Update ICU to 54.1 step 1

source/i18n/collationiterator.cpp

+/*
+*******************************************************************************
+* Copyright (C) 2010-2014, International Business Machines
+* Corporation and others.  All Rights Reserved.
+*******************************************************************************
+* collationiterator.cpp
+*
+* created on: 2010oct27
+* created by: Markus W. Scherer
+*/
+
+#include "utypeinfo.h"  // for 'typeid' to work
+
+#include "unicode/utypes.h"
+
+#if !UCONFIG_NO_COLLATION
+
+#include "unicode/ucharstrie.h"
+#include "unicode/ustringtrie.h"
+#include "charstr.h"
+#include "cmemory.h"
+#include "collation.h"
+#include "collationdata.h"
+#include "collationfcd.h"
+#include "collationiterator.h"
+#include "normalizer2impl.h"
+#include "uassert.h"
+#include "uvectr32.h"
+
+U_NAMESPACE_BEGIN
+
+CollationIterator::CEBuffer::~CEBuffer() {}
+
+UBool
+CollationIterator::CEBuffer::ensureAppendCapacity(int32_t appCap, UErrorCode &errorCode) {
+    int32_t capacity = buffer.getCapacity();
+    if((length + appCap) <= capacity) { return TRUE; }
+    if(U_FAILURE(errorCode)) { return FALSE; }
+    do {
+        if(capacity < 1000) {
+            capacity *= 4;
+        } else {
+            capacity *= 2;
+        }
+    } while(capacity < (length + appCap));
+    int64_t *p = buffer.resize(capacity, length);
+    if(p == NULL) {
+        errorCode = U_MEMORY_ALLOCATION_ERROR;
+        return FALSE;
+    }
+    return TRUE;
+}
+
+// State of combining marks skipped in discontiguous contraction.
+// We create a state object on first use and keep it around deactivated between uses.
+class SkippedState : public UMemory {
+public:
+    // Born active but empty.
+    SkippedState() : pos(0), skipLengthAtMatch(0) {}
+    void clear() {
+        oldBuffer.remove();
+        pos = 0;
+        // The newBuffer is reset by setFirstSkipped().
+    }
+
+    UBool isEmpty() const { return oldBuffer.isEmpty(); }
+
+    UBool hasNext() const { return pos < oldBuffer.length(); }
+
+    // Requires hasNext().
+    UChar32 next() {
+        UChar32 c = oldBuffer.char32At(pos);
+        pos += U16_LENGTH(c);
+        return c;
+    }
+
+    // Accounts for one more input code point read beyond the end of the marks buffer.
+    void incBeyond() {
+        U_ASSERT(!hasNext());
+        ++pos;
+    }
+
+    // Goes backward through the skipped-marks buffer.
+    // Returns the number of code points read beyond the skipped marks
+    // that need to be backtracked through normal input.
+    int32_t backwardNumCodePoints(int32_t n) {
+        int32_t length = oldBuffer.length();
+        int32_t beyond = pos - length;
+        if(beyond > 0) {
+            if(beyond >= n) {
+                // Not back far enough to re-enter the oldBuffer.
+                pos -= n;
+                return n;
+            } else {
+                // Back out all beyond-oldBuffer code points and re-enter the buffer.
+                pos = oldBuffer.moveIndex32(length, beyond - n);
+                return beyond;
+            }
+        } else {
+            // Go backwards from inside the oldBuffer.
+            pos = oldBuffer.moveIndex32(pos, -n);
+            return 0;
+        }
+    }
+
+    void setFirstSkipped(UChar32 c) {
+        skipLengthAtMatch = 0;
+        newBuffer.setTo(c);
+    }
+
+    void skip(UChar32 c) {
+        newBuffer.append(c);
+    }
+
+    void recordMatch() { skipLengthAtMatch = newBuffer.length(); }
+
+    // Replaces the characters we consumed with the newly skipped ones.
+    void replaceMatch() {
+        // Note: UnicodeString.replace() pins pos to at most length().
+        oldBuffer.replace(0, pos, newBuffer, 0, skipLengthAtMatch);
+        pos = 0;
+    }
+
+    void saveTrieState(const UCharsTrie &trie) { trie.saveState(state); }
+    void resetToTrieState(UCharsTrie &trie) const { trie.resetToState(state); }
+
+private:
+    // Combining marks skipped in previous discontiguous-contraction matching.
+    // After that discontiguous contraction was completed, we start reading them from here.
+    UnicodeString oldBuffer;
+    // Combining marks newly skipped in current discontiguous-contraction matching.
+    // These might have been read from the normal text or from the oldBuffer.
+    UnicodeString newBuffer;
+    // Reading index in oldBuffer,
+    // or counter for how many code points have been read beyond oldBuffer (pos-oldBuffer.length()).
+    int32_t pos;
+    // newBuffer.length() at the time of the last matching character.
+    // When a partial match fails, we back out skipped and partial-matching input characters.
+    int32_t skipLengthAtMatch;
+    // We save the trie state before we attempt to match a character,
+    // so that we can skip it and try the next one.
