+/*
+** This is the xFilter interface for the virtual table.  See
+** the virtual table xFilter method documentation for additional
+** information.
+**
+** If idxNum==QUERY_GENERIC then do a full table scan against
+** the %_content table.
+**
+** If idxNum==QUERY_ROWID then do a rowid lookup for a single entry
+** in the %_content table.
+**
+** If idxNum>=QUERY_FULLTEXT then use the full text index.  The
+** column on the left-hand side of the MATCH operator is column
+** number idxNum-QUERY_FULLTEXT, 0 indexed.  argv[0] is the right-hand
+** side of the MATCH operator.
+*/
+/* TODO(shess) Upgrade the cursor initialization and destruction to
+** account for fulltextFilter() being called multiple times on the
+** same cursor.  The current solution is very fragile.  Apply fix to
+** fts2 as appropriate.
+*/
+static int fulltextFilter(
+  sqlite3_vtab_cursor *pCursor,     /* The cursor used for this query */
+  int idxNum, const char *idxStr,   /* Which indexing scheme to use */
+  int argc, sqlite3_value **argv    /* Arguments for the indexing scheme */
+){
+  fulltext_cursor *c = (fulltext_cursor *) pCursor;
+  fulltext_vtab *v = cursor_vtab(c);
+  int rc;
+
+  TRACE(("FTS2 Filter %p\n",pCursor));
+
+  /* If the cursor has a statement that was not prepared according to
+  ** idxNum, clear it.  I believe all calls to fulltextFilter with a
+  ** given cursor will have the same idxNum , but in this case it's
+  ** easy to be safe.
+  */
+  if( c->pStmt && c->iCursorType!=idxNum ){
+    sqlite3_finalize(c->pStmt);
+    c->pStmt = NULL;
+  }
+
+  /* Get a fresh statement appropriate to idxNum. */
+  /* TODO(shess): Add a prepared-statement cache in the vt structure.
+  ** The cache must handle multiple open cursors.  Easier to cache the
+  ** statement variants at the vt to reduce malloc/realloc/free here.
+  ** Or we could have a StringBuffer variant which allowed stack
+  ** construction for small values.
+  */
+  if( !c->pStmt ){
+    char *zSql = sqlite3_mprintf("select rowid, * from %%_content %s",
+                                 idxNum==QUERY_GENERIC ? "" : "where rowid=?");
+    rc = sql_prepare(v->db, v->zDb, v->zName, &c->pStmt, zSql);
+    sqlite3_free(zSql);
+    if( rc!=SQLITE_OK ) return rc;
+    c->iCursorType = idxNum;
+  }else{
+    sqlite3_reset(c->pStmt);
+    assert( c->iCursorType==idxNum );
+  }
+
+  switch( idxNum ){
+    case QUERY_GENERIC:
+      break;
+
+    case QUERY_ROWID:
+      rc = sqlite3_bind_int64(c->pStmt, 1, sqlite3_value_int64(argv[0]));
+      if( rc!=SQLITE_OK ) return rc;
+      break;
+
+    default:   /* full-text search */
+    {
+      const char *zQuery = (const char *)sqlite3_value_text(argv[0]);
+      assert( idxNum<=QUERY_FULLTEXT+v->nColumn);
+      assert( argc==1 );
+      queryClear(&c->q);
+      if( c->result.nData!=0 ){
+        /* This case happens if the same cursor is used repeatedly. */
+        dlrDestroy(&c->reader);
+        dataBufferReset(&c->result);
+      }else{
+        dataBufferInit(&c->result, 0);
+      }
+      rc = fulltextQuery(v, idxNum-QUERY_FULLTEXT, zQuery, -1, &c->result, &c->q);
+      if( rc!=SQLITE_OK ) return rc;
+      if( c->result.nData!=0 ){
+        rc = dlrInit(&c->reader, DL_DOCIDS, c->result.pData, c->result.nData);
+        if( rc!=SQLITE_OK ) return rc;
+      }
+      break;
+    }
+  }
+
+  return fulltextNext(pCursor);
+}
+
+/* This is the xEof method of the virtual table.  The SQLite core
+** calls this routine to find out if it has reached the end of
+** a query's results set.
+*/
+static int fulltextEof(sqlite3_vtab_cursor *pCursor){
+  fulltext_cursor *c = (fulltext_cursor *) pCursor;
+  return c->eof;
+}
+
+/* This is the xColumn method of the virtual table.  The SQLite
+** core calls this method during a query when it needs the value
+** of a column from the virtual table.  This method needs to use
+** one of the sqlite3_result_*() routines to store the requested
+** value back in the pContext.
+*/
+static int fulltextColumn(sqlite3_vtab_cursor *pCursor,
+                          sqlite3_context *pContext, int idxCol){
+  fulltext_cursor *c = (fulltext_cursor *) pCursor;
+  fulltext_vtab *v = cursor_vtab(c);
+
+  if( idxCol<v->nColumn ){
+    sqlite3_value *pVal = sqlite3_column_value(c->pStmt, idxCol+1);
+    sqlite3_result_value(pContext, pVal);
+  }else if( idxCol==v->nColumn ){
+    /* The extra column whose name is the same as the table.
+    ** Return a blob which is a pointer to the cursor
+    */
+    sqlite3_result_blob(pContext, &c, sizeof(c), SQLITE_TRANSIENT);
+  }
+  return SQLITE_OK;
+}
+
+/* This is the xRowid method.  The SQLite core calls this routine to
+** retrive the rowid for the current row of the result set.  The
+** rowid should be written to *pRowid.
+*/
+static int fulltextRowid(sqlite3_vtab_cursor *pCursor, sqlite_int64 *pRowid){
+  fulltext_cursor *c = (fulltext_cursor *) pCursor;
+
+  *pRowid = sqlite3_column_int64(c->pStmt, 0);
+  return SQLITE_OK;
+}
+
+/* Add all terms in [zText] to pendingTerms table.  If [iColumn] > 0,
+** we also store positions and offsets in the hash table using that
+** column number.
+*/
+static int buildTerms(fulltext_vtab *v, sqlite_int64 iDocid,
+                      const char *zText, int iColumn){
+  sqlite3_tokenizer *pTokenizer = v->pTokenizer;
+  sqlite3_tokenizer_cursor *pCursor;
+  const char *pToken;
+  int nTokenBytes;
+  int iStartOffset, iEndOffset, iPosition;
+  int rc;
+
+  rc = pTokenizer->pModule->xOpen(pTokenizer, zText, -1, &pCursor);
+  if( rc!=SQLITE_OK ) return rc;
+
+  pCursor->pTokenizer = pTokenizer;
+  while( SQLITE_OK==(rc=pTokenizer->pModule->xNext(pCursor,
+                                                   &pToken, &nTokenBytes,
+                                                   &iStartOffset, &iEndOffset,
+                                                   &iPosition)) ){
+    DLCollector *p;
+    int nData;                   /* Size of doclist before our update. */
+
+    /* Positions can't be negative; we use -1 as a terminator
+     * internally.  Token can't be NULL or empty. */
+    if( iPosition<0 || pToken == NULL || nTokenBytes == 0 ){
+      rc = SQLITE_ERROR;
+      break;
+    }
+
+    p = fts2HashFind(&v->pendingTerms, pToken, nTokenBytes);
+    if( p==NULL ){
+      nData = 0;
+      p = dlcNew(iDocid, DL_DEFAULT);
+      fts2HashInsert(&v->pendingTerms, pToken, nTokenBytes, p);
+
+      /* Overhead for our hash table entry, the key, and the value. */
+      v->nPendingData += sizeof(struct fts2HashElem)+sizeof(*p)+nTokenBytes;
+    }else{
+      nData = p->b.nData;
+      if( p->dlw.iPrevDocid!=iDocid ) dlcNext(p, iDocid);
+    }
+    if( iColumn>=0 ){
+      dlcAddPos(p, iColumn, iPosition, iStartOffset, iEndOffset);
+    }
+
+    /* Accumulate data added by dlcNew or dlcNext, and dlcAddPos. */
+    v->nPendingData += p->b.nData-nData;
+  }
+
+  /* TODO(shess) Check return?  Should this be able to cause errors at
+  ** this point?  Actually, same question about sqlite3_finalize(),
+  ** though one could argue that failure there means that the data is
+  ** not durable.  *ponder*
+  */
+  pTokenizer->pModule->xClose(pCursor);
+  if( SQLITE_DONE == rc ) return SQLITE_OK;
+  return rc;
+}
+
+/* Add doclists for all terms in [pValues] to pendingTerms table. */
+static int insertTerms(fulltext_vtab *v, sqlite_int64 iRowid,
+                       sqlite3_value **pValues){
+  int i;
+  for(i = 0; i < v->nColumn ; ++i){
+    char *zText = (char*)sqlite3_value_text(pValues[i]);
+    int rc = buildTerms(v, iRowid, zText, i);
+    if( rc!=SQLITE_OK ) return rc;
+  }
+  return SQLITE_OK;
+}
+
+/* Add empty doclists for all terms in the given row's content to
+** pendingTerms.
+*/
+static int deleteTerms(fulltext_vtab *v, sqlite_int64 iRowid){
+  const char **pValues;
+  int i, rc;
+
+  /* TODO(shess) Should we allow such tables at all? */
+  if( DL_DEFAULT==DL_DOCIDS ) return SQLITE_ERROR;
+
+  rc = content_select(v, iRowid, &pValues);
+  if( rc!=SQLITE_OK ) return rc;
+
+  for(i = 0 ; i < v->nColumn; ++i) {
+    rc = buildTerms(v, iRowid, pValues[i], -1);
+    if( rc!=SQLITE_OK ) break;
+  }
+
+  freeStringArray(v->nColumn, pValues);
+  return SQLITE_OK;
+}
+
+/* TODO(shess) Refactor the code to remove this forward decl. */
+static int initPendingTerms(fulltext_vtab *v, sqlite_int64 iDocid);
+
+/* Insert a row into the %_content table; set *piRowid to be the ID of the
+** new row.  Add doclists for terms to pendingTerms.
+*/
+static int index_insert(fulltext_vtab *v, sqlite3_value *pRequestRowid,
+                        sqlite3_value **pValues, sqlite_int64 *piRowid){
+  int rc;
+
+  rc = content_insert(v, pRequestRowid, pValues);  /* execute an SQL INSERT */
+  if( rc!=SQLITE_OK ) return rc;
+
+  *piRowid = sqlite3_last_insert_rowid(v->db);
+  rc = initPendingTerms(v, *piRowid);
+  if( rc!=SQLITE_OK ) return rc;
+
+  return insertTerms(v, *piRowid, pValues);
+}
+
+/* Delete a row from the %_content table; add empty doclists for terms
+** to pendingTerms.
+*/
+static int index_delete(fulltext_vtab *v, sqlite_int64 iRow){
+  int rc = initPendingTerms(v, iRow);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = deleteTerms(v, iRow);
+  if( rc!=SQLITE_OK ) return rc;
+
+  return content_delete(v, iRow);  /* execute an SQL DELETE */
+}
+
+/* Update a row in the %_content table; add delete doclists to
+** pendingTerms for old terms not in the new data, add insert doclists
+** to pendingTerms for terms in the new data.
+*/
+static int index_update(fulltext_vtab *v, sqlite_int64 iRow,
+                        sqlite3_value **pValues){
+  int rc = initPendingTerms(v, iRow);
+  if( rc!=SQLITE_OK ) return rc;
+
+  /* Generate an empty doclist for each term that previously appeared in this
+   * row. */
+  rc = deleteTerms(v, iRow);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = content_update(v, pValues, iRow);  /* execute an SQL UPDATE */
+  if( rc!=SQLITE_OK ) return rc;
+
+  /* Now add positions for terms which appear in the updated row. */
+  return insertTerms(v, iRow, pValues);
+}
+
+/*******************************************************************/
+/* InteriorWriter is used to collect terms and block references into
+** interior nodes in %_segments.  See commentary at top of file for
+** format.
+*/
+
+/* How large interior nodes can grow. */
+#define INTERIOR_MAX 2048
+
+/* Minimum number of terms per interior node (except the root). This
+** prevents large terms from making the tree too skinny - must be >0
+** so that the tree always makes progress.  Note that the min tree
+** fanout will be INTERIOR_MIN_TERMS+1.
+*/
+#define INTERIOR_MIN_TERMS 7
+#if INTERIOR_MIN_TERMS<1
+# error INTERIOR_MIN_TERMS must be greater than 0.
+#endif
+
+/* ROOT_MAX controls how much data is stored inline in the segment
+** directory.
+*/
+/* TODO(shess) Push ROOT_MAX down to whoever is writing things.  It's
+** only here so that interiorWriterRootInfo() and leafWriterRootInfo()
+** can both see it, but if the caller passed it in, we wouldn't even
+** need a define.
+*/
+#define ROOT_MAX 1024
+#if ROOT_MAX<VARINT_MAX*2
+# error ROOT_MAX must have enough space for a header.
+#endif
+
+/* InteriorBlock stores a linked-list of interior blocks while a lower
+** layer is being constructed.
+*/
+typedef struct InteriorBlock {
+  DataBuffer term;           /* Leftmost term in block's subtree. */
+  DataBuffer data;           /* Accumulated data for the block. */
+  struct InteriorBlock *next;
+} InteriorBlock;
+
+static InteriorBlock *interiorBlockNew(int iHeight, sqlite_int64 iChildBlock,
+                                       const char *pTerm, int nTerm){
+  InteriorBlock *block = sqlite3_malloc(sizeof(InteriorBlock));
+  char c[VARINT_MAX+VARINT_MAX];
+  int n;
+
+  if( block ){
+    memset(block, 0, sizeof(*block));
+    dataBufferInit(&block->term, 0);
+    dataBufferReplace(&block->term, pTerm, nTerm);
+
+    n = putVarint(c, iHeight);
+    n += putVarint(c+n, iChildBlock);
+    dataBufferInit(&block->data, INTERIOR_MAX);
+    dataBufferReplace(&block->data, c, n);
+  }
+  return block;
+}
+
+#ifndef NDEBUG
+/* Verify that the data is readable as an interior node. */
+static void interiorBlockValidate(InteriorBlock *pBlock){
+  const char *pData = pBlock->data.pData;
+  int nData = pBlock->data.nData;
+  int n, iDummy;
+  sqlite_int64 iBlockid;
+
+  assert( nData>0 );
+  assert( pData!=0 );
+  assert( pData+nData>pData );
+
+  /* Must lead with height of node as a varint(n), n>0 */
+  n = getVarint32(pData, &iDummy);
+  assert( n>0 );
+  assert( iDummy>0 );
+  assert( n<nData );
+  pData += n;
+  nData -= n;
+
+  /* Must contain iBlockid. */
+  n = getVarint(pData, &iBlockid);
+  assert( n>0 );
+  assert( n<=nData );
+  pData += n;
+  nData -= n;
+
+  /* Zero or more terms of positive length */
+  if( nData!=0 ){
+    /* First term is not delta-encoded. */
+    n = getVarint32(pData, &iDummy);
+    assert( n>0 );
+    assert( iDummy>0 );
+    assert( n+iDummy>0);
+    assert( n+iDummy<=nData );
+    pData += n+iDummy;
+    nData -= n+iDummy;
+
+    /* Following terms delta-encoded. */
+    while( nData!=0 ){
+      /* Length of shared prefix. */
+      n = getVarint32(pData, &iDummy);
+      assert( n>0 );
+      assert( iDummy>=0 );
+      assert( n<nData );
+      pData += n;
+      nData -= n;
+
+      /* Length and data of distinct suffix. */
+      n = getVarint32(pData, &iDummy);
+      assert( n>0 );
+      assert( iDummy>0 );
+      assert( n+iDummy>0);
+      assert( n+iDummy<=nData );
+      pData += n+iDummy;
+      nData -= n+iDummy;
+    }
+  }
+}
+#define ASSERT_VALID_INTERIOR_BLOCK(x) interiorBlockValidate(x)
+#else
+#define ASSERT_VALID_INTERIOR_BLOCK(x) assert( 1 )
+#endif
+
+typedef struct InteriorWriter {
+  int iHeight;                   /* from 0 at leaves. */
+  InteriorBlock *first, *last;
+  struct InteriorWriter *parentWriter;
+
+  DataBuffer term;               /* Last term written to block "last". */
+  sqlite_int64 iOpeningChildBlock; /* First child block in block "last". */
+#ifndef NDEBUG
+  sqlite_int64 iLastChildBlock;  /* for consistency checks. */
+#endif
+} InteriorWriter;
+
+/* Initialize an interior node where pTerm[nTerm] marks the leftmost
+** term in the tree.  iChildBlock is the leftmost child block at the
+** next level down the tree.
