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Diffstat (limited to 'intl/icu/source/i18n/bocsu.h')
| -rw-r--r-- | intl/icu/source/i18n/bocsu.h | 161 |
1 files changed, 161 insertions, 0 deletions
diff --git a/intl/icu/source/i18n/bocsu.h b/intl/icu/source/i18n/bocsu.h new file mode 100644 index 000000000..56b03500b --- /dev/null +++ b/intl/icu/source/i18n/bocsu.h @@ -0,0 +1,161 @@ +// Copyright (C) 2016 and later: Unicode, Inc. and others. +// License & terms of use: http://www.unicode.org/copyright.html +/* +******************************************************************************* +* Copyright (C) 2001-2014, International Business Machines +* Corporation and others. All Rights Reserved. +******************************************************************************* +* file name: bocsu.h +* 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 + +U_NAMESPACE_BEGIN + +class ByteSink; + +U_NAMESPACE_END + +/* + * "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 UChar32 +u_writeIdenticalLevelRun(UChar32 prev, const UChar *s, int32_t length, icu::ByteSink &sink); + +#endif /* #if !UCONFIG_NO_COLLATION */ + +#endif |