+    UCharsTrie::State state;
+};
+
+CollationIterator::CollationIterator(const CollationIterator &other)
+        : UObject(other),
+          trie(other.trie),
+          data(other.data),
+          cesIndex(other.cesIndex),
+          skipped(NULL),
+          numCpFwd(other.numCpFwd),
+          isNumeric(other.isNumeric) {
+    UErrorCode errorCode = U_ZERO_ERROR;
+    int32_t length = other.ceBuffer.length;
+    if(length > 0 && ceBuffer.ensureAppendCapacity(length, errorCode)) {
+        for(int32_t i = 0; i < length; ++i) {
+            ceBuffer.set(i, other.ceBuffer.get(i));
+        }
+        ceBuffer.length = length;
+    } else {
+        cesIndex = 0;
+    }
+}
+
+CollationIterator::~CollationIterator() {
+    delete skipped;
+}
+
+UBool
+CollationIterator::operator==(const CollationIterator &other) const {
+    // Subclasses: Call this method and then add more specific checks.
+    // Compare the iterator state but not the collation data (trie & data fields):
+    // Assume that the caller compares the data.
+    // Ignore skipped since that should be unused between calls to nextCE().
+    // (It only stays around to avoid another memory allocation.)
+    if(!(typeid(*this) == typeid(other) &&
+            ceBuffer.length == other.ceBuffer.length &&
+            cesIndex == other.cesIndex &&
+            numCpFwd == other.numCpFwd &&
+            isNumeric == other.isNumeric)) {
+        return FALSE;
+    }
+    for(int32_t i = 0; i < ceBuffer.length; ++i) {
+        if(ceBuffer.get(i) != other.ceBuffer.get(i)) { return FALSE; }
+    }
+    return TRUE;
+}
+
+void
+CollationIterator::reset() {
+    cesIndex = ceBuffer.length = 0;
+    if(skipped != NULL) { skipped->clear(); }
+}
+
+int32_t
+CollationIterator::fetchCEs(UErrorCode &errorCode) {
+    while(U_SUCCESS(errorCode) && nextCE(errorCode) != Collation::NO_CE) {
+        // No need to loop for each expansion CE.
+        cesIndex = ceBuffer.length;
+    }
+    return ceBuffer.length;
+}
+
+uint32_t
+CollationIterator::handleNextCE32(UChar32 &c, UErrorCode &errorCode) {
+    c = nextCodePoint(errorCode);
+    return (c < 0) ? Collation::FALLBACK_CE32 : data->getCE32(c);
+}
+
+UChar
+CollationIterator::handleGetTrailSurrogate() {
+    return 0;
+}
+
+UBool
+CollationIterator::foundNULTerminator() {
+    return FALSE;
+}
+
+UBool
+CollationIterator::forbidSurrogateCodePoints() const {
+    return FALSE;
+}
+
+uint32_t
+CollationIterator::getDataCE32(UChar32 c) const {
+    return data->getCE32(c);
+}
+
+uint32_t
+CollationIterator::getCE32FromBuilderData(uint32_t /*ce32*/, UErrorCode &errorCode) {
+    if(U_SUCCESS(errorCode)) { errorCode = U_INTERNAL_PROGRAM_ERROR; }
+    return 0;
+}
+
+int64_t
+CollationIterator::nextCEFromCE32(const CollationData *d, UChar32 c, uint32_t ce32,
+                                  UErrorCode &errorCode) {
+    --ceBuffer.length;  // Undo ceBuffer.incLength().
+    appendCEsFromCE32(d, c, ce32, TRUE, errorCode);
+    if(U_SUCCESS(errorCode)) {
+        return ceBuffer.get(cesIndex++);
+    } else {
+        return Collation::NO_CE_PRIMARY;
+    }
+}
+
+void
+CollationIterator::appendCEsFromCE32(const CollationData *d, UChar32 c, uint32_t ce32,
+                                     UBool forward, UErrorCode &errorCode) {
+    while(Collation::isSpecialCE32(ce32)) {
+        switch(Collation::tagFromCE32(ce32)) {
+        case Collation::FALLBACK_TAG:
+        case Collation::RESERVED_TAG_3:
+            if(U_SUCCESS(errorCode)) { errorCode = U_INTERNAL_PROGRAM_ERROR; }
+            return;
+        case Collation::LONG_PRIMARY_TAG:
+            ceBuffer.append(Collation::ceFromLongPrimaryCE32(ce32), errorCode);
+            return;
+        case Collation::LONG_SECONDARY_TAG:
+            ceBuffer.append(Collation::ceFromLongSecondaryCE32(ce32), errorCode);
+            return;
+        case Collation::LATIN_EXPANSION_TAG:
+            if(ceBuffer.ensureAppendCapacity(2, errorCode)) {
+                ceBuffer.set(ceBuffer.length, Collation::latinCE0FromCE32(ce32));
+                ceBuffer.set(ceBuffer.length + 1, Collation::latinCE1FromCE32(ce32));
+                ceBuffer.length += 2;
+            }
+            return;
+        case Collation::EXPANSION32_TAG: {
+            const uint32_t *ce32s = d->ce32s + Collation::indexFromCE32(ce32);
+            int32_t length = Collation::lengthFromCE32(ce32);
+            if(ceBuffer.ensureAppendCapacity(length, errorCode)) {
+                do {
+                    ceBuffer.appendUnsafe(Collation::ceFromCE32(*ce32s++));
+                } while(--length > 0);
+            }
+            return;