+*/
+static void interiorWriterInit(int iHeight, const char *pTerm, int nTerm,
+                               sqlite_int64 iChildBlock,
+                               InteriorWriter *pWriter){
+  InteriorBlock *block;
+  assert( iHeight>0 );
+  CLEAR(pWriter);
+
+  pWriter->iHeight = iHeight;
+  pWriter->iOpeningChildBlock = iChildBlock;
+#ifndef NDEBUG
+  pWriter->iLastChildBlock = iChildBlock;
+#endif
+  block = interiorBlockNew(iHeight, iChildBlock, pTerm, nTerm);
+  pWriter->last = pWriter->first = block;
+  ASSERT_VALID_INTERIOR_BLOCK(pWriter->last);
+  dataBufferInit(&pWriter->term, 0);
+}
+
+/* Append the child node rooted at iChildBlock to the interior node,
+** with pTerm[nTerm] as the leftmost term in iChildBlock's subtree.
+*/
+static void interiorWriterAppend(InteriorWriter *pWriter,
+                                 const char *pTerm, int nTerm,
+                                 sqlite_int64 iChildBlock){
+  char c[VARINT_MAX+VARINT_MAX];
+  int n, nPrefix = 0;
+
+  ASSERT_VALID_INTERIOR_BLOCK(pWriter->last);
+
+  /* The first term written into an interior node is actually
+  ** associated with the second child added (the first child was added
+  ** in interiorWriterInit, or in the if clause at the bottom of this
+  ** function).  That term gets encoded straight up, with nPrefix left
+  ** at 0.
+  */
+  if( pWriter->term.nData==0 ){
+    n = putVarint(c, nTerm);
+  }else{
+    while( nPrefix<pWriter->term.nData &&
+           pTerm[nPrefix]==pWriter->term.pData[nPrefix] ){
+      nPrefix++;
+    }
+
+    n = putVarint(c, nPrefix);
+    n += putVarint(c+n, nTerm-nPrefix);
+  }
+
+#ifndef NDEBUG
+  pWriter->iLastChildBlock++;
+#endif
+  assert( pWriter->iLastChildBlock==iChildBlock );
+
+  /* Overflow to a new block if the new term makes the current block
+  ** too big, and the current block already has enough terms.
+  */
+  if( pWriter->last->data.nData+n+nTerm-nPrefix>INTERIOR_MAX &&
+      iChildBlock-pWriter->iOpeningChildBlock>INTERIOR_MIN_TERMS ){
+    pWriter->last->next = interiorBlockNew(pWriter->iHeight, iChildBlock,
+                                           pTerm, nTerm);
+    pWriter->last = pWriter->last->next;
+    pWriter->iOpeningChildBlock = iChildBlock;
+    dataBufferReset(&pWriter->term);
+  }else{
+    dataBufferAppend2(&pWriter->last->data, c, n,
+                      pTerm+nPrefix, nTerm-nPrefix);
+    dataBufferReplace(&pWriter->term, pTerm, nTerm);
+  }
+  ASSERT_VALID_INTERIOR_BLOCK(pWriter->last);
+}
+
+/* Free the space used by pWriter, including the linked-list of
+** InteriorBlocks, and parentWriter, if present.
+*/
+static int interiorWriterDestroy(InteriorWriter *pWriter){
+  InteriorBlock *block = pWriter->first;
+
+  while( block!=NULL ){
+    InteriorBlock *b = block;
+    block = block->next;
+    dataBufferDestroy(&b->term);
+    dataBufferDestroy(&b->data);
+    sqlite3_free(b);
+  }
+  if( pWriter->parentWriter!=NULL ){
+    interiorWriterDestroy(pWriter->parentWriter);
+    sqlite3_free(pWriter->parentWriter);
+  }
+  dataBufferDestroy(&pWriter->term);
+  SCRAMBLE(pWriter);
+  return SQLITE_OK;
+}
+
+/* If pWriter can fit entirely in ROOT_MAX, return it as the root info
+** directly, leaving *piEndBlockid unchanged.  Otherwise, flush
+** pWriter to %_segments, building a new layer of interior nodes, and
+** recursively ask for their root into.
+*/
+static int interiorWriterRootInfo(fulltext_vtab *v, InteriorWriter *pWriter,
+                                  char **ppRootInfo, int *pnRootInfo,
+                                  sqlite_int64 *piEndBlockid){
+  InteriorBlock *block = pWriter->first;
+  sqlite_int64 iBlockid = 0;
+  int rc;
+
+  /* If we can fit the segment inline */
+  if( block==pWriter->last && block->data.nData<ROOT_MAX ){
+    *ppRootInfo = block->data.pData;
+    *pnRootInfo = block->data.nData;
+    return SQLITE_OK;
+  }
+
+  /* Flush the first block to %_segments, and create a new level of
+  ** interior node.
+  */
+  ASSERT_VALID_INTERIOR_BLOCK(block);
+  rc = block_insert(v, block->data.pData, block->data.nData, &iBlockid);
+  if( rc!=SQLITE_OK ) return rc;
+  *piEndBlockid = iBlockid;
+
+  pWriter->parentWriter = sqlite3_malloc(sizeof(*pWriter->parentWriter));
+  interiorWriterInit(pWriter->iHeight+1,
+                     block->term.pData, block->term.nData,
+                     iBlockid, pWriter->parentWriter);
+
+  /* Flush additional blocks and append to the higher interior
+  ** node.
+  */
+  for(block=block->next; block!=NULL; block=block->next){
+    ASSERT_VALID_INTERIOR_BLOCK(block);
+    rc = block_insert(v, block->data.pData, block->data.nData, &iBlockid);
+    if( rc!=SQLITE_OK ) return rc;
+    *piEndBlockid = iBlockid;
+
+    interiorWriterAppend(pWriter->parentWriter,
+                         block->term.pData, block->term.nData, iBlockid);
+  }
+
+  /* Parent node gets the chance to be the root. */
+  return interiorWriterRootInfo(v, pWriter->parentWriter,
+                                ppRootInfo, pnRootInfo, piEndBlockid);
+}
+
+/****************************************************************/
+/* InteriorReader is used to read off the data from an interior node
+** (see comment at top of file for the format).
+*/
+typedef struct InteriorReader {
+  const char *pData;
+  int nData;
+
+  DataBuffer term;          /* previous term, for decoding term delta. */
+
+  sqlite_int64 iBlockid;
+} InteriorReader;
+
+static void interiorReaderDestroy(InteriorReader *pReader){
+  dataBufferDestroy(&pReader->term);
+  SCRAMBLE(pReader);
+}
+
+static int interiorReaderInit(const char *pData, int nData,
+                              InteriorReader *pReader){
+  int n, nTerm;
+
+  /* These conditions are checked and met by the callers. */
+  assert( nData>0 );
+  assert( pData[0]!='\0' );
+
+  CLEAR(pReader);
+
+  /* Decode the base blockid, and set the cursor to the first term. */
+  n = getVarintSafe(pData+1, &pReader->iBlockid, nData-1);
+  if( !n ) return SQLITE_CORRUPT_BKPT;
+  pReader->pData = pData+1+n;
+  pReader->nData = nData-(1+n);
+
+  /* A single-child interior node (such as when a leaf node was too
+  ** large for the segment directory) won't have any terms.
+  ** Otherwise, decode the first term.
+  */
+  if( pReader->nData==0 ){
+    dataBufferInit(&pReader->term, 0);
+  }else{
+    n = getVarint32Safe(pReader->pData, &nTerm, pReader->nData);
+    if( !n || nTerm<0 || nTerm>pReader->nData-n) return SQLITE_CORRUPT_BKPT;
+    dataBufferInit(&pReader->term, nTerm);
+    dataBufferReplace(&pReader->term, pReader->pData+n, nTerm);
+    pReader->pData += n+nTerm;
+    pReader->nData -= n+nTerm;
+  }
+  return SQLITE_OK;
+}
+
+static int interiorReaderAtEnd(InteriorReader *pReader){
+  return pReader->term.nData<=0;
+}
+
+static sqlite_int64 interiorReaderCurrentBlockid(InteriorReader *pReader){
+  return pReader->iBlockid;
+}
+
+static int interiorReaderTermBytes(InteriorReader *pReader){
+  assert( !interiorReaderAtEnd(pReader) );
+  return pReader->term.nData;
+}
+static const char *interiorReaderTerm(InteriorReader *pReader){
+  assert( !interiorReaderAtEnd(pReader) );
+  return pReader->term.pData;
+}
+
+/* Step forward to the next term in the node. */
+static int interiorReaderStep(InteriorReader *pReader){
+  assert( !interiorReaderAtEnd(pReader) );
+
+  /* If the last term has been read, signal eof, else construct the
+  ** next term.
+  */
+  if( pReader->nData==0 ){
+    dataBufferReset(&pReader->term);
+  }else{
+    int n, nPrefix, nSuffix;
+
+    n = getVarint32Safe(pReader->pData, &nPrefix, pReader->nData);
+    if( !n ) return SQLITE_CORRUPT_BKPT;
+    pReader->nData -= n;
+    pReader->pData += n;
+    n = getVarint32Safe(pReader->pData, &nSuffix, pReader->nData);
+    if( !n ) return SQLITE_CORRUPT_BKPT;
+    pReader->nData -= n;
+    pReader->pData += n;
+    if( nSuffix<0 || nSuffix>pReader->nData ) return SQLITE_CORRUPT_BKPT;
+    if( nPrefix<0 || nPrefix>pReader->term.nData ) return SQLITE_CORRUPT_BKPT;
+
+    /* Truncate the current term and append suffix data. */
+    pReader->term.nData = nPrefix;
+    dataBufferAppend(&pReader->term, pReader->pData, nSuffix);
+
+    pReader->pData += nSuffix;
+    pReader->nData -= nSuffix;
+  }
+  pReader->iBlockid++;
+  return SQLITE_OK;
+}
+
+/* Compare the current term to pTerm[nTerm], returning strcmp-style
+** results.  If isPrefix, equality means equal through nTerm bytes.
+*/
+static int interiorReaderTermCmp(InteriorReader *pReader,
+                                 const char *pTerm, int nTerm, int isPrefix){
+  const char *pReaderTerm = interiorReaderTerm(pReader);
+  int nReaderTerm = interiorReaderTermBytes(pReader);
+  int c, n = nReaderTerm<nTerm ? nReaderTerm : nTerm;
+
+  if( n==0 ){
+    if( nReaderTerm>0 ) return -1;
+    if( nTerm>0 ) return 1;
+    return 0;
+  }
+
+  c = memcmp(pReaderTerm, pTerm, n);
+  if( c!=0 ) return c;
+  if( isPrefix && n==nTerm ) return 0;
+  return nReaderTerm - nTerm;
+}
+
+/****************************************************************/
+/* LeafWriter is used to collect terms and associated doclist data
+** into leaf blocks in %_segments (see top of file for format info).
+** Expected usage is:
+**
+** LeafWriter writer;
+** leafWriterInit(0, 0, &writer);
+** while( sorted_terms_left_to_process ){
+**   // data is doclist data for that term.
+**   rc = leafWriterStep(v, &writer, pTerm, nTerm, pData, nData);
+**   if( rc!=SQLITE_OK ) goto err;
+** }
+** rc = leafWriterFinalize(v, &writer);
+**err:
+** leafWriterDestroy(&writer);
+** return rc;
+**
+** leafWriterStep() may write a collected leaf out to %_segments.
+** leafWriterFinalize() finishes writing any buffered data and stores
+** a root node in %_segdir.  leafWriterDestroy() frees all buffers and
+** InteriorWriters allocated as part of writing this segment.
+**
+** TODO(shess) Document leafWriterStepMerge().
+*/
+
+/* Put terms with data this big in their own block. */
+#define STANDALONE_MIN 1024
+
+/* Keep leaf blocks below this size. */
+#define LEAF_MAX 2048
+
+typedef struct LeafWriter {
+  int iLevel;
+  int idx;
+  sqlite_int64 iStartBlockid;     /* needed to create the root info */
+  sqlite_int64 iEndBlockid;       /* when we're done writing. */
+
+  DataBuffer term;                /* previous encoded term */
+  DataBuffer data;                /* encoding buffer */
+
+  /* bytes of first term in the current node which distinguishes that
+  ** term from the last term of the previous node.
+  */
+  int nTermDistinct;
+
+  InteriorWriter parentWriter;    /* if we overflow */
+  int has_parent;
+} LeafWriter;
+
+static void leafWriterInit(int iLevel, int idx, LeafWriter *pWriter){
+  CLEAR(pWriter);
+  pWriter->iLevel = iLevel;
+  pWriter->idx = idx;
+
+  dataBufferInit(&pWriter->term, 32);
+
+  /* Start out with a reasonably sized block, though it can grow. */
+  dataBufferInit(&pWriter->data, LEAF_MAX);
+}
+
+#ifndef NDEBUG
+/* Verify that the data is readable as a leaf node. */
+static void leafNodeValidate(const char *pData, int nData){
+  int n, iDummy;
+
+  if( nData==0 ) return;
+  assert( nData>0 );
+  assert( pData!=0 );
+  assert( pData+nData>pData );
+
+  /* Must lead with a varint(0) */
+  n = getVarint32(pData, &iDummy);
+  assert( iDummy==0 );
+  assert( n>0 );
+  assert( n<nData );
+  pData += n;
+  nData -= n;
+
+  /* Leading term length and data must fit in buffer. */
+  n = getVarint32(pData, &iDummy);
+  assert( n>0 );
+  assert( iDummy>0 );
+  assert( n+iDummy>0 );
+  assert( n+iDummy<nData );
+  pData += n+iDummy;
+  nData -= n+iDummy;
+
+  /* Leading term's doclist length and data must fit. */
+  n = getVarint32(pData, &iDummy);
+  assert( n>0 );
+  assert( iDummy>0 );
+  assert( n+iDummy>0 );
+  assert( n+iDummy<=nData );
+  ASSERT_VALID_DOCLIST(DL_DEFAULT, pData+n, iDummy, NULL);
+  pData += n+iDummy;
+  nData -= n+iDummy;
+
+  /* Verify that trailing terms and doclists also are readable. */
+  while( nData!=0 ){
+    n = getVarint32(pData, &iDummy);
+    assert( n>0 );
+    assert( iDummy>=0 );
+    assert( n<nData );
+    pData += n;
+    nData -= n;
+    n = getVarint32(pData, &iDummy);
+    assert( n>0 );
+    assert( iDummy>0 );
+    assert( n+iDummy>0 );
+    assert( n+iDummy<nData );
+    pData += n+iDummy;
+    nData -= n+iDummy;
+
+    n = getVarint32(pData, &iDummy);
+    assert( n>0 );
+    assert( iDummy>0 );
+    assert( n+iDummy>0 );
+    assert( n+iDummy<=nData );
+    ASSERT_VALID_DOCLIST(DL_DEFAULT, pData+n, iDummy, NULL);
+    pData += n+iDummy;
+    nData -= n+iDummy;
+  }
+}
+#define ASSERT_VALID_LEAF_NODE(p, n) leafNodeValidate(p, n)
+#else
+#define ASSERT_VALID_LEAF_NODE(p, n) assert( 1 )
+#endif
+
+/* Flush the current leaf node to %_segments, and adding the resulting
+** blockid and the starting term to the interior node which will
+** contain it.
+*/
+static int leafWriterInternalFlush(fulltext_vtab *v, LeafWriter *pWriter,
+                                   int iData, int nData){
+  sqlite_int64 iBlockid = 0;
+  const char *pStartingTerm;
+  int nStartingTerm, rc, n;
+
+  /* Must have the leading varint(0) flag, plus at least some
+  ** valid-looking data.
+  */
+  assert( nData>2 );
+  assert( iData>=0 );
+  assert( iData+nData<=pWriter->data.nData );
+  ASSERT_VALID_LEAF_NODE(pWriter->data.pData+iData, nData);
+
+  rc = block_insert(v, pWriter->data.pData+iData, nData, &iBlockid);
+  if( rc!=SQLITE_OK ) return rc;
+  assert( iBlockid!=0 );
+
+  /* Reconstruct the first term in the leaf for purposes of building
+  ** the interior node.
