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
160