+        }
+        case Collation::EXPANSION_TAG: {
+            const int64_t *ces = d->ces + Collation::indexFromCE32(ce32);
+            int32_t length = Collation::lengthFromCE32(ce32);
+            if(ceBuffer.ensureAppendCapacity(length, errorCode)) {
+                do {
+                    ceBuffer.appendUnsafe(*ces++);
+                } while(--length > 0);
+            }
+            return;
+        }
+        case Collation::BUILDER_DATA_TAG:
+            ce32 = getCE32FromBuilderData(ce32, errorCode);
+            if(U_FAILURE(errorCode)) { return; }
+            if(ce32 == Collation::FALLBACK_CE32) {
+                d = data->base;
+                ce32 = d->getCE32(c);
+            }
+            break;
+        case Collation::PREFIX_TAG:
+            if(forward) { backwardNumCodePoints(1, errorCode); }
+            ce32 = getCE32FromPrefix(d, ce32, errorCode);
+            if(forward) { forwardNumCodePoints(1, errorCode); }
+            break;
+        case Collation::CONTRACTION_TAG: {
+            const UChar *p = d->contexts + Collation::indexFromCE32(ce32);
+            uint32_t defaultCE32 = CollationData::readCE32(p);  // Default if no suffix match.
+            if(!forward) {
+                // Backward contractions are handled by previousCEUnsafe().
+                // c has contractions but they were not found.
+                ce32 = defaultCE32;
+                break;
+            }
+            UChar32 nextCp;
+            if(skipped == NULL && numCpFwd < 0) {
+                // Some portion of nextCE32FromContraction() pulled out here as an ASCII fast path,
+                // avoiding the function call and the nextSkippedCodePoint() overhead.
+                nextCp = nextCodePoint(errorCode);
+                if(nextCp < 0) {
+                    // No more text.
+                    ce32 = defaultCE32;
+                    break;
+                } else if((ce32 & Collation::CONTRACT_NEXT_CCC) != 0 &&
+                        !CollationFCD::mayHaveLccc(nextCp)) {
+                    // All contraction suffixes start with characters with lccc!=0
+                    // but the next code point has lccc==0.
+                    backwardNumCodePoints(1, errorCode);
+                    ce32 = defaultCE32;
+                    break;
+                }
+            } else {
+                nextCp = nextSkippedCodePoint(errorCode);
+                if(nextCp < 0) {
+                    // No more text.
+                    ce32 = defaultCE32;
+                    break;
+                } else if((ce32 & Collation::CONTRACT_NEXT_CCC) != 0 &&
+                        !CollationFCD::mayHaveLccc(nextCp)) {
+                    // All contraction suffixes start with characters with lccc!=0
+                    // but the next code point has lccc==0.
+                    backwardNumSkipped(1, errorCode);
+                    ce32 = defaultCE32;
+                    break;
+                }
+            }
+            ce32 = nextCE32FromContraction(d, ce32, p + 2, defaultCE32, nextCp, errorCode);
+            if(ce32 == Collation::NO_CE32) {
+                // CEs from a discontiguous contraction plus the skipped combining marks
+                // have been appended already.
+                return;
+            }
+            break;
+        }
+        case Collation::DIGIT_TAG:
+            if(isNumeric) {
+                appendNumericCEs(ce32, forward, errorCode);
+                return;
+            } else {
+                // Fetch the non-numeric-collation CE32 and continue.
+                ce32 = d->ce32s[Collation::indexFromCE32(ce32)];
+                break;
+            }
+        case Collation::U0000_TAG:
+            U_ASSERT(c == 0);
+            if(forward && foundNULTerminator()) {
+                // Handle NUL-termination. (Not needed in Java.)
+                ceBuffer.append(Collation::NO_CE, errorCode);
+                return;
+            } else {
+                // Fetch the normal ce32 for U+0000 and continue.
+                ce32 = d->ce32s[0];
+                break;
+            }
+        case Collation::HANGUL_TAG: {
+            const uint32_t *jamoCE32s = d->jamoCE32s;
+            c -= Hangul::HANGUL_BASE;
+            UChar32 t = c % Hangul::JAMO_T_COUNT;
+            c /= Hangul::JAMO_T_COUNT;
+            UChar32 v = c % Hangul::JAMO_V_COUNT;
+            c /= Hangul::JAMO_V_COUNT;
+            if((ce32 & Collation::HANGUL_NO_SPECIAL_JAMO) != 0) {
+                // None of the Jamo CE32s are isSpecialCE32().
+                // Avoid recursive function calls and per-Jamo tests.