+  */
+  n = getVarint32(pWriter->data.pData+iData+1, &nStartingTerm);
+  pStartingTerm = pWriter->data.pData+iData+1+n;
+  assert( pWriter->data.nData>iData+1+n+nStartingTerm );
+  assert( pWriter->nTermDistinct>0 );
+  assert( pWriter->nTermDistinct<=nStartingTerm );
+  nStartingTerm = pWriter->nTermDistinct;
+
+  if( pWriter->has_parent ){
+    interiorWriterAppend(&pWriter->parentWriter,
+                         pStartingTerm, nStartingTerm, iBlockid);
+  }else{
+    interiorWriterInit(1, pStartingTerm, nStartingTerm, iBlockid,
+                       &pWriter->parentWriter);
+    pWriter->has_parent = 1;
+  }
+
+  /* Track the span of this segment's leaf nodes. */
+  if( pWriter->iEndBlockid==0 ){
+    pWriter->iEndBlockid = pWriter->iStartBlockid = iBlockid;
+  }else{
+    pWriter->iEndBlockid++;
+    assert( iBlockid==pWriter->iEndBlockid );
+  }
+
+  return SQLITE_OK;
+}
+static int leafWriterFlush(fulltext_vtab *v, LeafWriter *pWriter){
+  int rc = leafWriterInternalFlush(v, pWriter, 0, pWriter->data.nData);
+  if( rc!=SQLITE_OK ) return rc;
+
+  /* Re-initialize the output buffer. */
+  dataBufferReset(&pWriter->data);
+
+  return SQLITE_OK;
+}
+
+/* Fetch the root info for the segment.  If the entire leaf fits
+** within ROOT_MAX, then it will be returned directly, otherwise it
+** will be flushed and the root info will be returned from the
+** interior node.  *piEndBlockid is set to the blockid of the last
+** interior or leaf node written to disk (0 if none are written at
+** all).
+*/
+static int leafWriterRootInfo(fulltext_vtab *v, LeafWriter *pWriter,
+                              char **ppRootInfo, int *pnRootInfo,
+                              sqlite_int64 *piEndBlockid){
+  /* we can fit the segment entirely inline */
+  if( !pWriter->has_parent && pWriter->data.nData<ROOT_MAX ){
+    *ppRootInfo = pWriter->data.pData;
+    *pnRootInfo = pWriter->data.nData;
+    *piEndBlockid = 0;
+    return SQLITE_OK;
+  }
+
+  /* Flush remaining leaf data. */
+  if( pWriter->data.nData>0 ){
+    int rc = leafWriterFlush(v, pWriter);
+    if( rc!=SQLITE_OK ) return rc;
+  }
+
+  /* We must have flushed a leaf at some point. */
+  assert( pWriter->has_parent );
+
+  /* Tenatively set the end leaf blockid as the end blockid.  If the
+  ** interior node can be returned inline, this will be the final
+  ** blockid, otherwise it will be overwritten by
+  ** interiorWriterRootInfo().
+  */
+  *piEndBlockid = pWriter->iEndBlockid;
+
+  return interiorWriterRootInfo(v, &pWriter->parentWriter,
+                                ppRootInfo, pnRootInfo, piEndBlockid);
+}
+
+/* Collect the rootInfo data and store it into the segment directory.
+** This has the effect of flushing the segment's leaf data to
+** %_segments, and also flushing any interior nodes to %_segments.
+*/
+static int leafWriterFinalize(fulltext_vtab *v, LeafWriter *pWriter){
+  sqlite_int64 iEndBlockid;
+  char *pRootInfo;
+  int rc, nRootInfo;
+
+  rc = leafWriterRootInfo(v, pWriter, &pRootInfo, &nRootInfo, &iEndBlockid);
+  if( rc!=SQLITE_OK ) return rc;
+
+  /* Don't bother storing an entirely empty segment. */
+  if( iEndBlockid==0 && nRootInfo==0 ) return SQLITE_OK;
+
+  return segdir_set(v, pWriter->iLevel, pWriter->idx,
+                    pWriter->iStartBlockid, pWriter->iEndBlockid,
+                    iEndBlockid, pRootInfo, nRootInfo);
+}
+
+static void leafWriterDestroy(LeafWriter *pWriter){
+  if( pWriter->has_parent ) interiorWriterDestroy(&pWriter->parentWriter);
+  dataBufferDestroy(&pWriter->term);
+  dataBufferDestroy(&pWriter->data);
+}
+
+/* Encode a term into the leafWriter, delta-encoding as appropriate.
+** Returns the length of the new term which distinguishes it from the
+** previous term, which can be used to set nTermDistinct when a node
+** boundary is crossed.
+*/
+static int leafWriterEncodeTerm(LeafWriter *pWriter,
+                                const char *pTerm, int nTerm){
+  char c[VARINT_MAX+VARINT_MAX];
+  int n, nPrefix = 0;
+
+  assert( nTerm>0 );
+  while( nPrefix<pWriter->term.nData &&
+         pTerm[nPrefix]==pWriter->term.pData[nPrefix] ){
+    nPrefix++;
+    /* Failing this implies that the terms weren't in order. */
+    assert( nPrefix<nTerm );
+  }
+
+  if( pWriter->data.nData==0 ){
+    /* Encode the node header and leading term as:
+    **  varint(0)
+    **  varint(nTerm)
+    **  char pTerm[nTerm]
+    */
+    n = putVarint(c, '\0');
+    n += putVarint(c+n, nTerm);
+    dataBufferAppend2(&pWriter->data, c, n, pTerm, nTerm);
+  }else{
+    /* Delta-encode the term as:
+    **  varint(nPrefix)
+    **  varint(nSuffix)
+    **  char pTermSuffix[nSuffix]
+    */
+    n = putVarint(c, nPrefix);
+    n += putVarint(c+n, nTerm-nPrefix);
+    dataBufferAppend2(&pWriter->data, c, n, pTerm+nPrefix, nTerm-nPrefix);
+  }
+  dataBufferReplace(&pWriter->term, pTerm, nTerm);
+
+  return nPrefix+1;
+}
+
+/* Used to avoid a memmove when a large amount of doclist data is in
+** the buffer.  This constructs a node and term header before
+** iDoclistData and flushes the resulting complete node using
+** leafWriterInternalFlush().
+*/
+static int leafWriterInlineFlush(fulltext_vtab *v, LeafWriter *pWriter,
+                                 const char *pTerm, int nTerm,
+                                 int iDoclistData){
+  char c[VARINT_MAX+VARINT_MAX];
+  int iData, n = putVarint(c, 0);
+  n += putVarint(c+n, nTerm);
+
+  /* There should always be room for the header.  Even if pTerm shared
+  ** a substantial prefix with the previous term, the entire prefix
+  ** could be constructed from earlier data in the doclist, so there
+  ** should be room.
+  */
+  assert( iDoclistData>=n+nTerm );
+
+  iData = iDoclistData-(n+nTerm);
+  memcpy(pWriter->data.pData+iData, c, n);
+  memcpy(pWriter->data.pData+iData+n, pTerm, nTerm);
+
+  return leafWriterInternalFlush(v, pWriter, iData, pWriter->data.nData-iData);
+}
+
+/* Push pTerm[nTerm] along with the doclist data to the leaf layer of
+** %_segments.
+*/
+static int leafWriterStepMerge(fulltext_vtab *v, LeafWriter *pWriter,
+                               const char *pTerm, int nTerm,
+                               DLReader *pReaders, int nReaders){
+  char c[VARINT_MAX+VARINT_MAX];
+  int iTermData = pWriter->data.nData, iDoclistData;
+  int i, nData, n, nActualData, nActual, rc, nTermDistinct;
+
+  ASSERT_VALID_LEAF_NODE(pWriter->data.pData, pWriter->data.nData);
+  nTermDistinct = leafWriterEncodeTerm(pWriter, pTerm, nTerm);
+
+  /* Remember nTermDistinct if opening a new node. */
+  if( iTermData==0 ) pWriter->nTermDistinct = nTermDistinct;
+
+  iDoclistData = pWriter->data.nData;
+
+  /* Estimate the length of the merged doclist so we can leave space
+  ** to encode it.
+  */
+  for(i=0, nData=0; i<nReaders; i++){
+    nData += dlrAllDataBytes(&pReaders[i]);
+  }
+  n = putVarint(c, nData);
+  dataBufferAppend(&pWriter->data, c, n);
+
+  rc = docListMerge(&pWriter->data, pReaders, nReaders);
+  if( rc!= SQLITE_OK ) return rc;
+  ASSERT_VALID_DOCLIST(DL_DEFAULT,
+                       pWriter->data.pData+iDoclistData+n,
+                       pWriter->data.nData-iDoclistData-n, NULL);
+
+  /* The actual amount of doclist data at this point could be smaller
+  ** than the length we encoded.  Additionally, the space required to
+  ** encode this length could be smaller.  For small doclists, this is
+  ** not a big deal, we can just use memmove() to adjust things.
+  */
+  nActualData = pWriter->data.nData-(iDoclistData+n);
+  nActual = putVarint(c, nActualData);
+  assert( nActualData<=nData );
+  assert( nActual<=n );
+
+  /* If the new doclist is big enough for force a standalone leaf
+  ** node, we can immediately flush it inline without doing the
+  ** memmove().
+  */
+  /* TODO(shess) This test matches leafWriterStep(), which does this
+  ** test before it knows the cost to varint-encode the term and
+  ** doclist lengths.  At some point, change to
+  ** pWriter->data.nData-iTermData>STANDALONE_MIN.
+  */
+  if( nTerm+nActualData>STANDALONE_MIN ){
+    /* Push leaf node from before this term. */
+    if( iTermData>0 ){
+      rc = leafWriterInternalFlush(v, pWriter, 0, iTermData);
+      if( rc!=SQLITE_OK ) return rc;
+
+      pWriter->nTermDistinct = nTermDistinct;
+    }
+
+    /* Fix the encoded doclist length. */
+    iDoclistData += n - nActual;
+    memcpy(pWriter->data.pData+iDoclistData, c, nActual);
+
+    /* Push the standalone leaf node. */
+    rc = leafWriterInlineFlush(v, pWriter, pTerm, nTerm, iDoclistData);
+    if( rc!=SQLITE_OK ) return rc;
+
+    /* Leave the node empty. */
+    dataBufferReset(&pWriter->data);
+
+    return rc;
+  }
+
+  /* At this point, we know that the doclist was small, so do the
+  ** memmove if indicated.
+  */
+  if( nActual<n ){
+    memmove(pWriter->data.pData+iDoclistData+nActual,
+            pWriter->data.pData+iDoclistData+n,
+            pWriter->data.nData-(iDoclistData+n));
+    pWriter->data.nData -= n-nActual;
+  }
+
+  /* Replace written length with actual length. */
+  memcpy(pWriter->data.pData+iDoclistData, c, nActual);
+
+  /* If the node is too large, break things up. */
+  /* TODO(shess) This test matches leafWriterStep(), which does this
+  ** test before it knows the cost to varint-encode the term and
+  ** doclist lengths.  At some point, change to
+  ** pWriter->data.nData>LEAF_MAX.
+  */
+  if( iTermData+nTerm+nActualData>LEAF_MAX ){
+    /* Flush out the leading data as a node */
+    rc = leafWriterInternalFlush(v, pWriter, 0, iTermData);
+    if( rc!=SQLITE_OK ) return rc;
+
+    pWriter->nTermDistinct = nTermDistinct;
+
+    /* Rebuild header using the current term */
+    n = putVarint(pWriter->data.pData, 0);
+    n += putVarint(pWriter->data.pData+n, nTerm);
+    memcpy(pWriter->data.pData+n, pTerm, nTerm);
+    n += nTerm;
+
+    /* There should always be room, because the previous encoding
+    ** included all data necessary to construct the term.
+    */
+    assert( n<iDoclistData );
+    /* So long as STANDALONE_MIN is half or less of LEAF_MAX, the
+    ** following memcpy() is safe (as opposed to needing a memmove).
+    */
+    assert( 2*STANDALONE_MIN<=LEAF_MAX );
+    assert( n+pWriter->data.nData-iDoclistData<iDoclistData );
+    memcpy(pWriter->data.pData+n,
+           pWriter->data.pData+iDoclistData,
+           pWriter->data.nData-iDoclistData);
+    pWriter->data.nData -= iDoclistData-n;
+  }
+  ASSERT_VALID_LEAF_NODE(pWriter->data.pData, pWriter->data.nData);
+
+  return SQLITE_OK;
+}
+
+/* Push pTerm[nTerm] along with the doclist data to the leaf layer of
+** %_segments.
+*/
+/* TODO(shess) Revise writeZeroSegment() so that doclists are
+** constructed directly in pWriter->data.
+*/
+static int leafWriterStep(fulltext_vtab *v, LeafWriter *pWriter,
+                          const char *pTerm, int nTerm,
+                          const char *pData, int nData){
+  int rc;
+  DLReader reader;
+
+  rc = dlrInit(&reader, DL_DEFAULT, pData, nData);
+  if( rc!=SQLITE_OK ) return rc;
+  rc = leafWriterStepMerge(v, pWriter, pTerm, nTerm, &reader, 1);
+  dlrDestroy(&reader);
+
+  return rc;
+}
+
+
+/****************************************************************/
+/* LeafReader is used to iterate over an individual leaf node. */
+typedef struct LeafReader {
+  DataBuffer term;          /* copy of current term. */
+
+  const char *pData;        /* data for current term. */
+  int nData;
+} LeafReader;
+
+static void leafReaderDestroy(LeafReader *pReader){
+  dataBufferDestroy(&pReader->term);
+  SCRAMBLE(pReader);
+}
+
+static int leafReaderAtEnd(LeafReader *pReader){
+  return pReader->nData<=0;
+}
+
+/* Access the current term. */
+static int leafReaderTermBytes(LeafReader *pReader){
+  return pReader->term.nData;
+}
+static const char *leafReaderTerm(LeafReader *pReader){
+  assert( pReader->term.nData>0 );
+  return pReader->term.pData;
+}
+
+/* Access the doclist data for the current term. */
+static int leafReaderDataBytes(LeafReader *pReader){
+  int nData;
+  assert( pReader->term.nData>0 );
+  getVarint32(pReader->pData, &nData);
+  return nData;
+}
+static const char *leafReaderData(LeafReader *pReader){
+  int n, nData;
+  assert( pReader->term.nData>0 );
+  n = getVarint32Safe(pReader->pData, &nData, pReader->nData);
+  if( !n || nData>pReader->nData-n ) return NULL;
+  return pReader->pData+n;
+}
+
+static int leafReaderInit(const char *pData, int nData, LeafReader *pReader){
+  int nTerm, n;
+
+  /* All callers check this precondition. */
+  assert( nData>0 );
+  assert( pData[0]=='\0' );
+
+  CLEAR(pReader);
+
+  /* Read the first term, skipping the header byte. */
+  n = getVarint32Safe(pData+1, &nTerm, nData-1);
+  if( !n || nTerm<0 || nTerm>nData-1-n ) return SQLITE_CORRUPT_BKPT;
+  dataBufferInit(&pReader->term, nTerm);
+  dataBufferReplace(&pReader->term, pData+1+n, nTerm);
+
+  /* Position after the first term. */
+  pReader->pData = pData+1+n+nTerm;
+  pReader->nData = nData-1-n-nTerm;
+  return SQLITE_OK;
+}
+
+/* Step the reader forward to the next term. */
+static int leafReaderStep(LeafReader *pReader){
+  int n, nData, nPrefix, nSuffix;
+  assert( !leafReaderAtEnd(pReader) );
+
+  /* Skip previous entry's data block. */
+  n = getVarint32Safe(pReader->pData, &nData, pReader->nData);
+  if( !n || nData<0 || nData>pReader->nData-n ) return SQLITE_CORRUPT_BKPT;
+  pReader->pData += n+nData;
+  pReader->nData -= n+nData;
+
+  if( !leafReaderAtEnd(pReader) ){
+    /* Construct the new term using a prefix from the old term plus a
+    ** suffix from the leaf data.
+    */
+    n = getVarint32Safe(pReader->pData, &nPrefix, pReader->nData);
+    if( !n ) return SQLITE_CORRUPT_BKPT;
+    pReader->nData -= n;
+    pReader->pData += n;
+    n = getVarint32Safe(pReader->pData, &nSuffix, pReader->nData);
+    if( !n ) return SQLITE_CORRUPT_BKPT;
+    pReader->nData -= n;
+    pReader->pData += n;
+    if( nSuffix<0 || nSuffix>pReader->nData ) return SQLITE_CORRUPT_BKPT;
+    if( nPrefix<0 || nPrefix>pReader->term.nData ) return SQLITE_CORRUPT_BKPT;
+    pReader->term.nData = nPrefix;
+    dataBufferAppend(&pReader->term, pReader->pData, nSuffix);
+
+    pReader->pData += nSuffix;
+    pReader->nData -= nSuffix;
+  }
+  return SQLITE_OK;
+}
+
+/* strcmp-style comparison of pReader's current term against pTerm.
