icuSources/i18n/bocsu.h

   1 /*
   2 *******************************************************************************
   3 *   Copyright (C) 2001-2014, International Business Machines
   4 *   Corporation and others.  All Rights Reserved.
   5 *******************************************************************************
   6 *   file name:  bocsu.h
   7 *   encoding:   US-ASCII
   8 *   tab size:   8 (not used)
   9 *   indentation:4
  10 *
  11 *   Author: Markus W. Scherer
  12 *
  13 *   Modification history:
  14 *   05/18/2001  weiv    Made into separate module
  15 */
  16
  17 #ifndef BOCSU_H
  18 #define BOCSU_H
  19
  20 #include "unicode/utypes.h"
  21
  22 #if !UCONFIG_NO_COLLATION
  23
  24 U_NAMESPACE_BEGIN
  25
  26 class ByteSink;
  27
  28 U_NAMESPACE_END
  29
  30 /*
  31  * "BOCSU"
  32  * Binary Ordered Compression Scheme for Unicode
  33  *
  34  * Specific application:
  35  *
  36  * Encode a Unicode string for the identical level of a sort key.
  37  * Restrictions:
  38  * - byte stream (unsigned 8-bit bytes)
  39  * - lexical order of the identical-level run must be
  40  *   the same as code point order for the string
  41  * - avoid byte values 0, 1, 2
  42  *
  43  * Method: Slope Detection
  44  * Remember the previous code point (initial 0).
  45  * For each cp in the string, encode the difference to the previous one.
  46  *
  47  * With a compact encoding of differences, this yields good results for
  48  * small scripts and UTF-like results otherwise.
  49  *
  50  * Encoding of differences:
  51  * - Similar to a UTF, encoding the length of the byte sequence in the lead bytes.
  52  * - Does not need to be friendly for decoding or random access
  53  *   (trail byte values may overlap with lead/single byte values).
  54  * - The signedness must be encoded as the most significant part.
  55  *
  56  * We encode differences with few bytes if their absolute values are small.
  57  * For correct ordering, we must treat the entire value range -10ffff..+10ffff
  58  * in ascending order, which forbids encoding the sign and the absolute value separately.
  59  * Instead, we split the lead byte range in the middle and encode non-negative values
  60  * going up and negative values going down.
  61  *
  62  * For very small absolute values, the difference is added to a middle byte value
  63  * for single-byte encoded differences.
  64  * For somewhat larger absolute values, the difference is divided by the number
  65  * of byte values available, the modulo is used for one trail byte, and the remainder
  66  * is added to a lead byte avoiding the single-byte range.
  67  * For large absolute values, the difference is similarly encoded in three bytes.
  68  *
  69  * This encoding does not use byte values 0, 1, 2, but uses all other byte values
  70  * for lead/single bytes so that the middle range of single bytes is as large
  71  * as possible.
  72  * Note that the lead byte ranges overlap some, but that the sequences as a whole
  73  * are well ordered. I.e., even if the lead byte is the same for sequences of different
  74  * lengths, the trail bytes establish correct order.
  75  * It would be possible to encode slightly larger ranges for each length (>1) by
  76  * subtracting the lower bound of the range. However, that would also slow down the
  77  * calculation.
  78  *
  79  * For the actual string encoding, an optimization moves the previous code point value
  80  * to the middle of its Unicode script block to minimize the differences in
  81  * same-script text runs.
  82  */
  83
  84 /* Do not use byte values 0, 1, 2 because they are separators in sort keys. */
  85 #define SLOPE_MIN           3
  86 #define SLOPE_MAX           0xff
  87 #define SLOPE_MIDDLE        0x81
  88
  89 #define SLOPE_TAIL_COUNT    (SLOPE_MAX-SLOPE_MIN+1)
  90
  91 #define SLOPE_MAX_BYTES     4
  92
  93 /*
  94  * Number of lead bytes:
  95  * 1        middle byte for 0
  96  * 2*80=160 single bytes for !=0
  97  * 2*42=84  for double-byte values
  98  * 2*3=6    for 3-byte values
  99  * 2*1=2    for 4-byte values
 100  *
 101  * The sum must be <=SLOPE_TAIL_COUNT.
 102  *
 103  * Why these numbers?
 104  * - There should be >=128 single-byte values to cover 128-blocks
 105  *   with small scripts.
 106  * - There should be >=20902 single/double-byte values to cover Unihan.
 107  * - It helps CJK Extension B some if there are 3-byte values that cover
 108  *   the distance between them and Unihan.
 109  *   This also helps to jump among distant places in the BMP.
 110  * - Four-byte values are necessary to cover the rest of Unicode.
 111  *
 112  * Symmetrical lead byte counts are for convenience.
 113  * With an equal distribution of even and odd differences there is also
 114  * no advantage to asymmetrical lead byte counts.
 115  */
 116 #define SLOPE_SINGLE        80
 117 #define SLOPE_LEAD_2        42
 118 #define SLOPE_LEAD_3        3
 119 #define SLOPE_LEAD_4        1
 120
 121 /* The difference value range for single-byters. */
 122 #define SLOPE_REACH_POS_1   SLOPE_SINGLE
 123 #define SLOPE_REACH_NEG_1   (-SLOPE_SINGLE)
 124
 125 /* The difference value range for double-byters. */
 126 #define SLOPE_REACH_POS_2   (SLOPE_LEAD_2*SLOPE_TAIL_COUNT+(SLOPE_LEAD_2-1))
 127 #define SLOPE_REACH_NEG_2   (-SLOPE_REACH_POS_2-1)
 128
 129 /* The difference value range for 3-byters. */
 130 #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))
 131 #define SLOPE_REACH_NEG_3   (-SLOPE_REACH_POS_3-1)
 132
 133 /* The lead byte start values. */
 134 #define SLOPE_START_POS_2   (SLOPE_MIDDLE+SLOPE_SINGLE+1)
 135 #define SLOPE_START_POS_3   (SLOPE_START_POS_2+SLOPE_LEAD_2)
 136
 137 #define SLOPE_START_NEG_2   (SLOPE_MIDDLE+SLOPE_REACH_NEG_1)
 138 #define SLOPE_START_NEG_3   (SLOPE_START_NEG_2-SLOPE_LEAD_2)
 139
 140 /*
 141  * Integer division and modulo with negative numerators
 142  * yields negative modulo results and quotients that are one more than
 143  * what we need here.
 144  */
 145 #define NEGDIVMOD(n, d, m) { \
 146     (m)=(n)%(d); \
 147     (n)/=(d); \
 148     if((m)<0) { \
 149         --(n); \
 150         (m)+=(d); \
 151     } \
 152 }
 153
 154 U_CFUNC UChar32
 155 u_writeIdenticalLevelRun(UChar32 prev, const UChar *s, int32_t length, icu::ByteSink &sink);
 156
 157 #endif /* #if !UCONFIG_NO_COLLATION */
 158
 159 #endif