+                if(ceBuffer.ensureAppendCapacity(t == 0 ? 2 : 3, errorCode)) {
+                    ceBuffer.set(ceBuffer.length, Collation::ceFromCE32(jamoCE32s[c]));
+                    ceBuffer.set(ceBuffer.length + 1, Collation::ceFromCE32(jamoCE32s[19 + v]));
+                    ceBuffer.length += 2;
+                    if(t != 0) {
+                        ceBuffer.appendUnsafe(Collation::ceFromCE32(jamoCE32s[39 + t]));
+                    }
+                }
+                return;
+            } else {
+                // We should not need to compute each Jamo code point.
+                // In particular, there should be no offset or implicit ce32.
+                appendCEsFromCE32(d, U_SENTINEL, jamoCE32s[c], forward, errorCode);
+                appendCEsFromCE32(d, U_SENTINEL, jamoCE32s[19 + v], forward, errorCode);
+                if(t == 0) { return; }
+                // offset 39 = 19 + 21 - 1:
+                // 19 = JAMO_L_COUNT
+                // 21 = JAMO_T_COUNT
+                // -1 = omit t==0
+                ce32 = jamoCE32s[39 + t];
+                c = U_SENTINEL;
+                break;
+            }
+        }
+        case Collation::LEAD_SURROGATE_TAG: {
+            U_ASSERT(forward);  // Backward iteration should never see lead surrogate code _unit_ data.
+            U_ASSERT(U16_IS_LEAD(c));
+            UChar trail;
+            if(U16_IS_TRAIL(trail = handleGetTrailSurrogate())) {
+                c = U16_GET_SUPPLEMENTARY(c, trail);
+                ce32 &= Collation::LEAD_TYPE_MASK;
+                if(ce32 == Collation::LEAD_ALL_UNASSIGNED) {
+                    ce32 = Collation::UNASSIGNED_CE32;  // unassigned-implicit
+                } else if(ce32 == Collation::LEAD_ALL_FALLBACK ||
+                        (ce32 = d->getCE32FromSupplementary(c)) == Collation::FALLBACK_CE32) {
+                    // fall back to the base data
+                    d = d->base;
+                    ce32 = d->getCE32FromSupplementary(c);
+                }
+            } else {
+                // c is an unpaired surrogate.
+                ce32 = Collation::UNASSIGNED_CE32;
+            }
+            break;
+        }
+        case Collation::OFFSET_TAG:
+            U_ASSERT(c >= 0);
+            ceBuffer.append(d->getCEFromOffsetCE32(c, ce32), errorCode);
+            return;
+        case Collation::IMPLICIT_TAG:
+            U_ASSERT(c >= 0);
+            if(U_IS_SURROGATE(c) && forbidSurrogateCodePoints()) {
+                ce32 = Collation::FFFD_CE32;
+                break;
+            } else {
+                ceBuffer.append(Collation::unassignedCEFromCodePoint(c), errorCode);
+                return;
+            }
+        }
+    }
+    ceBuffer.append(Collation::ceFromSimpleCE32(ce32), errorCode);
+}
+
+uint32_t
+CollationIterator::getCE32FromPrefix(const CollationData *d, uint32_t ce32,
+                                     UErrorCode &errorCode) {
+    const UChar *p = d->contexts + Collation::indexFromCE32(ce32);
+    ce32 = CollationData::readCE32(p);  // Default if no prefix match.
+    p += 2;
+    // Number of code points read before the original code point.
+    int32_t lookBehind = 0;
+    UCharsTrie prefixes(p);
+    for(;;) {
+        UChar32 c = previousCodePoint(errorCode);
+        if(c < 0) { break; }
+        ++lookBehind;
+        UStringTrieResult match = prefixes.nextForCodePoint(c);
+        if(USTRINGTRIE_HAS_VALUE(match)) {
+            ce32 = (uint32_t)prefixes.getValue();
+        }
+        if(!USTRINGTRIE_HAS_NEXT(match)) { break; }
+    }
+    forwardNumCodePoints(lookBehind, errorCode);
+    return ce32;
+}
+
+UChar32
+CollationIterator::nextSkippedCodePoint(UErrorCode &errorCode) {
+    if(skipped != NULL && skipped->hasNext()) { return skipped->next(); }
+    if(numCpFwd == 0) { return U_SENTINEL; }
+    UChar32 c = nextCodePoint(errorCode);
+    if(skipped != NULL && !skipped->isEmpty() && c >= 0) { skipped->incBeyond(); }
+    if(numCpFwd > 0 && c >= 0) { --numCpFwd; }
+    return c;
+}
+
+void
+CollationIterator::backwardNumSkipped(int32_t n, UErrorCode &errorCode) {
+    if(skipped != NULL && !skipped->isEmpty()) {
+        n = skipped->backwardNumCodePoints(n);
+    }
+    backwardNumCodePoints(n, errorCode);
+    if(numCpFwd >= 0) { numCpFwd += n; }
+}
+
+uint32_t
+CollationIterator::nextCE32FromContraction(const CollationData *d, uint32_t contractionCE32,
+                                           const UChar *p, uint32_t ce32, UChar32 c,
+                                           UErrorCode &errorCode) {
+    // c: next code point after the original one
+
+    // Number of code points read beyond the original code point.