+** If isPrefix, equality means equal through nTerm bytes.
+*/
+static int leafReaderTermCmp(LeafReader *pReader,
+                             const char *pTerm, int nTerm, int isPrefix){
+  int c, n = pReader->term.nData<nTerm ? pReader->term.nData : nTerm;
+  if( n==0 ){
+    if( pReader->term.nData>0 ) return -1;
+    if(nTerm>0 ) return 1;
+    return 0;
+  }
+
+  c = memcmp(pReader->term.pData, pTerm, n);
+  if( c!=0 ) return c;
+  if( isPrefix && n==nTerm ) return 0;
+  return pReader->term.nData - nTerm;
+}
+
+
+/****************************************************************/
+/* LeavesReader wraps LeafReader to allow iterating over the entire
+** leaf layer of the tree.
+*/
+typedef struct LeavesReader {
+  int idx;                  /* Index within the segment. */
+
+  sqlite3_stmt *pStmt;      /* Statement we're streaming leaves from. */
+  int eof;                  /* we've seen SQLITE_DONE from pStmt. */
+
+  LeafReader leafReader;    /* reader for the current leaf. */
+  DataBuffer rootData;      /* root data for inline. */
+} LeavesReader;
+
+/* Access the current term. */
+static int leavesReaderTermBytes(LeavesReader *pReader){
+  assert( !pReader->eof );
+  return leafReaderTermBytes(&pReader->leafReader);
+}
+static const char *leavesReaderTerm(LeavesReader *pReader){
+  assert( !pReader->eof );
+  return leafReaderTerm(&pReader->leafReader);
+}
+
+/* Access the doclist data for the current term. */
+static int leavesReaderDataBytes(LeavesReader *pReader){
+  assert( !pReader->eof );
+  return leafReaderDataBytes(&pReader->leafReader);
+}
+static const char *leavesReaderData(LeavesReader *pReader){
+  assert( !pReader->eof );
+  return leafReaderData(&pReader->leafReader);
+}
+
+static int leavesReaderAtEnd(LeavesReader *pReader){
+  return pReader->eof;
+}
+
+/* loadSegmentLeaves() may not read all the way to SQLITE_DONE, thus
+** leaving the statement handle open, which locks the table.
+*/
+/* TODO(shess) This "solution" is not satisfactory.  Really, there
+** should be check-in function for all statement handles which
+** arranges to call sqlite3_reset().  This most likely will require
+** modification to control flow all over the place, though, so for now
+** just punt.
+**
+** Note the the current system assumes that segment merges will run to
+** completion, which is why this particular probably hasn't arisen in
+** this case.  Probably a brittle assumption.
+*/
+static int leavesReaderReset(LeavesReader *pReader){
+  return sqlite3_reset(pReader->pStmt);
+}
+
+static void leavesReaderDestroy(LeavesReader *pReader){
+  /* If idx is -1, that means we're using a non-cached statement
+  ** handle in the optimize() case, so we need to release it.
+  */
+  if( pReader->pStmt!=NULL && pReader->idx==-1 ){
+    sqlite3_finalize(pReader->pStmt);
+  }
+  leafReaderDestroy(&pReader->leafReader);
+  dataBufferDestroy(&pReader->rootData);
+  SCRAMBLE(pReader);
+}
+
+/* Initialize pReader with the given root data (if iStartBlockid==0
+** the leaf data was entirely contained in the root), or from the
+** stream of blocks between iStartBlockid and iEndBlockid, inclusive.
+*/
+/* TODO(shess): Figure out a means of indicating how many leaves are
+** expected, for purposes of detecting corruption.
+*/
+static int leavesReaderInit(fulltext_vtab *v,
+                            int idx,
+                            sqlite_int64 iStartBlockid,
+                            sqlite_int64 iEndBlockid,
+                            const char *pRootData, int nRootData,
+                            LeavesReader *pReader){
+  CLEAR(pReader);
+  pReader->idx = idx;
+
+  dataBufferInit(&pReader->rootData, 0);
+  if( iStartBlockid==0 ){
+    int rc;
+    /* Corrupt if this can't be a leaf node. */
+    if( pRootData==NULL || nRootData<1 || pRootData[0]!='\0' ){
+      return SQLITE_CORRUPT_BKPT;
+    }
+    /* Entire leaf level fit in root data. */
+    dataBufferReplace(&pReader->rootData, pRootData, nRootData);
+    rc = leafReaderInit(pReader->rootData.pData, pReader->rootData.nData,
+                        &pReader->leafReader);
+    if( rc!=SQLITE_OK ){
+      dataBufferDestroy(&pReader->rootData);
+      return rc;
+    }
+  }else{
+    sqlite3_stmt *s;
+    int rc = sql_get_leaf_statement(v, idx, &s);
+    if( rc!=SQLITE_OK ) return rc;
+
+    rc = sqlite3_bind_int64(s, 1, iStartBlockid);
+    if( rc!=SQLITE_OK ) goto err;
+
+    rc = sqlite3_bind_int64(s, 2, iEndBlockid);
+    if( rc!=SQLITE_OK ) goto err;
+
+    rc = sqlite3_step(s);
+
+    /* Corrupt if interior node referenced missing leaf node. */
+    if( rc==SQLITE_DONE ){
+      rc = SQLITE_CORRUPT_BKPT;
+      goto err;
+    }
+
+    if( rc!=SQLITE_ROW ) goto err;
+    rc = SQLITE_OK;
+
+    /* Corrupt if leaf data isn't a blob. */
+    if( sqlite3_column_type(s, 0)!=SQLITE_BLOB ){
+      rc = SQLITE_CORRUPT_BKPT;
+    }else{
+      const char *pLeafData = sqlite3_column_blob(s, 0);
+      int nLeafData = sqlite3_column_bytes(s, 0);
+
+      /* Corrupt if this can't be a leaf node. */
+      if( pLeafData==NULL || nLeafData<1 || pLeafData[0]!='\0' ){
+        rc = SQLITE_CORRUPT_BKPT;
+      }else{
+        rc = leafReaderInit(pLeafData, nLeafData, &pReader->leafReader);
+      }
+    }
+
+ err:
+    if( rc!=SQLITE_OK ){
+      if( idx==-1 ){
+        sqlite3_finalize(s);
+      }else{
+        sqlite3_reset(s);
+      }
+      return rc;
+    }
+
+    pReader->pStmt = s;
+  }
+  return SQLITE_OK;
+}
+
+/* Step the current leaf forward to the next term.  If we reach the
+** end of the current leaf, step forward to the next leaf block.
+*/
+static int leavesReaderStep(fulltext_vtab *v, LeavesReader *pReader){
+  int rc;
+  assert( !leavesReaderAtEnd(pReader) );
+  rc = leafReaderStep(&pReader->leafReader);
+  if( rc!=SQLITE_OK ) return rc;
+
+  if( leafReaderAtEnd(&pReader->leafReader) ){
+    if( pReader->rootData.pData ){
+      pReader->eof = 1;
+      return SQLITE_OK;
+    }
+    rc = sqlite3_step(pReader->pStmt);
+    if( rc!=SQLITE_ROW ){
+      pReader->eof = 1;
+      return rc==SQLITE_DONE ? SQLITE_OK : rc;
+    }
+
+    /* Corrupt if leaf data isn't a blob. */
+    if( sqlite3_column_type(pReader->pStmt, 0)!=SQLITE_BLOB ){
+      return SQLITE_CORRUPT_BKPT;
+    }else{
+      LeafReader tmp;
+      const char *pLeafData = sqlite3_column_blob(pReader->pStmt, 0);
+      int nLeafData = sqlite3_column_bytes(pReader->pStmt, 0);
+
+      /* Corrupt if this can't be a leaf node. */
+      if( pLeafData==NULL || nLeafData<1 || pLeafData[0]!='\0' ){
+        return SQLITE_CORRUPT_BKPT;
+      }
+
+      rc = leafReaderInit(pLeafData, nLeafData, &tmp);
+      if( rc!=SQLITE_OK ) return rc;
+      leafReaderDestroy(&pReader->leafReader);
+      pReader->leafReader = tmp;
+    }
+  }
+  return SQLITE_OK;
+}
+
+/* Order LeavesReaders by their term, ignoring idx.  Readers at eof
+** always sort to the end.
+*/
+static int leavesReaderTermCmp(LeavesReader *lr1, LeavesReader *lr2){
+  if( leavesReaderAtEnd(lr1) ){
+    if( leavesReaderAtEnd(lr2) ) return 0;
+    return 1;
+  }
+  if( leavesReaderAtEnd(lr2) ) return -1;
+
+  return leafReaderTermCmp(&lr1->leafReader,
+                           leavesReaderTerm(lr2), leavesReaderTermBytes(lr2),
+                           0);
+}
+
+/* Similar to leavesReaderTermCmp(), with additional ordering by idx
+** so that older segments sort before newer segments.
+*/
+static int leavesReaderCmp(LeavesReader *lr1, LeavesReader *lr2){
+  int c = leavesReaderTermCmp(lr1, lr2);
+  if( c!=0 ) return c;
+  return lr1->idx-lr2->idx;
+}
+
+/* Assume that pLr[1]..pLr[nLr] are sorted.  Bubble pLr[0] into its
+** sorted position.
+*/
+static void leavesReaderReorder(LeavesReader *pLr, int nLr){
+  while( nLr>1 && leavesReaderCmp(pLr, pLr+1)>0 ){
+    LeavesReader tmp = pLr[0];
+    pLr[0] = pLr[1];
+    pLr[1] = tmp;
+    nLr--;
+    pLr++;
+  }
+}
+
+/* Initializes pReaders with the segments from level iLevel, returning
+** the number of segments in *piReaders.  Leaves pReaders in sorted
+** order.
+*/
+static int leavesReadersInit(fulltext_vtab *v, int iLevel,
+                             LeavesReader *pReaders, int *piReaders){
+  sqlite3_stmt *s;
+  int i, rc = sql_get_statement(v, SEGDIR_SELECT_LEVEL_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int(s, 1, iLevel);
+  if( rc!=SQLITE_OK ) return rc;
+
+  i = 0;
+  while( (rc = sqlite3_step(s))==SQLITE_ROW ){
+    sqlite_int64 iStart = sqlite3_column_int64(s, 0);
+    sqlite_int64 iEnd = sqlite3_column_int64(s, 1);
+    const char *pRootData = sqlite3_column_blob(s, 2);
+    int nRootData = sqlite3_column_bytes(s, 2);
+    sqlite_int64 iIndex = sqlite3_column_int64(s, 3);
+
+    /* Corrupt if we get back different types than we stored. */
+    /* Also corrupt if the index is not sequential starting at 0. */
+    if( sqlite3_column_type(s, 0)!=SQLITE_INTEGER ||
+        sqlite3_column_type(s, 1)!=SQLITE_INTEGER ||
+        sqlite3_column_type(s, 2)!=SQLITE_BLOB ||
+        i!=iIndex ||
+        i>=MERGE_COUNT ){
+      rc = SQLITE_CORRUPT_BKPT;
+      break;
+    }
+
+    rc = leavesReaderInit(v, i, iStart, iEnd, pRootData, nRootData,
+                          &pReaders[i]);
+    if( rc!=SQLITE_OK ) break;
+
+    i++;
+  }
+  if( rc!=SQLITE_DONE ){
+    while( i-->0 ){
+      leavesReaderDestroy(&pReaders[i]);
+    }
+    sqlite3_reset(s);          /* So we don't leave a lock. */
+    return rc;
+  }
+
+  *piReaders = i;
+
+  /* Leave our results sorted by term, then age. */
+  while( i-- ){
+    leavesReaderReorder(pReaders+i, *piReaders-i);
+  }
+  return SQLITE_OK;
+}
+
+/* Merge doclists from pReaders[nReaders] into a single doclist, which
+** is written to pWriter.  Assumes pReaders is ordered oldest to
+** newest.
+*/
+/* TODO(shess) Consider putting this inline in segmentMerge(). */
+static int leavesReadersMerge(fulltext_vtab *v,
+                              LeavesReader *pReaders, int nReaders,
+                              LeafWriter *pWriter){
+  DLReader dlReaders[MERGE_COUNT];
+  const char *pTerm = leavesReaderTerm(pReaders);
+  int i, nTerm = leavesReaderTermBytes(pReaders);
+  int rc;
+
+  assert( nReaders<=MERGE_COUNT );
+
+  for(i=0; i<nReaders; i++){
+    const char *pData = leavesReaderData(pReaders+i);
+    if( pData==NULL ){
+      rc = SQLITE_CORRUPT_BKPT;
+      break;
+    }
+    rc = dlrInit(&dlReaders[i], DL_DEFAULT,
+                 pData,
+                 leavesReaderDataBytes(pReaders+i));
+    if( rc!=SQLITE_OK ) break;
+  }
+  if( rc!=SQLITE_OK ){
+    while( i-->0 ){ 
+      dlrDestroy(&dlReaders[i]);
+    }
+    return rc;
+  }
+
+  return leafWriterStepMerge(v, pWriter, pTerm, nTerm, dlReaders, nReaders);
+}
+
+/* Forward ref due to mutual recursion with segdirNextIndex(). */
+static int segmentMerge(fulltext_vtab *v, int iLevel);
+
+/* Put the next available index at iLevel into *pidx.  If iLevel
+** already has MERGE_COUNT segments, they are merged to a higher
+** level to make room.
+*/
+static int segdirNextIndex(fulltext_vtab *v, int iLevel, int *pidx){
+  int rc = segdir_max_index(v, iLevel, pidx);
+  if( rc==SQLITE_DONE ){              /* No segments at iLevel. */
+    *pidx = 0;
+  }else if( rc==SQLITE_ROW ){
+    if( *pidx==(MERGE_COUNT-1) ){
+      rc = segmentMerge(v, iLevel);
+      if( rc!=SQLITE_OK ) return rc;
+      *pidx = 0;
+    }else{
+      (*pidx)++;
+    }
+  }else{
+    return rc;
+  }
+  return SQLITE_OK;
+}
+
+/* Merge MERGE_COUNT segments at iLevel into a new segment at
+** iLevel+1.  If iLevel+1 is already full of segments, those will be
+** merged to make room.
+*/
+static int segmentMerge(fulltext_vtab *v, int iLevel){
+  LeafWriter writer;
+  LeavesReader lrs[MERGE_COUNT];
+  int i, rc, idx = 0;
+
+  /* Determine the next available segment index at the next level,
+  ** merging as necessary.
+  */
+  rc = segdirNextIndex(v, iLevel+1, &idx);
+  if( rc!=SQLITE_OK ) return rc;
+
+  /* TODO(shess) This assumes that we'll always see exactly
+  ** MERGE_COUNT segments to merge at a given level.  That will be
+  ** broken if we allow the developer to request preemptive or
+  ** deferred merging.
+  */
+  memset(&lrs, '\0', sizeof(lrs));
+  rc = leavesReadersInit(v, iLevel, lrs, &i);
+  if( rc!=SQLITE_OK ) return rc;
+
+  leafWriterInit(iLevel+1, idx, &writer);
+
+  if( i!=MERGE_COUNT ){
+    rc = SQLITE_CORRUPT_BKPT;
+    goto err;
+  }
+
+  /* Since leavesReaderReorder() pushes readers at eof to the end,
+  ** when the first reader is empty, all will be empty.
+  */
+  while( !leavesReaderAtEnd(lrs) ){
+    /* Figure out how many readers share their next term. */
+    for(i=1; i<MERGE_COUNT && !leavesReaderAtEnd(lrs+i); i++){
+      if( 0!=leavesReaderTermCmp(lrs, lrs+i) ) break;
+    }
+
+    rc = leavesReadersMerge(v, lrs, i, &writer);
+    if( rc!=SQLITE_OK ) goto err;
+
+    /* Step forward those that were merged. */
+    while( i-->0 ){
+      rc = leavesReaderStep(v, lrs+i);
+      if( rc!=SQLITE_OK ) goto err;
+
+      /* Reorder by term, then by age. */
+      leavesReaderReorder(lrs+i, MERGE_COUNT-i);
+    }
+  }
+
+  for(i=0; i<MERGE_COUNT; i++){
+    leavesReaderDestroy(&lrs[i]);
+  }
+
+  rc = leafWriterFinalize(v, &writer);
+  leafWriterDestroy(&writer);
+  if( rc!=SQLITE_OK ) return rc;
+
+  /* Delete the merged segment data. */
+  return segdir_delete(v, iLevel);
+
+ err:
+  for(i=0; i<MERGE_COUNT; i++){
+    leavesReaderDestroy(&lrs[i]);
+  }
+  leafWriterDestroy(&writer);
+  return rc;
+}
+
+/* Accumulate the union of *acc and *pData into *acc. */
+static int docListAccumulateUnion(DataBuffer *acc,
+                                  const char *pData, int nData) {
+  DataBuffer tmp = *acc;
+  int rc;
+  dataBufferInit(acc, tmp.nData+nData);
+  rc = docListUnion(tmp.pData, tmp.nData, pData, nData, acc);
+  dataBufferDestroy(&tmp);
+  return rc;
+}
+
+/* TODO(shess) It might be interesting to explore different merge
+** strategies, here.  For instance, since this is a sorted merge, we
+** could easily merge many doclists in parallel.  With some
+** comprehension of the storage format, we could merge all of the
+** doclists within a leaf node directly from the leaf node's storage.
