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icu46/source/i18n/bocsu.h

+/*
+*******************************************************************************
+*   Copyright (C) 2001-2003, International Business Machines
+*   Corporation and others.  All Rights Reserved.
+*******************************************************************************
+*   file name:  bocsu.c
+*   encoding:   US-ASCII
+*   tab size:   8 (not used)
+*   indentation:4
+*
+*   Author: Markus W. Scherer
+*
+*   Modification history:
+*   05/18/2001  weiv    Made into separate module
+*/
+
+#ifndef BOCSU_H
+#define BOCSU_H
+
+#include "unicode/utypes.h"
+
+#if !UCONFIG_NO_COLLATION
+
+/*
+ * "BOCSU"
+ * Binary Ordered Compression Scheme for Unicode
+ *
+ * Specific application:
+ *
+ * Encode a Unicode string for the identical level of a sort key.
+ * Restrictions:
+ * - byte stream (unsigned 8-bit bytes)
+ * - lexical order of the identical-level run must be
+ *   the same as code point order for the string
+ * - avoid byte values 0, 1, 2
+ *
+ * Method: Slope Detection
+ * Remember the previous code point (initial 0).
+ * For each cp in the string, encode the difference to the previous one.
+ *
+ * With a compact encoding of differences, this yields good results for
+ * small scripts and UTF-like results otherwise.
+ *
+ * Encoding of differences:
+ * - Similar to a UTF, encoding the length of the byte sequence in the lead bytes.
+ * - Does not need to be friendly for decoding or random access
+ *   (trail byte values may overlap with lead/single byte values).
+ * - The signedness must be encoded as the most significant part.
+ *
+ * We encode differences with few bytes if their absolute values are small.
+ * For correct ordering, we must treat the entire value range -10ffff..+10ffff
+ * in ascending order, which forbids encoding the sign and the absolute value separately.
+ * Instead, we split the lead byte range in the middle and encode non-negative values
+ * going up and negative values going down.
+ *
+ * For very small absolute values, the difference is added to a middle byte value
+ * for single-byte encoded differences.
+ * For somewhat larger absolute values, the difference is divided by the number
+ * of byte values available, the modulo is used for one trail byte, and the remainder
+ * is added to a lead byte avoiding the single-byte range.
+ * For large absolute values, the difference is similarly encoded in three bytes.
+ *
+ * This encoding does not use byte values 0, 1, 2, but uses all other byte values
+ * for lead/single bytes so that the middle range of single bytes is as large
+ * as possible.
+ * Note that the lead byte ranges overlap some, but that the sequences as a whole
+ * are well ordered. I.e., even if the lead byte is the same for sequences of different
+ * lengths, the trail bytes establish correct order.
+ * It would be possible to encode slightly larger ranges for each length (>1) by
+ * subtracting the lower bound of the range. However, that would also slow down the
+ * calculation.
+ *
+ * For the actual string encoding, an optimization moves the previous code point value
+ * to the middle of its Unicode script block to minimize the differences in
+ * same-script text runs.
+ */
+
+/* Do not use byte values 0, 1, 2 because they are separators in sort keys. */
+#define SLOPE_MIN           3
+#define SLOPE_MAX           0xff
+#define SLOPE_MIDDLE        0x81
+
+#define SLOPE_TAIL_COUNT    (SLOPE_MAX-SLOPE_MIN+1)
+
+#define SLOPE_MAX_BYTES     4
+
+/*
+ * Number of lead bytes:
+ * 1        middle byte for 0
+ * 2*80=160 single bytes for !=0
+ * 2*42=84  for double-byte values
+ * 2*3=6    for 3-byte values
+ * 2*1=2    for 4-byte values
+ *
+ * The sum must be <=SLOPE_TAIL_COUNT.
+ *
+ * Why these numbers?
+ * - There should be >=128 single-byte values to cover 128-blocks
+ *   with small scripts.
+ * - There should be >=20902 single/double-byte values to cover Unihan.
+ * - It helps CJK Extension B some if there are 3-byte values that cover
+ *   the distance between them and Unihan.
+ *   This also helps to jump among distant places in the BMP.
+ * - Four-byte values are necessary to cover the rest of Unicode.
+ *
+ * Symmetrical lead byte counts are for convenience.
+ * With an equal distribution of even and odd differences there is also
+ * no advantage to asymmetrical lead byte counts.
+ */
+#define SLOPE_SINGLE        80
+#define SLOPE_LEAD_2        42
+#define SLOPE_LEAD_3        3
+#define SLOPE_LEAD_4        1
+
+/* The difference value range for single-byters. */
+#define SLOPE_REACH_POS_1   SLOPE_SINGLE
+#define SLOPE_REACH_NEG_1   (-SLOPE_SINGLE)
+
+/* The difference value range for double-byters. */
+#define SLOPE_REACH_POS_2   (SLOPE_LEAD_2*SLOPE_TAIL_COUNT+(SLOPE_LEAD_2-1))
+#define SLOPE_REACH_NEG_2   (-SLOPE_REACH_POS_2-1)
+
+/* The difference value range for 3-byters. */
+#define SLOPE_REACH_POS_3   (SLOPE_LEAD_3*SLOPE_TAIL_COUNT*SLOPE_TAIL_COUNT+(SLOPE_LEAD_3-1)*SLOPE_TAIL_COUNT+(SLOPE_TAIL_COUNT-1))
+#define SLOPE_REACH_NEG_3   (-SLOPE_REACH_POS_3-1)
+
+/* The lead byte start values. */
+#define SLOPE_START_POS_2   (SLOPE_MIDDLE+SLOPE_SINGLE+1)
+#define SLOPE_START_POS_3   (SLOPE_START_POS_2+SLOPE_LEAD_2)
+
+#define SLOPE_START_NEG_2   (SLOPE_MIDDLE+SLOPE_REACH_NEG_1)
+#define SLOPE_START_NEG_3   (SLOPE_START_NEG_2-SLOPE_LEAD_2)
+
+/*
+ * Integer division and modulo with negative numerators
+ * yields negative modulo results and quotients that are one more than
+ * what we need here.
+ */
+#define NEGDIVMOD(n, d, m) { \
+    (m)=(n)%(d); \
+    (n)/=(d); \
+    if((m)<0) { \
+        --(n); \
+        (m)+=(d); \
+    } \
+}
+
+U_CFUNC int32_t
+u_writeIdenticalLevelRun(const UChar *s, int32_t length, uint8_t *p);
+
+U_CFUNC int32_t
+u_writeIdenticalLevelRunTwoChars(UChar32 first, UChar32 second, uint8_t *p);
+
+U_CFUNC int32_t
+u_lengthOfIdenticalLevelRun(const UChar *s, int32_t length);
+
+U_CFUNC uint8_t *
+u_writeDiff(int32_t diff, uint8_t *p);
+
+#endif /* #if !UCONFIG_NO_COLLATION */
+
+#endif