+    // Needed for discontiguous contraction matching.
+    int32_t lookAhead = 1;
+    // Number of code points read since the last match (initially only c).
+    int32_t sinceMatch = 1;
+    // Normally we only need a contiguous match,
+    // and therefore need not remember the suffixes state from before a mismatch for retrying.
+    // If we are already processing skipped combining marks, then we do track the state.
+    UCharsTrie suffixes(p);
+    if(skipped != NULL && !skipped->isEmpty()) { skipped->saveTrieState(suffixes); }
+    UStringTrieResult match = suffixes.firstForCodePoint(c);
+    for(;;) {
+        UChar32 nextCp;
+        if(USTRINGTRIE_HAS_VALUE(match)) {
+            ce32 = (uint32_t)suffixes.getValue();
+            if(!USTRINGTRIE_HAS_NEXT(match) || (c = nextSkippedCodePoint(errorCode)) < 0) {
+                return ce32;
+            }
+            if(skipped != NULL && !skipped->isEmpty()) { skipped->saveTrieState(suffixes); }
+            sinceMatch = 1;
+        } else if(match == USTRINGTRIE_NO_MATCH || (nextCp = nextSkippedCodePoint(errorCode)) < 0) {
+            // No match for c, or partial match (USTRINGTRIE_NO_VALUE) and no further text.
+            // Back up if necessary, and try a discontiguous contraction.
+            if((contractionCE32 & Collation::CONTRACT_TRAILING_CCC) != 0 &&
+                    // Discontiguous contraction matching extends an existing match.
+                    // If there is no match yet, then there is nothing to do.
+                    ((contractionCE32 & Collation::CONTRACT_SINGLE_CP_NO_MATCH) == 0 ||
+                        sinceMatch < lookAhead)) {
+                // The last character of at least one suffix has lccc!=0,
+                // allowing for discontiguous contractions.
+                // UCA S2.1.1 only processes non-starters immediately following
+                // "a match in the table" (sinceMatch=1).
+                if(sinceMatch > 1) {
+                    // Return to the state after the last match.
+                    // (Return to sinceMatch=0 and re-fetch the first partially-matched character.)
+                    backwardNumSkipped(sinceMatch, errorCode);
+                    c = nextSkippedCodePoint(errorCode);
+                    lookAhead -= sinceMatch - 1;
+                    sinceMatch = 1;
+                }
+                if(d->getFCD16(c) > 0xff) {
+                    return nextCE32FromDiscontiguousContraction(
+                        d, suffixes, ce32, lookAhead, c, errorCode);
+                }
+            }
+            break;
+        } else {
+            // Continue after partial match (USTRINGTRIE_NO_VALUE) for c.
+            // It does not have a result value, therefore it is not itself "a match in the table".
+            // If a partially-matched c has ccc!=0 then
+            // it might be skipped in discontiguous contraction.
+            c = nextCp;
+            ++sinceMatch;
+        }
+        ++lookAhead;
+        match = suffixes.nextForCodePoint(c);
+    }
+    backwardNumSkipped(sinceMatch, errorCode);
+    return ce32;
+}
+
+uint32_t
+CollationIterator::nextCE32FromDiscontiguousContraction(
+        const CollationData *d, UCharsTrie &suffixes, uint32_t ce32,
+        int32_t lookAhead, UChar32 c,
+        UErrorCode &errorCode) {
+    if(U_FAILURE(errorCode)) { return 0; }
+
+    // UCA section 3.3.2 Contractions:
+    // Contractions that end with non-starter characters
+    // are known as discontiguous contractions.
+    // ... discontiguous contractions must be detected in input text
+    // whenever the final sequence of non-starter characters could be rearranged
+    // so as to make a contiguous matching sequence that is canonically equivalent.
+
+    // UCA: http://www.unicode.org/reports/tr10/#S2.1
+    // S2.1 Find the longest initial substring S at each point that has a match in the table.
+    // S2.1.1 If there are any non-starters following S, process each non-starter C.
+    // S2.1.2 If C is not blocked from S, find if S + C has a match in the table.
+    //     Note: A non-starter in a string is called blocked
+    //     if there is another non-starter of the same canonical combining class or zero
+    //     between it and the last character of canonical combining class 0.
+    // S2.1.3 If there is a match, replace S by S + C, and remove C.
+
+    // First: Is a discontiguous contraction even possible?
+    uint16_t fcd16 = d->getFCD16(c);
+    U_ASSERT(fcd16 > 0xff);  // The caller checked this already, as a shortcut.
+    UChar32 nextCp = nextSkippedCodePoint(errorCode);
+    if(nextCp < 0) {
+        // No further text.
+        backwardNumSkipped(1, errorCode);
+        return ce32;
+    }
+    ++lookAhead;
+    uint8_t prevCC = (uint8_t)fcd16;
+    fcd16 = d->getFCD16(nextCp);
+    if(fcd16 <= 0xff) {
+        // The next code point after c is a starter (S2.1.1 "process each non-starter").