+** It may be worthwhile to merge smaller doclists before larger
+** doclists, since they can be traversed more quickly - but the
+** results may have less overlap, making them more expensive in a
+** different way.
+*/
+
+/* Scan pReader for pTerm/nTerm, and merge the term's doclist over
+** *out (any doclists with duplicate docids overwrite those in *out).
+** Internal function for loadSegmentLeaf().
+*/
+static int loadSegmentLeavesInt(fulltext_vtab *v, LeavesReader *pReader,
+                                const char *pTerm, int nTerm, int isPrefix,
+                                DataBuffer *out){
+  /* doclist data is accumulated into pBuffers similar to how one does
+  ** increment in binary arithmetic.  If index 0 is empty, the data is
+  ** stored there.  If there is data there, it is merged and the
+  ** results carried into position 1, with further merge-and-carry
+  ** until an empty position is found.
+  */
+  DataBuffer *pBuffers = NULL;
+  int nBuffers = 0, nMaxBuffers = 0, rc;
+
+  assert( nTerm>0 );
+
+  for(rc=SQLITE_OK; rc==SQLITE_OK && !leavesReaderAtEnd(pReader);
+      rc=leavesReaderStep(v, pReader)){
+    /* TODO(shess) Really want leavesReaderTermCmp(), but that name is
+    ** already taken to compare the terms of two LeavesReaders.  Think
+    ** on a better name.  [Meanwhile, break encapsulation rather than
+    ** use a confusing name.]
+    */
+    int c = leafReaderTermCmp(&pReader->leafReader, pTerm, nTerm, isPrefix);
+    if( c>0 ) break;      /* Past any possible matches. */
+    if( c==0 ){
+      int iBuffer, nData;
+      const char *pData = leavesReaderData(pReader);
+      if( pData==NULL ){
+        rc = SQLITE_CORRUPT_BKPT;
+        break;
+      }
+      nData = leavesReaderDataBytes(pReader);
+
+      /* Find the first empty buffer. */
+      for(iBuffer=0; iBuffer<nBuffers; ++iBuffer){
+        if( 0==pBuffers[iBuffer].nData ) break;
+      }
+
+      /* Out of buffers, add an empty one. */
+      if( iBuffer==nBuffers ){
+        if( nBuffers==nMaxBuffers ){
+          DataBuffer *p;
+          nMaxBuffers += 20;
+
+          /* Manual realloc so we can handle NULL appropriately. */
+          p = sqlite3_malloc(nMaxBuffers*sizeof(*pBuffers));
+          if( p==NULL ){
+            rc = SQLITE_NOMEM;
+            break;
+          }
+
+          if( nBuffers>0 ){
+            assert(pBuffers!=NULL);
+            memcpy(p, pBuffers, nBuffers*sizeof(*pBuffers));
+            sqlite3_free(pBuffers);
+          }
+          pBuffers = p;
+        }
+        dataBufferInit(&(pBuffers[nBuffers]), 0);
+        nBuffers++;
+      }
+
+      /* At this point, must have an empty at iBuffer. */
+      assert(iBuffer<nBuffers && pBuffers[iBuffer].nData==0);
+
+      /* If empty was first buffer, no need for merge logic. */
+      if( iBuffer==0 ){
+        dataBufferReplace(&(pBuffers[0]), pData, nData);
+      }else{
+        /* pAcc is the empty buffer the merged data will end up in. */
+        DataBuffer *pAcc = &(pBuffers[iBuffer]);
+        DataBuffer *p = &(pBuffers[0]);
+
+        /* Handle position 0 specially to avoid need to prime pAcc
+        ** with pData/nData.
+        */
+        dataBufferSwap(p, pAcc);
+        rc = docListAccumulateUnion(pAcc, pData, nData);
+        if( rc!=SQLITE_OK ) goto err;
+
+        /* Accumulate remaining doclists into pAcc. */
+        for(++p; p<pAcc; ++p){
+          rc = docListAccumulateUnion(pAcc, p->pData, p->nData);
+          if( rc!=SQLITE_OK ) goto err;
+
+          /* dataBufferReset() could allow a large doclist to blow up
+          ** our memory requirements.
+          */
+          if( p->nCapacity<1024 ){
+            dataBufferReset(p);
+          }else{
+            dataBufferDestroy(p);
+            dataBufferInit(p, 0);
+          }
+        }
+      }
+    }
+  }
+
+  /* Union all the doclists together into *out. */
+  /* TODO(shess) What if *out is big?  Sigh. */
+  if( rc==SQLITE_OK && nBuffers>0 ){
+    int iBuffer;
+    for(iBuffer=0; iBuffer<nBuffers; ++iBuffer){
+      if( pBuffers[iBuffer].nData>0 ){
+        if( out->nData==0 ){
+          dataBufferSwap(out, &(pBuffers[iBuffer]));
+        }else{
+          rc = docListAccumulateUnion(out, pBuffers[iBuffer].pData,
+                                      pBuffers[iBuffer].nData);
+          if( rc!=SQLITE_OK ) break;
+        }
+      }
+    }
+  }
+
+err:
+  while( nBuffers-- ){
+    dataBufferDestroy(&(pBuffers[nBuffers]));
+  }
+  if( pBuffers!=NULL ) sqlite3_free(pBuffers);
+
+  return rc;
+}
+
+/* Call loadSegmentLeavesInt() with pData/nData as input. */
+static int loadSegmentLeaf(fulltext_vtab *v, const char *pData, int nData,
+                           const char *pTerm, int nTerm, int isPrefix,
+                           DataBuffer *out){
+  LeavesReader reader;
+  int rc;
+
+  assert( nData>1 );
+  assert( *pData=='\0' );
+  rc = leavesReaderInit(v, 0, 0, 0, pData, nData, &reader);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = loadSegmentLeavesInt(v, &reader, pTerm, nTerm, isPrefix, out);
+  leavesReaderReset(&reader);
+  leavesReaderDestroy(&reader);
+  return rc;
+}
+
+/* Call loadSegmentLeavesInt() with the leaf nodes from iStartLeaf to
+** iEndLeaf (inclusive) as input, and merge the resulting doclist into
+** out.
+*/
+static int loadSegmentLeaves(fulltext_vtab *v,
+                             sqlite_int64 iStartLeaf, sqlite_int64 iEndLeaf,
+                             const char *pTerm, int nTerm, int isPrefix,
+                             DataBuffer *out){
+  int rc;
+  LeavesReader reader;
+
+  assert( iStartLeaf<=iEndLeaf );
+  rc = leavesReaderInit(v, 0, iStartLeaf, iEndLeaf, NULL, 0, &reader);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = loadSegmentLeavesInt(v, &reader, pTerm, nTerm, isPrefix, out);
+  leavesReaderReset(&reader);
+  leavesReaderDestroy(&reader);
+  return rc;
+}
+
+/* Taking pData/nData as an interior node, find the sequence of child
+** nodes which could include pTerm/nTerm/isPrefix.  Note that the
+** interior node terms logically come between the blocks, so there is
+** one more blockid than there are terms (that block contains terms >=
+** the last interior-node term).
+*/
+/* TODO(shess) The calling code may already know that the end child is
+** not worth calculating, because the end may be in a later sibling
+** node.  Consider whether breaking symmetry is worthwhile.  I suspect
+** it is not worthwhile.
+*/
+static int getChildrenContaining(const char *pData, int nData,
+                                 const char *pTerm, int nTerm, int isPrefix,
+                                 sqlite_int64 *piStartChild,
+                                 sqlite_int64 *piEndChild){
+  InteriorReader reader;
+  int rc;
+
+  assert( nData>1 );
+  assert( *pData!='\0' );
+  rc = interiorReaderInit(pData, nData, &reader);
+  if( rc!=SQLITE_OK ) return rc;
+
+  /* Scan for the first child which could contain pTerm/nTerm. */
+  while( !interiorReaderAtEnd(&reader) ){
+    if( interiorReaderTermCmp(&reader, pTerm, nTerm, 0)>0 ) break;
+    rc = interiorReaderStep(&reader);
+    if( rc!=SQLITE_OK ){
+      interiorReaderDestroy(&reader);
+      return rc;
+    }
+  }
+  *piStartChild = interiorReaderCurrentBlockid(&reader);
+
+  /* Keep scanning to find a term greater than our term, using prefix
+  ** comparison if indicated.  If isPrefix is false, this will be the
+  ** same blockid as the starting block.
+  */
+  while( !interiorReaderAtEnd(&reader) ){
+    if( interiorReaderTermCmp(&reader, pTerm, nTerm, isPrefix)>0 ) break;
+    rc = interiorReaderStep(&reader);
+    if( rc!=SQLITE_OK ){
+      interiorReaderDestroy(&reader);
+      return rc;
+    }
+  }
+  *piEndChild = interiorReaderCurrentBlockid(&reader);
+
+  interiorReaderDestroy(&reader);
+
+  /* Children must ascend, and if !prefix, both must be the same. */
+  assert( *piEndChild>=*piStartChild );
+  assert( isPrefix || *piStartChild==*piEndChild );
+  return rc;
+}
+
+/* Read block at iBlockid and pass it with other params to
+** getChildrenContaining().
+*/
+static int loadAndGetChildrenContaining(
+  fulltext_vtab *v,
+  sqlite_int64 iBlockid,
+  const char *pTerm, int nTerm, int isPrefix,
+  sqlite_int64 *piStartChild, sqlite_int64 *piEndChild
+){
+  sqlite3_stmt *s = NULL;
+  int rc;
+
+  assert( iBlockid!=0 );
+  assert( pTerm!=NULL );
+  assert( nTerm!=0 );        /* TODO(shess) Why not allow this? */
+  assert( piStartChild!=NULL );
+  assert( piEndChild!=NULL );
+
+  rc = sql_get_statement(v, BLOCK_SELECT_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_bind_int64(s, 1, iBlockid);
+  if( rc!=SQLITE_OK ) return rc;
+
+  rc = sqlite3_step(s);
+  /* Corrupt if interior node references missing child node. */
+  if( rc==SQLITE_DONE ) return SQLITE_CORRUPT_BKPT;
+  if( rc!=SQLITE_ROW ) return rc;
+
+  /* Corrupt if child node isn't a blob. */
+  if( sqlite3_column_type(s, 0)!=SQLITE_BLOB ){
+    sqlite3_reset(s);         /* So we don't leave a lock. */
+    return SQLITE_CORRUPT_BKPT;
+  }else{
+    const char *pData = sqlite3_column_blob(s, 0);
+    int nData = sqlite3_column_bytes(s, 0);
+
+    /* Corrupt if child is not a valid interior node. */
+    if( pData==NULL || nData<1 || pData[0]=='\0' ){
+      sqlite3_reset(s);         /* So we don't leave a lock. */
+      return SQLITE_CORRUPT_BKPT;
+    }
+
+    rc = getChildrenContaining(pData, nData, pTerm, nTerm,
+                               isPrefix, piStartChild, piEndChild);
+    if( rc!=SQLITE_OK ){
+      sqlite3_reset(s);
+      return rc;
+    }
+  }
+
+  /* We expect only one row.  We must execute another sqlite3_step()
+   * to complete the iteration; otherwise the table will remain
+   * locked. */
+  rc = sqlite3_step(s);
+  if( rc==SQLITE_ROW ) return SQLITE_ERROR;
+  if( rc!=SQLITE_DONE ) return rc;
+
+  return SQLITE_OK;
+}
+
+/* Traverse the tree represented by pData[nData] looking for
+** pTerm[nTerm], placing its doclist into *out.  This is internal to
+** loadSegment() to make error-handling cleaner.
+*/
+static int loadSegmentInt(fulltext_vtab *v, const char *pData, int nData,
+                          sqlite_int64 iLeavesEnd,
+                          const char *pTerm, int nTerm, int isPrefix,
+                          DataBuffer *out){
+  /* Special case where root is a leaf. */
+  if( *pData=='\0' ){
+    return loadSegmentLeaf(v, pData, nData, pTerm, nTerm, isPrefix, out);
+  }else{
+    int rc;
+    sqlite_int64 iStartChild, iEndChild;
+
+    /* Process pData as an interior node, then loop down the tree
+    ** until we find the set of leaf nodes to scan for the term.
+    */
+    rc = getChildrenContaining(pData, nData, pTerm, nTerm, isPrefix,
+                               &iStartChild, &iEndChild);
+    if( rc!=SQLITE_OK ) return rc;
+    while( iStartChild>iLeavesEnd ){
+      sqlite_int64 iNextStart, iNextEnd;
+      rc = loadAndGetChildrenContaining(v, iStartChild, pTerm, nTerm, isPrefix,
+                                        &iNextStart, &iNextEnd);
+      if( rc!=SQLITE_OK ) return rc;
+
+      /* If we've branched, follow the end branch, too. */
+      if( iStartChild!=iEndChild ){
+        sqlite_int64 iDummy;
+        rc = loadAndGetChildrenContaining(v, iEndChild, pTerm, nTerm, isPrefix,
+                                          &iDummy, &iNextEnd);
+        if( rc!=SQLITE_OK ) return rc;
+      }
+
+      assert( iNextStart<=iNextEnd );
+      iStartChild = iNextStart;
+      iEndChild = iNextEnd;
+    }
+    assert( iStartChild<=iLeavesEnd );
+    assert( iEndChild<=iLeavesEnd );
+
+    /* Scan through the leaf segments for doclists. */
+    return loadSegmentLeaves(v, iStartChild, iEndChild,
+                             pTerm, nTerm, isPrefix, out);
+  }
+}
+
+/* Call loadSegmentInt() to collect the doclist for pTerm/nTerm, then
+** merge its doclist over *out (any duplicate doclists read from the
+** segment rooted at pData will overwrite those in *out).
+*/
+/* TODO(shess) Consider changing this to determine the depth of the
+** leaves using either the first characters of interior nodes (when
+** ==1, we're one level above the leaves), or the first character of
+** the root (which will describe the height of the tree directly).
+** Either feels somewhat tricky to me.
+*/
+/* TODO(shess) The current merge is likely to be slow for large
+** doclists (though it should process from newest/smallest to
+** oldest/largest, so it may not be that bad).  It might be useful to
+** modify things to allow for N-way merging.  This could either be
+** within a segment, with pairwise merges across segments, or across
+** all segments at once.
+*/
+static int loadSegment(fulltext_vtab *v, const char *pData, int nData,
+                       sqlite_int64 iLeavesEnd,
+                       const char *pTerm, int nTerm, int isPrefix,
+                       DataBuffer *out){
+  DataBuffer result;
+  int rc;
+
+  /* Corrupt if segment root can't be valid. */
+  if( pData==NULL || nData<1 ) return SQLITE_CORRUPT_BKPT;
+
+  /* This code should never be called with buffered updates. */
+  assert( v->nPendingData<0 );
+
+  dataBufferInit(&result, 0);
+  rc = loadSegmentInt(v, pData, nData, iLeavesEnd,
+                      pTerm, nTerm, isPrefix, &result);
+  if( rc==SQLITE_OK && result.nData>0 ){
+    if( out->nData==0 ){
+      DataBuffer tmp = *out;
+      *out = result;
+      result = tmp;
+    }else{
+      DataBuffer merged;
+      DLReader readers[2];
+
+      rc = dlrInit(&readers[0], DL_DEFAULT, out->pData, out->nData);
+      if( rc==SQLITE_OK ){
+        rc = dlrInit(&readers[1], DL_DEFAULT, result.pData, result.nData);
+        if( rc==SQLITE_OK ){
+          dataBufferInit(&merged, out->nData+result.nData);
+          rc = docListMerge(&merged, readers, 2);
+          dataBufferDestroy(out);
+          *out = merged;
+          dlrDestroy(&readers[1]);
+        }
+        dlrDestroy(&readers[0]);
+      }
+    }
+  }
+
+  dataBufferDestroy(&result);
+  return rc;
+}
+
+/* Scan the database and merge together the posting lists for the term
+** into *out.