+        backwardNumSkipped(2, errorCode);
+        return ce32;
+    }
+
+    // We have read and matched (lookAhead-2) code points,
+    // read non-matching c and peeked ahead at nextCp.
+    // Return to the state before the mismatch and continue matching with nextCp.
+    if(skipped == NULL || skipped->isEmpty()) {
+        if(skipped == NULL) {
+            skipped = new SkippedState();
+            if(skipped == NULL) {
+                errorCode = U_MEMORY_ALLOCATION_ERROR;
+                return 0;
+            }
+        }
+        suffixes.reset();
+        if(lookAhead > 2) {
+            // Replay the partial match so far.
+            backwardNumCodePoints(lookAhead, errorCode);
+            suffixes.firstForCodePoint(nextCodePoint(errorCode));
+            for(int32_t i = 3; i < lookAhead; ++i) {
+                suffixes.nextForCodePoint(nextCodePoint(errorCode));
+            }
+            // Skip c (which did not match) and nextCp (which we will try now).
+            forwardNumCodePoints(2, errorCode);
+        }
+        skipped->saveTrieState(suffixes);
+    } else {
+        // Reset to the trie state before the failed match of c.
+        skipped->resetToTrieState(suffixes);
+    }
+
+    skipped->setFirstSkipped(c);
+    // Number of code points read since the last match (at this point: c and nextCp).
+    int32_t sinceMatch = 2;
+    c = nextCp;
+    for(;;) {
+        UStringTrieResult match;
+        // "If C is not blocked from S, find if S + C has a match in the table." (S2.1.2)
+        if(prevCC < (fcd16 >> 8) && USTRINGTRIE_HAS_VALUE(match = suffixes.nextForCodePoint(c))) {
+            // "If there is a match, replace S by S + C, and remove C." (S2.1.3)
+            // Keep prevCC unchanged.
+            ce32 = (uint32_t)suffixes.getValue();
+            sinceMatch = 0;
+            skipped->recordMatch();
+            if(!USTRINGTRIE_HAS_NEXT(match)) { break; }
+            skipped->saveTrieState(suffixes);
+        } else {
+            // No match for "S + C", skip C.
+            skipped->skip(c);
+            skipped->resetToTrieState(suffixes);
+            prevCC = (uint8_t)fcd16;
+        }
+        if((c = nextSkippedCodePoint(errorCode)) < 0) { break; }
+        ++sinceMatch;
+        fcd16 = d->getFCD16(c);
+        if(fcd16 <= 0xff) {
+            // The next code point after c is a starter (S2.1.1 "process each non-starter").
+            break;
+        }
+    }
+    backwardNumSkipped(sinceMatch, errorCode);
+    UBool isTopDiscontiguous = skipped->isEmpty();
+    skipped->replaceMatch();
+    if(isTopDiscontiguous && !skipped->isEmpty()) {
+        // We did get a match after skipping one or more combining marks,
+        // and we are not in a recursive discontiguous contraction.
+        // Append CEs from the contraction ce32
+        // and then from the combining marks that we skipped before the match.
+        c = U_SENTINEL;
+        for(;;) {
+            appendCEsFromCE32(d, c, ce32, TRUE, errorCode);
+            // Fetch CE32s for skipped combining marks from the normal data, with fallback,
+            // rather than from the CollationData where we found the contraction.
+            if(!skipped->hasNext()) { break; }
+            c = skipped->next();
+            ce32 = getDataCE32(c);
+            if(ce32 == Collation::FALLBACK_CE32) {
+                d = data->base;
+                ce32 = d->getCE32(c);
+            } else {
+                d = data;
+            }
+            // Note: A nested discontiguous-contraction match
+            // replaces consumed combining marks with newly skipped ones
+            // and resets the reading position to the beginning.
+        }
+        skipped->clear();
+        ce32 = Collation::NO_CE32;  // Signal to the caller that the result is in the ceBuffer.
+    }
+    return ce32;
+}
+
+void
+CollationIterator::appendNumericCEs(uint32_t ce32, UBool forward, UErrorCode &errorCode) {
+    // Collect digits.
+    CharString digits;
+    if(forward) {
+        for(;;) {
+            char digit = Collation::digitFromCE32(ce32);
+            digits.append(digit, errorCode);
+            if(numCpFwd == 0) { break; }
+            UChar32 c = nextCodePoint(errorCode);
+            if(c < 0) { break; }
+            ce32 = data->getCE32(c);
+            if(ce32 == Collation::FALLBACK_CE32) {
+                ce32 = data->base->getCE32(c);
+            }
+            if(!Collation::hasCE32Tag(ce32, Collation::DIGIT_TAG)) {
+                backwardNumCodePoints(1, errorCode);
+                break;
+            }
+            if(numCpFwd > 0) { --numCpFwd; }
+        }
+    } else {
+        for(;;) {
+            char digit = Collation::digitFromCE32(ce32);
+            digits.append(digit, errorCode);
+            UChar32 c = previousCodePoint(errorCode);
+            if(c < 0) { break; }
+            ce32 = data->getCE32(c);
+            if(ce32 == Collation::FALLBACK_CE32) {
+                ce32 = data->base->getCE32(c);
+            }
+            if(!Collation::hasCE32Tag(ce32, Collation::DIGIT_TAG)) {
+                forwardNumCodePoints(1, errorCode);
+                break;
+            }
+        }
+        // Reverse the digit string.