+*/
+static int termSelect(fulltext_vtab *v, int iColumn,
+                      const char *pTerm, int nTerm, int isPrefix,
+                      DocListType iType, DataBuffer *out){
+  DataBuffer doclist;
+  sqlite3_stmt *s;
+  int rc = sql_get_statement(v, SEGDIR_SELECT_ALL_STMT, &s);
+  if( rc!=SQLITE_OK ) return rc;
+
+  /* This code should never be called with buffered updates. */
+  assert( v->nPendingData<0 );
+
+  dataBufferInit(&doclist, 0);
+
+  /* Traverse the segments from oldest to newest so that newer doclist
+  ** elements for given docids overwrite older elements.
+  */
+  while( (rc = sqlite3_step(s))==SQLITE_ROW ){
+    const char *pData = sqlite3_column_blob(s, 2);
+    const int nData = sqlite3_column_bytes(s, 2);
+    const sqlite_int64 iLeavesEnd = sqlite3_column_int64(s, 1);
+
+    /* Corrupt if we get back different types than we stored. */
+    if( sqlite3_column_type(s, 1)!=SQLITE_INTEGER ||
+        sqlite3_column_type(s, 2)!=SQLITE_BLOB ){
+      rc = SQLITE_CORRUPT_BKPT;
+      goto err;
+    }
+
+    rc = loadSegment(v, pData, nData, iLeavesEnd, pTerm, nTerm, isPrefix,
+                     &doclist);
+    if( rc!=SQLITE_OK ) goto err;
+  }
+  if( rc==SQLITE_DONE ){
+    rc = SQLITE_OK;
+    if( doclist.nData!=0 ){
+      /* TODO(shess) The old term_select_all() code applied the column
+      ** restrict as we merged segments, leading to smaller buffers.
+      ** This is probably worthwhile to bring back, once the new storage
+      ** system is checked in.
+      */
+      if( iColumn==v->nColumn) iColumn = -1;
+      rc = docListTrim(DL_DEFAULT, doclist.pData, doclist.nData,
+                       iColumn, iType, out);
+    }
+  }
+
+ err:
+  sqlite3_reset(s);         /* So we don't leave a lock. */
+  dataBufferDestroy(&doclist);
+  return rc;
+}
+
+/****************************************************************/
+/* Used to hold hashtable data for sorting. */
+typedef struct TermData {
+  const char *pTerm;
+  int nTerm;
+  DLCollector *pCollector;
+} TermData;
+
+/* Orders TermData elements in strcmp fashion ( <0 for less-than, 0
+** for equal, >0 for greater-than).
+*/
+static int termDataCmp(const void *av, const void *bv){
+  const TermData *a = (const TermData *)av;
+  const TermData *b = (const TermData *)bv;
+  int n = a->nTerm<b->nTerm ? a->nTerm : b->nTerm;
+  int c = memcmp(a->pTerm, b->pTerm, n);
+  if( c!=0 ) return c;
+  return a->nTerm-b->nTerm;
+}
+
+/* Order pTerms data by term, then write a new level 0 segment using
+** LeafWriter.
+*/
+static int writeZeroSegment(fulltext_vtab *v, fts2Hash *pTerms){
+  fts2HashElem *e;
+  int idx, rc, i, n;
+  TermData *pData;
+  LeafWriter writer;
+  DataBuffer dl;
+
+  /* Determine the next index at level 0, merging as necessary. */
+  rc = segdirNextIndex(v, 0, &idx);
+  if( rc!=SQLITE_OK ) return rc;
+
+  n = fts2HashCount(pTerms);
+  pData = sqlite3_malloc(n*sizeof(TermData));
+
+  for(i = 0, e = fts2HashFirst(pTerms); e; i++, e = fts2HashNext(e)){
+    assert( i<n );
+    pData[i].pTerm = fts2HashKey(e);
+    pData[i].nTerm = fts2HashKeysize(e);
+    pData[i].pCollector = fts2HashData(e);
+  }
+  assert( i==n );
+
+  /* TODO(shess) Should we allow user-defined collation sequences,
+  ** here?  I think we only need that once we support prefix searches.
+  */
+  if( n>1 ) qsort(pData, n, sizeof(*pData), termDataCmp);
+
+  /* TODO(shess) Refactor so that we can write directly to the segment
+  ** DataBuffer, as happens for segment merges.
+  */
+  leafWriterInit(0, idx, &writer);
+  dataBufferInit(&dl, 0);
+  for(i=0; i<n; i++){
+    dataBufferReset(&dl);
+    dlcAddDoclist(pData[i].pCollector, &dl);
+    rc = leafWriterStep(v, &writer,
+                        pData[i].pTerm, pData[i].nTerm, dl.pData, dl.nData);
+    if( rc!=SQLITE_OK ) goto err;
+  }
+  rc = leafWriterFinalize(v, &writer);
+
+ err:
+  dataBufferDestroy(&dl);
+  sqlite3_free(pData);
+  leafWriterDestroy(&writer);
+  return rc;
+}
+
+/* If pendingTerms has data, free it. */
+static int clearPendingTerms(fulltext_vtab *v){
+  if( v->nPendingData>=0 ){
+    fts2HashElem *e;
+    for(e=fts2HashFirst(&v->pendingTerms); e; e=fts2HashNext(e)){
+      dlcDelete(fts2HashData(e));
+    }
+    fts2HashClear(&v->pendingTerms);
+    v->nPendingData = -1;
+  }
+  return SQLITE_OK;
+}
+
+/* If pendingTerms has data, flush it to a level-zero segment, and
+** free it.
+*/
+static int flushPendingTerms(fulltext_vtab *v){
+  if( v->nPendingData>=0 ){
+    int rc = writeZeroSegment(v, &v->pendingTerms);
+    if( rc==SQLITE_OK ) clearPendingTerms(v);
+    return rc;
+  }
+  return SQLITE_OK;
+}
+
+/* If pendingTerms is "too big", or docid is out of order, flush it.
+** Regardless, be certain that pendingTerms is initialized for use.
+*/
+static int initPendingTerms(fulltext_vtab *v, sqlite_int64 iDocid){
+  /* TODO(shess) Explore whether partially flushing the buffer on
+  ** forced-flush would provide better performance.  I suspect that if
+  ** we ordered the doclists by size and flushed the largest until the
+  ** buffer was half empty, that would let the less frequent terms
+  ** generate longer doclists.
+  */
+  if( iDocid<=v->iPrevDocid || v->nPendingData>kPendingThreshold ){
+    int rc = flushPendingTerms(v);
+    if( rc!=SQLITE_OK ) return rc;
+  }
+  if( v->nPendingData<0 ){
+    fts2HashInit(&v->pendingTerms, FTS2_HASH_STRING, 1);
+    v->nPendingData = 0;
+  }
+  v->iPrevDocid = iDocid;
+  return SQLITE_OK;
+}
+
+/* This function implements the xUpdate callback; it is the top-level entry
+ * point for inserting, deleting or updating a row in a full-text table. */
+static int fulltextUpdate(sqlite3_vtab *pVtab, int nArg, sqlite3_value **ppArg,
+                   sqlite_int64 *pRowid){
+  fulltext_vtab *v = (fulltext_vtab *) pVtab;
+  int rc;
+
+  TRACE(("FTS2 Update %p\n", pVtab));
+
+  if( nArg<2 ){
+    rc = index_delete(v, sqlite3_value_int64(ppArg[0]));
+    if( rc==SQLITE_OK ){
+      /* If we just deleted the last row in the table, clear out the
+      ** index data.
+      */
+      rc = content_exists(v);
+      if( rc==SQLITE_ROW ){
+        rc = SQLITE_OK;
+      }else if( rc==SQLITE_DONE ){
+        /* Clear the pending terms so we don't flush a useless level-0
+        ** segment when the transaction closes.
+        */
+        rc = clearPendingTerms(v);
+        if( rc==SQLITE_OK ){
+          rc = segdir_delete_all(v);
+        }
+      }
+    }
+  } else if( sqlite3_value_type(ppArg[0]) != SQLITE_NULL ){
+    /* An update:
+     * ppArg[0] = old rowid
+     * ppArg[1] = new rowid
+     * ppArg[2..2+v->nColumn-1] = values
+     * ppArg[2+v->nColumn] = value for magic column (we ignore this)
+     */
+    sqlite_int64 rowid = sqlite3_value_int64(ppArg[0]);
+    if( sqlite3_value_type(ppArg[1]) != SQLITE_INTEGER ||
+      sqlite3_value_int64(ppArg[1]) != rowid ){
+      rc = SQLITE_ERROR;  /* we don't allow changing the rowid */
+    } else {
+      assert( nArg==2+v->nColumn+1);
+      rc = index_update(v, rowid, &ppArg[2]);
+    }
+  } else {
+    /* An insert:
+     * ppArg[1] = requested rowid
+     * ppArg[2..2+v->nColumn-1] = values
+     * ppArg[2+v->nColumn] = value for magic column (we ignore this)
+     */
+    assert( nArg==2+v->nColumn+1);
+    rc = index_insert(v, ppArg[1], &ppArg[2], pRowid);
+  }
+
+  return rc;
+}
+
+static int fulltextSync(sqlite3_vtab *pVtab){
+  TRACE(("FTS2 xSync()\n"));
+  return flushPendingTerms((fulltext_vtab *)pVtab);
+}
+
+static int fulltextBegin(sqlite3_vtab *pVtab){
+  fulltext_vtab *v = (fulltext_vtab *) pVtab;
+  TRACE(("FTS2 xBegin()\n"));
+
+  /* Any buffered updates should have been cleared by the previous
+  ** transaction.
+  */
+  assert( v->nPendingData<0 );
+  return clearPendingTerms(v);
+}
+
+static int fulltextCommit(sqlite3_vtab *pVtab){
+  fulltext_vtab *v = (fulltext_vtab *) pVtab;
+  TRACE(("FTS2 xCommit()\n"));
+
+  /* Buffered updates should have been cleared by fulltextSync(). */
+  assert( v->nPendingData<0 );
+  return clearPendingTerms(v);
+}
+
+static int fulltextRollback(sqlite3_vtab *pVtab){
+  TRACE(("FTS2 xRollback()\n"));
+  return clearPendingTerms((fulltext_vtab *)pVtab);
+}
+
+/*
+** Implementation of the snippet() function for FTS2
+*/
+static void snippetFunc(
+  sqlite3_context *pContext,
+  int argc,
+  sqlite3_value **argv
+){
+  fulltext_cursor *pCursor;
+  if( argc<1 ) return;
+  if( sqlite3_value_type(argv[0])!=SQLITE_BLOB ||
+      sqlite3_value_bytes(argv[0])!=sizeof(pCursor) ){
+    sqlite3_result_error(pContext, "illegal first argument to html_snippet",-1);
+  }else{
+    const char *zStart = "<b>";
+    const char *zEnd = "</b>";
+    const char *zEllipsis = "<b>...</b>";
+    memcpy(&pCursor, sqlite3_value_blob(argv[0]), sizeof(pCursor));
+    if( argc>=2 ){
+      zStart = (const char*)sqlite3_value_text(argv[1]);
+      if( argc>=3 ){
+        zEnd = (const char*)sqlite3_value_text(argv[2]);
+        if( argc>=4 ){
+          zEllipsis = (const char*)sqlite3_value_text(argv[3]);
+        }
+      }
+    }
+    snippetAllOffsets(pCursor);
+    snippetText(pCursor, zStart, zEnd, zEllipsis);
+    sqlite3_result_text(pContext, pCursor->snippet.zSnippet,
+                        pCursor->snippet.nSnippet, SQLITE_STATIC);
+  }
+}
+
+/*
+** Implementation of the offsets() function for FTS2
+*/
+static void snippetOffsetsFunc(
+  sqlite3_context *pContext,
+  int argc,
+  sqlite3_value **argv
+){
+  fulltext_cursor *pCursor;
+  if( argc<1 ) return;
+  if( sqlite3_value_type(argv[0])!=SQLITE_BLOB ||
+      sqlite3_value_bytes(argv[0])!=sizeof(pCursor) ){
+    sqlite3_result_error(pContext, "illegal first argument to offsets",-1);
+  }else{
+    memcpy(&pCursor, sqlite3_value_blob(argv[0]), sizeof(pCursor));
+    snippetAllOffsets(pCursor);
+    snippetOffsetText(&pCursor->snippet);
+    sqlite3_result_text(pContext,
+                        pCursor->snippet.zOffset, pCursor->snippet.nOffset,
+                        SQLITE_STATIC);
+  }
+}
+
+/* OptLeavesReader is nearly identical to LeavesReader, except that
+** where LeavesReader is geared towards the merging of complete
+** segment levels (with exactly MERGE_COUNT segments), OptLeavesReader
+** is geared towards implementation of the optimize() function, and
+** can merge all segments simultaneously.  This version may be
+** somewhat less efficient than LeavesReader because it merges into an
+** accumulator rather than doing an N-way merge, but since segment
+** size grows exponentially (so segment count logrithmically) this is
+** probably not an immediate problem.
+*/
+/* TODO(shess): Prove that assertion, or extend the merge code to
+** merge tree fashion (like the prefix-searching code does).
+*/
+/* TODO(shess): OptLeavesReader and LeavesReader could probably be
+** merged with little or no loss of performance for LeavesReader.  The
+** merged code would need to handle >MERGE_COUNT segments, and would
+** also need to be able to optionally optimize away deletes.
+*/
+typedef struct OptLeavesReader {
+  /* Segment number, to order readers by age. */
+  int segment;
+  LeavesReader reader;
+} OptLeavesReader;
+
+static int optLeavesReaderAtEnd(OptLeavesReader *pReader){
+  return leavesReaderAtEnd(&pReader->reader);
+}
+static int optLeavesReaderTermBytes(OptLeavesReader *pReader){
+  return leavesReaderTermBytes(&pReader->reader);
+}
+static const char *optLeavesReaderData(OptLeavesReader *pReader){
+  return leavesReaderData(&pReader->reader);
+}
+static int optLeavesReaderDataBytes(OptLeavesReader *pReader){
+  return leavesReaderDataBytes(&pReader->reader);
+}
+static const char *optLeavesReaderTerm(OptLeavesReader *pReader){
+  return leavesReaderTerm(&pReader->reader);
+}
+static int optLeavesReaderStep(fulltext_vtab *v, OptLeavesReader *pReader){
+  return leavesReaderStep(v, &pReader->reader);
+}
+static int optLeavesReaderTermCmp(OptLeavesReader *lr1, OptLeavesReader *lr2){
+  return leavesReaderTermCmp(&lr1->reader, &lr2->reader);
+}
+/* Order by term ascending, segment ascending (oldest to newest), with
+** exhausted readers to the end.
+*/
+static int optLeavesReaderCmp(OptLeavesReader *lr1, OptLeavesReader *lr2){
+  int c = optLeavesReaderTermCmp(lr1, lr2);
+  if( c!=0 ) return c;
+  return lr1->segment-lr2->segment;
+}
+/* Bubble pLr[0] to appropriate place in pLr[1..nLr-1].  Assumes that
+** pLr[1..nLr-1] is already sorted.
+*/
+static void optLeavesReaderReorder(OptLeavesReader *pLr, int nLr){
+  while( nLr>1 && optLeavesReaderCmp(pLr, pLr+1)>0 ){
+    OptLeavesReader tmp = pLr[0];
+    pLr[0] = pLr[1];
+    pLr[1] = tmp;
+    nLr--;
+    pLr++;
+  }
+}
+
+/* optimize() helper function.  Put the readers in order and iterate
+** through them, merging doclists for matching terms into pWriter.
+** Returns SQLITE_OK on success, or the SQLite error code which
+** prevented success.
+*/
+static int optimizeInternal(fulltext_vtab *v,
+                            OptLeavesReader *readers, int nReaders,
+                            LeafWriter *pWriter){
+  int i, rc = SQLITE_OK;
+  DataBuffer doclist, merged, tmp;
+  const char *pData;
+
+  /* Order the readers. */
+  i = nReaders;
+  while( i-- > 0 ){
+    optLeavesReaderReorder(&readers[i], nReaders-i);
+  }
+
+  dataBufferInit(&doclist, LEAF_MAX);
+  dataBufferInit(&merged, LEAF_MAX);
+
+  /* Exhausted readers bubble to the end, so when the first reader is
+  ** at eof, all are at eof.
+  */
+  while( !optLeavesReaderAtEnd(&readers[0]) ){
+
+    /* Figure out how many readers share the next term. */
+    for(i=1; i<nReaders && !optLeavesReaderAtEnd(&readers[i]); i++){
+      if( 0!=optLeavesReaderTermCmp(&readers[0], &readers[i]) ) break;
+    }
+
+    pData = optLeavesReaderData(&readers[0]);
+    if( pData==NULL ){
+      rc = SQLITE_CORRUPT_BKPT;
+      break;
+    }
+
+    /* Special-case for no merge. */
+    if( i==1 ){
+      /* Trim deletions from the doclist. */
+      dataBufferReset(&merged);
+      rc = docListTrim(DL_DEFAULT,
+                       pData,
+                       optLeavesReaderDataBytes(&readers[0]),
+                       -1, DL_DEFAULT, &merged);
+      if( rc!= SQLITE_OK ) break;
+    }else{
+      DLReader dlReaders[MERGE_COUNT];
+      int iReader, nReaders;
+
+      /* Prime the pipeline with the first reader's doclist.  After
+      ** one pass index 0 will reference the accumulated doclist.
+      */
+      rc = dlrInit(&dlReaders[0], DL_DEFAULT,
+                   pData,
+                   optLeavesReaderDataBytes(&readers[0]));
+      if( rc!=SQLITE_OK ) break;
+      iReader = 1;
+
+      assert( iReader<i );  /* Must execute the loop at least once. */
+      while( iReader<i ){
+        /* Merge 16 inputs per pass. */
+        for( nReaders=1; iReader<i && nReaders<MERGE_COUNT;
+             iReader++, nReaders++ ){
+          pData = optLeavesReaderData(&readers[iReader]);
+          if( pData == NULL ){
+            rc = SQLITE_CORRUPT_BKPT;
+            break;
+          }
+          rc = dlrInit(&dlReaders[nReaders], DL_DEFAULT,
+                       pData,
+                       optLeavesReaderDataBytes(&readers[iReader]));
+          if( rc != SQLITE_OK ) break;
+        }
+
+        /* Merge doclists and swap result into accumulator. */
+        if( rc==SQLITE_OK ){
+          dataBufferReset(&merged);
+          rc = docListMerge(&merged, dlReaders, nReaders);
+          tmp = merged;
+          merged = doclist;
+          doclist = tmp;
+        }
+
+        while( nReaders-- > 0 ){
+          dlrDestroy(&dlReaders[nReaders]);
+        }
+
+        if( rc!=SQLITE_OK ) goto err;
+
+        /* Accumulated doclist to reader 0 for next pass. */
+        rc = dlrInit(&dlReaders[0], DL_DEFAULT, doclist.pData, doclist.nData);
+        if( rc!=SQLITE_OK ) goto err;
+      }
+
+      /* Destroy reader that was left in the pipeline. */
+      dlrDestroy(&dlReaders[0]);
+
+      /* Trim deletions from the doclist. */
+      dataBufferReset(&merged);
+      rc = docListTrim(DL_DEFAULT, doclist.pData, doclist.nData,
+                       -1, DL_DEFAULT, &merged);
+      if( rc!=SQLITE_OK ) goto err;
+    }
+
+    /* Only pass doclists with hits (skip if all hits deleted). */
+    if( merged.nData>0 ){
+      rc = leafWriterStep(v, pWriter,
+                          optLeavesReaderTerm(&readers[0]),
+                          optLeavesReaderTermBytes(&readers[0]),
+                          merged.pData, merged.nData);
+      if( rc!=SQLITE_OK ) goto err;
+    }
+
+    /* Step merged readers to next term and reorder. */
+    while( i-- > 0 ){
+      rc = optLeavesReaderStep(v, &readers[i]);
+      if( rc!=SQLITE_OK ) goto err;
+
+      optLeavesReaderReorder(&readers[i], nReaders-i);
+    }
+  }
+
+ err:
+  dataBufferDestroy(&doclist);
+  dataBufferDestroy(&merged);
+  return rc;
+}
+
+/* Implement optimize() function for FTS3.  optimize(t) merges all
+** segments in the fts index into a single segment.  't' is the magic
+** table-named column.
+*/
+static void optimizeFunc(sqlite3_context *pContext,
+                         int argc, sqlite3_value **argv){
+  fulltext_cursor *pCursor;
+  if( argc>1 ){
+    sqlite3_result_error(pContext, "excess arguments to optimize()",-1);
+  }else if( sqlite3_value_type(argv[0])!=SQLITE_BLOB ||
+            sqlite3_value_bytes(argv[0])!=sizeof(pCursor) ){
+    sqlite3_result_error(pContext, "illegal first argument to optimize",-1);
+  }else{
+    fulltext_vtab *v;
+    int i, rc, iMaxLevel;
+    OptLeavesReader *readers;
+    int nReaders;
+    LeafWriter writer;
+    sqlite3_stmt *s;
+
+    memcpy(&pCursor, sqlite3_value_blob(argv[0]), sizeof(pCursor));
+    v = cursor_vtab(pCursor);
+
+    /* Flush any buffered updates before optimizing. */
+    rc = flushPendingTerms(v);
+    if( rc!=SQLITE_OK ) goto err;
+
+    rc = segdir_count(v, &nReaders, &iMaxLevel);
+    if( rc!=SQLITE_OK ) goto err;
+    if( nReaders==0 || nReaders==1 ){
+      sqlite3_result_text(pContext, "Index already optimal", -1,
+                          SQLITE_STATIC);
+      return;
+    }
+
+    rc = sql_get_statement(v, SEGDIR_SELECT_ALL_STMT, &s);
+    if( rc!=SQLITE_OK ) goto err;
+
+    readers = sqlite3_malloc(nReaders*sizeof(readers[0]));
+    if( readers==NULL ) goto err;
+
+    /* Note that there will already be a segment at this position
+    ** until we call segdir_delete() on iMaxLevel.
+    */
+    leafWriterInit(iMaxLevel, 0, &writer);
+
+    i = 0;
+    while( (rc = sqlite3_step(s))==SQLITE_ROW ){
+      sqlite_int64 iStart = sqlite3_column_int64(s, 0);
+      sqlite_int64 iEnd = sqlite3_column_int64(s, 1);
+      const char *pRootData = sqlite3_column_blob(s, 2);
+      int nRootData = sqlite3_column_bytes(s, 2);
+
+      /* Corrupt if we get back different types than we stored. */
+      if( sqlite3_column_type(s, 0)!=SQLITE_INTEGER ||
+          sqlite3_column_type(s, 1)!=SQLITE_INTEGER ||
+          sqlite3_column_type(s, 2)!=SQLITE_BLOB ){
+        rc = SQLITE_CORRUPT_BKPT;
+        break;
+      }
+
+      assert( i<nReaders );
+      rc = leavesReaderInit(v, -1, iStart, iEnd, pRootData, nRootData,
+                            &readers[i].reader);
+      if( rc!=SQLITE_OK ) break;
+
+      readers[i].segment = i;
+      i++;
+    }
+
+    /* If we managed to successfully read them all, optimize them. */
+    if( rc==SQLITE_DONE ){
+      assert( i==nReaders );
+      rc = optimizeInternal(v, readers, nReaders, &writer);
+    }else{
+      sqlite3_reset(s);      /* So we don't leave a lock. */
+    }
+
+    while( i-- > 0 ){
+      leavesReaderDestroy(&readers[i].reader);
+    }
+    sqlite3_free(readers);
+
+    /* If we've successfully gotten to here, delete the old segments
+    ** and flush the interior structure of the new segment.
+    */
+    if( rc==SQLITE_OK ){
+      for( i=0; i<=iMaxLevel; i++ ){
+        rc = segdir_delete(v, i);
+        if( rc!=SQLITE_OK ) break;
+      }
+
+      if( rc==SQLITE_OK ) rc = leafWriterFinalize(v, &writer);
+    }
+
+    leafWriterDestroy(&writer);
+
+    if( rc!=SQLITE_OK ) goto err;
+
+    sqlite3_result_text(pContext, "Index optimized", -1, SQLITE_STATIC);
+    return;
+
+    /* TODO(shess): Error-handling needs to be improved along the
+    ** lines of the dump_ functions.
+    */
+ err:
+    {
+      char buf[512];
+      sqlite3_snprintf(sizeof(buf), buf, "Error in optimize: %s",
+                       sqlite3_errmsg(sqlite3_context_db_handle(pContext)));
+      sqlite3_result_error(pContext, buf, -1);
+    }
+  }
+}
+
+#ifdef SQLITE_TEST
+/* Generate an error of the form "<prefix>: <msg>".  If msg is NULL,
+** pull the error from the context's db handle.
+*/
+static void generateError(sqlite3_context *pContext,
+                          const char *prefix, const char *msg){
+  char buf[512];
+  if( msg==NULL ) msg = sqlite3_errmsg(sqlite3_context_db_handle(pContext));
+  sqlite3_snprintf(sizeof(buf), buf, "%s: %s", prefix, msg);
+  sqlite3_result_error(pContext, buf, -1);
+}
+
+/* Helper function to collect the set of terms in the segment into
+** pTerms.  The segment is defined by the leaf nodes between
+** iStartBlockid and iEndBlockid, inclusive, or by the contents of
+** pRootData if iStartBlockid is 0 (in which case the entire segment
+** fit in a leaf).
+*/
+static int collectSegmentTerms(fulltext_vtab *v, sqlite3_stmt *s,
+                               fts2Hash *pTerms){
+  const sqlite_int64 iStartBlockid = sqlite3_column_int64(s, 0);
+  const sqlite_int64 iEndBlockid = sqlite3_column_int64(s, 1);
+  const char *pRootData = sqlite3_column_blob(s, 2);
+  const int nRootData = sqlite3_column_bytes(s, 2);
+  int rc;
+  LeavesReader reader;
+
+  /* Corrupt if we get back different types than we stored. */
+  if( sqlite3_column_type(s, 0)!=SQLITE_INTEGER ||
+      sqlite3_column_type(s, 1)!=SQLITE_INTEGER ||
+      sqlite3_column_type(s, 2)!=SQLITE_BLOB ){
+    return SQLITE_CORRUPT_BKPT;
+  }
+
+  rc = leavesReaderInit(v, 0, iStartBlockid, iEndBlockid,
+                        pRootData, nRootData, &reader);
+  if( rc!=SQLITE_OK ) return rc;
+
+  while( rc==SQLITE_OK && !leavesReaderAtEnd(&reader) ){
+    const char *pTerm = leavesReaderTerm(&reader);
+    const int nTerm = leavesReaderTermBytes(&reader);
+    void *oldValue = sqlite3Fts2HashFind(pTerms, pTerm, nTerm);
+    void *newValue = (void *)((char *)oldValue+1);
+
+    /* From the comment before sqlite3Fts2HashInsert in fts2_hash.c,
+    ** the data value passed is returned in case of malloc failure.
+    */
+    if( newValue==sqlite3Fts2HashInsert(pTerms, pTerm, nTerm, newValue) ){
+      rc = SQLITE_NOMEM;
+    }else{
+      rc = leavesReaderStep(v, &reader);
+    }
+  }
+
+  leavesReaderDestroy(&reader);
+  return rc;
+}
+
+/* Helper function to build the result string for dump_terms(). */
+static int generateTermsResult(sqlite3_context *pContext, fts2Hash *pTerms){
+  int iTerm, nTerms, nResultBytes, iByte;
+  char *result;
+  TermData *pData;
+  fts2HashElem *e;
+
+  /* Iterate pTerms to generate an array of terms in pData for
+  ** sorting.
+  */
+  nTerms = fts2HashCount(pTerms);
+  assert( nTerms>0 );
+  pData = sqlite3_malloc(nTerms*sizeof(TermData));
+  if( pData==NULL ) return SQLITE_NOMEM;
+
+  nResultBytes = 0;
+  for(iTerm = 0, e = fts2HashFirst(pTerms); e; iTerm++, e = fts2HashNext(e)){
+    nResultBytes += fts2HashKeysize(e)+1;   /* Term plus trailing space */
+    assert( iTerm<nTerms );
+    pData[iTerm].pTerm = fts2HashKey(e);
+    pData[iTerm].nTerm = fts2HashKeysize(e);
+    pData[iTerm].pCollector = fts2HashData(e);  /* unused */
+  }
+  assert( iTerm==nTerms );
+
+  assert( nResultBytes>0 );   /* nTerms>0, nResultsBytes must be, too. */
+  result = sqlite3_malloc(nResultBytes);
+  if( result==NULL ){
+    sqlite3_free(pData);
+    return SQLITE_NOMEM;
+  }
+
+  if( nTerms>1 ) qsort(pData, nTerms, sizeof(*pData), termDataCmp);
+
+  /* Read the terms in order to build the result. */
+  iByte = 0;
+  for(iTerm=0; iTerm<nTerms; ++iTerm){
+    memcpy(result+iByte, pData[iTerm].pTerm, pData[iTerm].nTerm);
+    iByte += pData[iTerm].nTerm;
+    result[iByte++] = ' ';
+  }
+  assert( iByte==nResultBytes );
+  assert( result[nResultBytes-1]==' ' );
+  result[nResultBytes-1] = '\0';
+
+  /* Passes away ownership of result. */
+  sqlite3_result_text(pContext, result, nResultBytes-1, sqlite3_free);
+  sqlite3_free(pData);
+  return SQLITE_OK;
+}
+
+/* Implements dump_terms() for use in inspecting the fts2 index from
+** tests.  TEXT result containing the ordered list of terms joined by
+** spaces.  dump_terms(t, level, idx) dumps the terms for the segment
+** specified by level, idx (in %_segdir), while dump_terms(t) dumps
+** all terms in the index.  In both cases t is the fts table's magic
+** table-named column.
+*/
+static void dumpTermsFunc(
+  sqlite3_context *pContext,
+  int argc, sqlite3_value **argv
+){
+  fulltext_cursor *pCursor;
+  if( argc!=3 && argc!=1 ){
+    generateError(pContext, "dump_terms", "incorrect arguments");
+  }else if( sqlite3_value_type(argv[0])!=SQLITE_BLOB ||
+            sqlite3_value_bytes(argv[0])!=sizeof(pCursor) ){
+    generateError(pContext, "dump_terms", "illegal first argument");
+  }else{
+    fulltext_vtab *v;
+    fts2Hash terms;
+    sqlite3_stmt *s = NULL;
+    int rc;
+
+    memcpy(&pCursor, sqlite3_value_blob(argv[0]), sizeof(pCursor));
+    v = cursor_vtab(pCursor);
+
+    /* If passed only the cursor column, get all segments.  Otherwise
+    ** get the segment described by the following two arguments.
+    */
+    if( argc==1 ){
+      rc = sql_get_statement(v, SEGDIR_SELECT_ALL_STMT, &s);
+    }else{
+      rc = sql_get_statement(v, SEGDIR_SELECT_SEGMENT_STMT, &s);
+      if( rc==SQLITE_OK ){
+        rc = sqlite3_bind_int(s, 1, sqlite3_value_int(argv[1]));
+        if( rc==SQLITE_OK ){
+          rc = sqlite3_bind_int(s, 2, sqlite3_value_int(argv[2]));
+        }
+      }
+    }
+
+    if( rc!=SQLITE_OK ){
+      generateError(pContext, "dump_terms", NULL);
+      return;
+    }
+
+    /* Collect the terms for each segment. */
+    sqlite3Fts2HashInit(&terms, FTS2_HASH_STRING, 1);
+    while( (rc = sqlite3_step(s))==SQLITE_ROW ){
+      rc = collectSegmentTerms(v, s, &terms);
+      if( rc!=SQLITE_OK ) break;
+    }
+
+    if( rc!=SQLITE_DONE ){
+      sqlite3_reset(s);
+      generateError(pContext, "dump_terms", NULL);
+    }else{
+      const int nTerms = fts2HashCount(&terms);
+      if( nTerms>0 ){
+        rc = generateTermsResult(pContext, &terms);
+        if( rc==SQLITE_NOMEM ){
+          generateError(pContext, "dump_terms", "out of memory");
+        }else{
+          assert( rc==SQLITE_OK );
+        }
+      }else if( argc==3 ){
+        /* The specific segment asked for could not be found. */
+        generateError(pContext, "dump_terms", "segment not found");
+      }else{
+        /* No segments found. */
+        /* TODO(shess): It should be impossible to reach this.  This
+        ** case can only happen for an empty table, in which case
+        ** SQLite has no rows to call this function on.
+        */
+        sqlite3_result_null(pContext);
+      }
+    }
+    sqlite3Fts2HashClear(&terms);
+  }
+}
+
+/* Expand the DL_DEFAULT doclist in pData into a text result in
+** pContext.