+        char *p = digits.data();
+        char *q = p + digits.length() - 1;
+        while(p < q) {
+            char digit = *p;
+            *p++ = *q;
+            *q-- = digit;
+        }
+    }
+    if(U_FAILURE(errorCode)) { return; }
+    int32_t pos = 0;
+    do {
+        // Skip leading zeros.
+        while(pos < (digits.length() - 1) && digits[pos] == 0) { ++pos; }
+        // Write a sequence of CEs for at most 254 digits at a time.
+        int32_t segmentLength = digits.length() - pos;
+        if(segmentLength > 254) { segmentLength = 254; }
+        appendNumericSegmentCEs(digits.data() + pos, segmentLength, errorCode);
+        pos += segmentLength;
+    } while(U_SUCCESS(errorCode) && pos < digits.length());
+}
+
+void
+CollationIterator::appendNumericSegmentCEs(const char *digits, int32_t length, UErrorCode &errorCode) {
+    U_ASSERT(1 <= length && length <= 254);
+    U_ASSERT(length == 1 || digits[0] != 0);
+    uint32_t numericPrimary = data->numericPrimary;
+    // Note: We use primary byte values 2..255: digits are not compressible.
+    if(length <= 7) {
+        // Very dense encoding for small numbers.
+        int32_t value = digits[0];
+        for(int32_t i = 1; i < length; ++i) {
+            value = value * 10 + digits[i];
+        }
+        // Primary weight second byte values:
+        //     74 byte values   2.. 75 for small numbers in two-byte primary weights.
+        //     40 byte values  76..115 for medium numbers in three-byte primary weights.
+        //     16 byte values 116..131 for large numbers in four-byte primary weights.
+        //    124 byte values 132..255 for very large numbers with 4..127 digit pairs.
+        int32_t firstByte = 2;
+        int32_t numBytes = 74;
+        if(value < numBytes) {
+            // Two-byte primary for 0..73, good for day & month numbers etc.
+            uint32_t primary = numericPrimary | ((firstByte + value) << 16);
+            ceBuffer.append(Collation::makeCE(primary), errorCode);
+            return;
+        }
+        value -= numBytes;
+        firstByte += numBytes;
+        numBytes = 40;
+        if(value < numBytes * 254) {
+            // Three-byte primary for 74..10233=74+40*254-1, good for year numbers and more.
+            uint32_t primary = numericPrimary |
+                ((firstByte + value / 254) << 16) | ((2 + value % 254) << 8);
+            ceBuffer.append(Collation::makeCE(primary), errorCode);
+            return;
+        }
+        value -= numBytes * 254;
+        firstByte += numBytes;
+        numBytes = 16;
+        if(value < numBytes * 254 * 254) {
+            // Four-byte primary for 10234..1042489=10234+16*254*254-1.
+            uint32_t primary = numericPrimary | (2 + value % 254);
+            value /= 254;
+            primary |= (2 + value % 254) << 8;
+            value /= 254;
+            primary |= (firstByte + value % 254) << 16;
+            ceBuffer.append(Collation::makeCE(primary), errorCode);
+            return;
+        }
+        // original value > 1042489
+    }
+    U_ASSERT(length >= 7);
+
+    // The second primary byte value 132..255 indicates the number of digit pairs (4..127),
+    // then we generate primary bytes with those pairs.
+    // Omit trailing 00 pairs.
+    // Decrement the value for the last pair.
+
+    // Set the exponent. 4 pairs->132, 5 pairs->133, ..., 127 pairs->255.
+    int32_t numPairs = (length + 1) / 2;
+    uint32_t primary = numericPrimary | ((132 - 4 + numPairs) << 16);
+    // Find the length without trailing 00 pairs.
+    while(digits[length - 1] == 0 && digits[length - 2] == 0) {
+        length -= 2;
+    }
+    // Read the first pair.
+    uint32_t pair;
+    int32_t pos;
+    if(length & 1) {
+        // Only "half a pair" if we have an odd number of digits.
+        pair = digits[0];
+        pos = 1;
+    } else {
+        pair = digits[0] * 10 + digits[1];
+        pos = 2;
+    }
+    pair = 11 + 2 * pair;
+    // Add the pairs of digits between pos and length.
+    int32_t shift = 8;
+    while(pos < length) {
+        if(shift == 0) {
+            // Every three pairs/bytes we need to store a 4-byte-primary CE
+            // and start with a new CE with the '0' primary lead byte.
+            primary |= pair;
+            ceBuffer.append(Collation::makeCE(primary), errorCode);
+            primary = numericPrimary;
+            shift = 16;
+        } else {
+            primary |= pair << shift;
+            shift -= 8;
+        }
+        pair = 11 + 2 * (digits[pos] * 10 + digits[pos + 1]);
+        pos += 2;
+    }
+    primary |= (pair - 1) << shift;
+    ceBuffer.append(Collation::makeCE(primary), errorCode);
+}
+
+int64_t
+CollationIterator::previousCE(UVector32 &offsets, UErrorCode &errorCode) {
+    if(ceBuffer.length > 0) {
+        // Return the previous buffered CE.
+        return ceBuffer.get(--ceBuffer.length);
+    }
+    offsets.removeAllElements();
+    int32_t limitOffset = getOffset();
+    UChar32 c = previousCodePoint(errorCode);
+    if(c < 0) { return Collation::NO_CE; }
+    if(data->isUnsafeBackward(c, isNumeric)) {
+        return previousCEUnsafe(c, offsets, errorCode);
+    }
+    // Simple, safe-backwards iteration:
+    // Get a CE going backwards, handle prefixes but no contractions.
+    uint32_t ce32 = data->getCE32(c);
+    const CollationData *d;
+    if(ce32 == Collation::FALLBACK_CE32) {
+        d = data->base;
+        ce32 = d->getCE32(c);
+    } else {
+        d = data;
+    }
+    if(Collation::isSimpleOrLongCE32(ce32)) {
+        return Collation::ceFromCE32(ce32);
+    }
+    appendCEsFromCE32(d, c, ce32, FALSE, errorCode);
+    if(U_SUCCESS(errorCode)) {
+        if(ceBuffer.length > 1) {
+            offsets.addElement(getOffset(), errorCode);
+            // For an expansion, the offset of each non-initial CE is the limit offset,
+            // consistent with forward iteration.
+            while(offsets.size() <= ceBuffer.length) {
+                offsets.addElement(limitOffset, errorCode);
+            };
+        }
+        return ceBuffer.get(--ceBuffer.length);
+    } else {
+        return Collation::NO_CE_PRIMARY;
+    }
+}
+
+int64_t
+CollationIterator::previousCEUnsafe(UChar32 c, UVector32 &offsets, UErrorCode &errorCode) {
+    // We just move through the input counting safe and unsafe code points
+    // without collecting the unsafe-backward substring into a buffer and
+    // switching to it.
+    // This is to keep the logic simple. Otherwise we would have to handle
+    // prefix matching going before the backward buffer, switching
+    // to iteration and back, etc.
+    // In the most important case of iterating over a normal string,
+    // reading from the string itself is already maximally fast.
+    // The only drawback there is that after getting the CEs we always
+    // skip backward to the safe character rather than switching out
+    // of a backwardBuffer.
+    // But this should not be the common case for previousCE(),
+    // and correctness and maintainability are more important than
+    // complex optimizations.
+    // Find the first safe character before c.
+    int32_t numBackward = 1;
+    while((c = previousCodePoint(errorCode)) >= 0) {
+        ++numBackward;
+        if(!data->isUnsafeBackward(c, isNumeric)) {
+            break;
+        }
+    }
+    // Set the forward iteration limit.
+    // Note: This counts code points.
+    // We cannot enforce a limit in the middle of a surrogate pair or similar.
+    numCpFwd = numBackward;
+    // Reset the forward iterator.
+    cesIndex = 0;
+    U_ASSERT(ceBuffer.length == 0);
+    // Go forward and collect the CEs.
+    int32_t offset = getOffset();
+    while(numCpFwd > 0) {
+        // nextCE() normally reads one code point.
+        // Contraction matching and digit specials read more and check numCpFwd.
+        --numCpFwd;
+        // Append one or more CEs to the ceBuffer.
+        (void)nextCE(errorCode);
+        U_ASSERT(U_FAILURE(errorCode) || ceBuffer.get(ceBuffer.length - 1) != Collation::NO_CE);
+        // No need to loop for getting each expansion CE from nextCE().
+        cesIndex = ceBuffer.length;
+        // However, we need to write an offset for each CE.
+        // This is for CollationElementIterator::getOffset() to return
+        // intermediate offsets from the unsafe-backwards segment.
+        U_ASSERT(offsets.size() < ceBuffer.length);
+        offsets.addElement(offset, errorCode);
+        // For an expansion, the offset of each non-initial CE is the limit offset,
+        // consistent with forward iteration.
+        offset = getOffset();
+        while(offsets.size() < ceBuffer.length) {
+            offsets.addElement(offset, errorCode);
+        };
+    }
+    U_ASSERT(offsets.size() == ceBuffer.length);
+    // End offset corresponding to just after the unsafe-backwards segment.
+    offsets.addElement(offset, errorCode);
+    // Reset the forward iteration limit
+    // and move backward to before the segment for which we fetched CEs.
+    numCpFwd = -1;
+    backwardNumCodePoints(numBackward, errorCode);
+    // Use the collected CEs and return the last one.
+    cesIndex = 0;  // Avoid cesIndex > ceBuffer.length when that gets decremented.
+    if(U_SUCCESS(errorCode)) {
+        return ceBuffer.get(--ceBuffer.length);
+    } else {
+        return Collation::NO_CE_PRIMARY;
+    }
+}
+
+U_NAMESPACE_END
+
+#endif  // !UCONFIG_NO_COLLATION