+*/
+static void createDoclistResult(sqlite3_context *pContext,
+                                const char *pData, int nData){
+  DataBuffer dump;
+  DLReader dlReader;
+  int rc;
+
+  assert( pData!=NULL && nData>0 );
+
+  rc = dlrInit(&dlReader, DL_DEFAULT, pData, nData);
+  if( rc!=SQLITE_OK ) return rc;
+  dataBufferInit(&dump, 0);
+  for( ; rc==SQLITE_OK && !dlrAtEnd(&dlReader); rc = dlrStep(&dlReader) ){
+    char buf[256];
+    PLReader plReader;
+
+    rc = plrInit(&plReader, &dlReader);
+    if( rc!=SQLITE_OK ) break;
+    if( DL_DEFAULT==DL_DOCIDS || plrAtEnd(&plReader) ){
+      sqlite3_snprintf(sizeof(buf), buf, "[%lld] ", dlrDocid(&dlReader));
+      dataBufferAppend(&dump, buf, strlen(buf));
+    }else{
+      int iColumn = plrColumn(&plReader);
+
+      sqlite3_snprintf(sizeof(buf), buf, "[%lld %d[",
+                       dlrDocid(&dlReader), iColumn);
+      dataBufferAppend(&dump, buf, strlen(buf));
+
+      for( ; !plrAtEnd(&plReader); rc = plrStep(&plReader) ){
+        if( rc!=SQLITE_OK ) break;
+        if( plrColumn(&plReader)!=iColumn ){
+          iColumn = plrColumn(&plReader);
+          sqlite3_snprintf(sizeof(buf), buf, "] %d[", iColumn);
+          assert( dump.nData>0 );
+          dump.nData--;                     /* Overwrite trailing space. */
+          assert( dump.pData[dump.nData]==' ');
+          dataBufferAppend(&dump, buf, strlen(buf));
+        }
+        if( DL_DEFAULT==DL_POSITIONS_OFFSETS ){
+          sqlite3_snprintf(sizeof(buf), buf, "%d,%d,%d ",
+                           plrPosition(&plReader),
+                           plrStartOffset(&plReader), plrEndOffset(&plReader));
+        }else if( DL_DEFAULT==DL_POSITIONS ){
+          sqlite3_snprintf(sizeof(buf), buf, "%d ", plrPosition(&plReader));
+        }else{
+          assert( NULL=="Unhandled DL_DEFAULT value");
+        }
+        dataBufferAppend(&dump, buf, strlen(buf));
+      }
+      plrDestroy(&plReader);
+      if( rc!= SQLITE_OK ) break;
+
+      assert( dump.nData>0 );
+      dump.nData--;                     /* Overwrite trailing space. */
+      assert( dump.pData[dump.nData]==' ');
+      dataBufferAppend(&dump, "]] ", 3);
+    }
+  }
+  dlrDestroy(&dlReader);
+  if( rc!=SQLITE_OK ){
+    dataBufferDestroy(&dump);
+    return rc;
+  }
+
+  assert( dump.nData>0 );
+  dump.nData--;                     /* Overwrite trailing space. */
+  assert( dump.pData[dump.nData]==' ');
+  dump.pData[dump.nData] = '\0';
+  assert( dump.nData>0 );
+
+  /* Passes ownership of dump's buffer to pContext. */
+  sqlite3_result_text(pContext, dump.pData, dump.nData, sqlite3_free);
+  dump.pData = NULL;
+  dump.nData = dump.nCapacity = 0;
+  return SQLITE_OK;
+}
+
+/* Implements dump_doclist() for use in inspecting the fts2 index from
+** tests.  TEXT result containing a string representation of the
+** doclist for the indicated term.  dump_doclist(t, term, level, idx)
+** dumps the doclist for term from the segment specified by level, idx
+** (in %_segdir), while dump_doclist(t, term) dumps the logical
+** doclist for the term across all segments.  The per-segment doclist
+** can contain deletions, while the full-index doclist will not
+** (deletions are omitted).
+**
+** Result formats differ with the setting of DL_DEFAULTS.  Examples:
+**
+** DL_DOCIDS: [1] [3] [7]
+** DL_POSITIONS: [1 0[0 4] 1[17]] [3 1[5]]
+** DL_POSITIONS_OFFSETS: [1 0[0,0,3 4,23,26] 1[17,102,105]] [3 1[5,20,23]]
+**
+** In each case the number after the outer '[' is the docid.  In the
+** latter two cases, the number before the inner '[' is the column
+** associated with the values within.  For DL_POSITIONS the numbers
+** within are the positions, for DL_POSITIONS_OFFSETS they are the
+** position, the start offset, and the end offset.
+*/
+static void dumpDoclistFunc(
+  sqlite3_context *pContext,
+  int argc, sqlite3_value **argv
+){
+  fulltext_cursor *pCursor;
+  if( argc!=2 && argc!=4 ){
+    generateError(pContext, "dump_doclist", "incorrect arguments");
+  }else if( sqlite3_value_type(argv[0])!=SQLITE_BLOB ||
+            sqlite3_value_bytes(argv[0])!=sizeof(pCursor) ){
+    generateError(pContext, "dump_doclist", "illegal first argument");
+  }else if( sqlite3_value_text(argv[1])==NULL ||
+            sqlite3_value_text(argv[1])[0]=='\0' ){
+    generateError(pContext, "dump_doclist", "empty second argument");
+  }else{
+    const char *pTerm = (const char *)sqlite3_value_text(argv[1]);
+    const int nTerm = strlen(pTerm);
+    fulltext_vtab *v;
+    int rc;
+    DataBuffer doclist;
+
+    memcpy(&pCursor, sqlite3_value_blob(argv[0]), sizeof(pCursor));
+    v = cursor_vtab(pCursor);
+
+    dataBufferInit(&doclist, 0);
+
+    /* termSelect() yields the same logical doclist that queries are
+    ** run against.
+    */
+    if( argc==2 ){
+      rc = termSelect(v, v->nColumn, pTerm, nTerm, 0, DL_DEFAULT, &doclist);
+    }else{
+      sqlite3_stmt *s = NULL;
+
+      /* Get our specific segment's information. */
+      rc = sql_get_statement(v, SEGDIR_SELECT_SEGMENT_STMT, &s);
+      if( rc==SQLITE_OK ){
+        rc = sqlite3_bind_int(s, 1, sqlite3_value_int(argv[2]));
+        if( rc==SQLITE_OK ){
+          rc = sqlite3_bind_int(s, 2, sqlite3_value_int(argv[3]));
+        }
+      }
+
+      if( rc==SQLITE_OK ){
+        rc = sqlite3_step(s);
+
+        if( rc==SQLITE_DONE ){
+          dataBufferDestroy(&doclist);
+          generateError(pContext, "dump_doclist", "segment not found");
+          return;
+        }
+
+        /* Found a segment, load it into doclist. */
+        if( rc==SQLITE_ROW ){
+          const sqlite_int64 iLeavesEnd = sqlite3_column_int64(s, 1);
+          const char *pData = sqlite3_column_blob(s, 2);
+          const int nData = sqlite3_column_bytes(s, 2);
+
+          /* loadSegment() is used by termSelect() to load each
+          ** segment's data.
+          */
+          rc = loadSegment(v, pData, nData, iLeavesEnd, pTerm, nTerm, 0,
+                           &doclist);
+          if( rc==SQLITE_OK ){
+            rc = sqlite3_step(s);
+
+            /* Should not have more than one matching segment. */
+            if( rc!=SQLITE_DONE ){
+              sqlite3_reset(s);
+              dataBufferDestroy(&doclist);
+              generateError(pContext, "dump_doclist", "invalid segdir");
+              return;
+            }
+            rc = SQLITE_OK;
+          }
+        }
+      }
+
+      sqlite3_reset(s);
+    }
+
+    if( rc==SQLITE_OK ){
+      if( doclist.nData>0 ){
+        createDoclistResult(pContext, doclist.pData, doclist.nData);
+      }else{
+        /* TODO(shess): This can happen if the term is not present, or
+        ** if all instances of the term have been deleted and this is
+        ** an all-index dump.  It may be interesting to distinguish
+        ** these cases.
+        */
+        sqlite3_result_text(pContext, "", 0, SQLITE_STATIC);
+      }
+    }else if( rc==SQLITE_NOMEM ){
+      /* Handle out-of-memory cases specially because if they are
+      ** generated in fts2 code they may not be reflected in the db
+      ** handle.
+      */
+      /* TODO(shess): Handle this more comprehensively.
+      ** sqlite3ErrStr() has what I need, but is internal.
+      */
+      generateError(pContext, "dump_doclist", "out of memory");
+    }else{
+      generateError(pContext, "dump_doclist", NULL);
+    }
+
+    dataBufferDestroy(&doclist);
+  }
+}
+#endif
+
+/*
+** This routine implements the xFindFunction method for the FTS2
+** virtual table.
+*/
+static int fulltextFindFunction(
+  sqlite3_vtab *pVtab,
+  int nArg,
+  const char *zName,
+  void (**pxFunc)(sqlite3_context*,int,sqlite3_value**),
+  void **ppArg
+){
+  if( strcmp(zName,"snippet")==0 ){
+    *pxFunc = snippetFunc;
+    return 1;
+  }else if( strcmp(zName,"offsets")==0 ){
+    *pxFunc = snippetOffsetsFunc;
+    return 1;
+  }else if( strcmp(zName,"optimize")==0 ){
+    *pxFunc = optimizeFunc;
+    return 1;
+#ifdef SQLITE_TEST
+    /* NOTE(shess): These functions are present only for testing
+    ** purposes.  No particular effort is made to optimize their
+    ** execution or how they build their results.
+    */
+  }else if( strcmp(zName,"dump_terms")==0 ){
+    /* fprintf(stderr, "Found dump_terms\n"); */
+    *pxFunc = dumpTermsFunc;
+    return 1;
+  }else if( strcmp(zName,"dump_doclist")==0 ){
+    /* fprintf(stderr, "Found dump_doclist\n"); */
+    *pxFunc = dumpDoclistFunc;
+    return 1;
+#endif
+  }
+  return 0;
+}
+
+/*
+** Rename an fts2 table.
+*/
+static int fulltextRename(
+  sqlite3_vtab *pVtab,
+  const char *zName
+){
+  fulltext_vtab *p = (fulltext_vtab *)pVtab;
+  int rc = SQLITE_NOMEM;
+  char *zSql = sqlite3_mprintf(
+    "ALTER TABLE %Q.'%q_content'  RENAME TO '%q_content';"
+    "ALTER TABLE %Q.'%q_segments' RENAME TO '%q_segments';"
+    "ALTER TABLE %Q.'%q_segdir'   RENAME TO '%q_segdir';"
+    , p->zDb, p->zName, zName 
+    , p->zDb, p->zName, zName 
+    , p->zDb, p->zName, zName
+  );
+  if( zSql ){
+    rc = sqlite3_exec(p->db, zSql, 0, 0, 0);
+    sqlite3_free(zSql);
+  }
+  return rc;
+}
+
+static const sqlite3_module fts2Module = {
+  /* iVersion      */ 0,
+  /* xCreate       */ fulltextCreate,
+  /* xConnect      */ fulltextConnect,
+  /* xBestIndex    */ fulltextBestIndex,
+  /* xDisconnect   */ fulltextDisconnect,
+  /* xDestroy      */ fulltextDestroy,
+  /* xOpen         */ fulltextOpen,
+  /* xClose        */ fulltextClose,
+  /* xFilter       */ fulltextFilter,
+  /* xNext         */ fulltextNext,
+  /* xEof          */ fulltextEof,
+  /* xColumn       */ fulltextColumn,
+  /* xRowid        */ fulltextRowid,
+  /* xUpdate       */ fulltextUpdate,
+  /* xBegin        */ fulltextBegin,
+  /* xSync         */ fulltextSync,
+  /* xCommit       */ fulltextCommit,
+  /* xRollback     */ fulltextRollback,
+  /* xFindFunction */ fulltextFindFunction,
+  /* xRename */       fulltextRename,
+};
+
+static void hashDestroy(void *p){
+  fts2Hash *pHash = (fts2Hash *)p;
+  sqlite3Fts2HashClear(pHash);
+  sqlite3_free(pHash);
+}
+
+/*
+** The fts2 built-in tokenizers - "simple" and "porter" - are implemented
+** in files fts2_tokenizer1.c and fts2_porter.c respectively. The following
+** two forward declarations are for functions declared in these files
+** used to retrieve the respective implementations.
+**
+** Calling sqlite3Fts2SimpleTokenizerModule() sets the value pointed
+** to by the argument to point a the "simple" tokenizer implementation.
+** Function ...PorterTokenizerModule() sets *pModule to point to the
+** porter tokenizer/stemmer implementation.
+*/
+void sqlite3Fts2SimpleTokenizerModule(sqlite3_tokenizer_module const**ppModule);
+void sqlite3Fts2PorterTokenizerModule(sqlite3_tokenizer_module const**ppModule);
+void sqlite3Fts2IcuTokenizerModule(sqlite3_tokenizer_module const**ppModule);
+
+int sqlite3Fts2InitHashTable(sqlite3 *, fts2Hash *, const char *);
+
+/*
+** Initialise the fts2 extension. If this extension is built as part
+** of the sqlite library, then this function is called directly by
+** SQLite. If fts2 is built as a dynamically loadable extension, this
+** function is called by the sqlite3_extension_init() entry point.
+*/
+int sqlite3Fts2Init(sqlite3 *db){
+  int rc = SQLITE_OK;
+  fts2Hash *pHash = 0;
+  const sqlite3_tokenizer_module *pSimple = 0;
+  const sqlite3_tokenizer_module *pPorter = 0;
+  const sqlite3_tokenizer_module *pIcu = 0;
+
+  sqlite3Fts2SimpleTokenizerModule(&pSimple);
+  sqlite3Fts2PorterTokenizerModule(&pPorter);
+#ifdef SQLITE_ENABLE_ICU
+  sqlite3Fts2IcuTokenizerModule(&pIcu);
+#endif
+
+  /* Allocate and initialise the hash-table used to store tokenizers. */
+  pHash = sqlite3_malloc(sizeof(fts2Hash));
+  if( !pHash ){
+    rc = SQLITE_NOMEM;
+  }else{
+    sqlite3Fts2HashInit(pHash, FTS2_HASH_STRING, 1);
+  }
+
+  /* Load the built-in tokenizers into the hash table */
+  if( rc==SQLITE_OK ){
+    if( sqlite3Fts2HashInsert(pHash, "simple", 7, (void *)pSimple)
+     || sqlite3Fts2HashInsert(pHash, "porter", 7, (void *)pPorter) 
+     || (pIcu && sqlite3Fts2HashInsert(pHash, "icu", 4, (void *)pIcu))
+    ){
+      rc = SQLITE_NOMEM;
+    }
+  }
+
+  /* Create the virtual table wrapper around the hash-table and overload 
+  ** the two scalar functions. If this is successful, register the
+  ** module with sqlite.
+  */
+  if( SQLITE_OK==rc 
+#if GEARS_FTS2_CHANGES && !SQLITE_TEST
+      /* fts2_tokenizer() disabled for security reasons. */
+#else
+   && SQLITE_OK==(rc = sqlite3Fts2InitHashTable(db, pHash, "fts2_tokenizer"))
+#endif
+   && SQLITE_OK==(rc = sqlite3_overload_function(db, "snippet", -1))
+   && SQLITE_OK==(rc = sqlite3_overload_function(db, "offsets", -1))
+   && SQLITE_OK==(rc = sqlite3_overload_function(db, "optimize", -1))
+#ifdef SQLITE_TEST
+   && SQLITE_OK==(rc = sqlite3_overload_function(db, "dump_terms", -1))
+   && SQLITE_OK==(rc = sqlite3_overload_function(db, "dump_doclist", -1))
+#endif
+  ){
+    return sqlite3_create_module_v2(
+        db, "fts2", &fts2Module, (void *)pHash, hashDestroy
+    );
+  }
+
+  /* An error has occurred. Delete the hash table and return the error code. */
+  assert( rc!=SQLITE_OK );
+  if( pHash ){
+    sqlite3Fts2HashClear(pHash);
+    sqlite3_free(pHash);
+  }
+  return rc;
+}
+
+#if !SQLITE_CORE
+int sqlite3_extension_init(
+  sqlite3 *db, 
+  char **pzErrMsg,
+  const sqlite3_api_routines *pApi
+){
+  SQLITE_EXTENSION_INIT2(pApi)
+  return sqlite3Fts2Init(db);
+}
+#endif
+
+#endif /* !defined(SQLITE_CORE) || defined(SQLITE_ENABLE_FTS2) */