| /** |
| ******************************************************************************* |
| * Copyright (C) 1996-2014, International Business Machines Corporation and |
| * others. All Rights Reserved. |
| ******************************************************************************* |
| */ |
| package com.ibm.icu.impl.coll; |
| |
| import com.ibm.icu.util.ByteArrayWrapper; |
| |
| /** |
| * <p>Binary Ordered Compression Scheme for Unicode</p> |
| * |
| * <p>Users are strongly encouraged to read the ICU paper on |
| * <a href="http://www.icu-project.org/docs/papers/binary_ordered_compression_for_unicode.html"> |
| * BOCU</a> before attempting to use this class.</p> |
| * |
| * <p>BOCU is used to compress unicode text into a stream of unsigned |
| * bytes. For many kinds of text the compression compares favorably |
| * to UTF-8, and for some kinds of text (such as CJK) it does better. |
| * The resulting bytes will compare in the same order as the original |
| * code points. The byte stream does not contain the values 0, 1, or |
| * 2.</p> |
| * |
| * <p>One example of a use of BOCU is in |
| * com.ibm.icu.text.Collator#getCollationKey(String) for a RuleBasedCollator object with |
| * collation strength IDENTICAL. The result CollationKey will consist of the |
| * collation order of the source string followed by the BOCU result of the |
| * source string. |
| * </p> |
| * |
| * <p>Unlike a UTF encoding, BOCU-compressed text is not suitable for |
| * random access.</p> |
| * |
| * <p>Method: Slope Detection<br> Remember the previous code point |
| * (initial 0). For each code point in the string, encode the |
| * difference with the previous one. Similar to a UTF, the length of |
| * the byte sequence is encoded in the lead bytes. Unlike a UTF, the |
| * trail byte values may overlap with lead/single byte values. The |
| * signedness of the difference must be encoded as the most |
| * significant part.</p> |
| * |
| * <p>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.</p> |
| * |
| * <p>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. (Syn Wee, I need examples |
| * here.)</p> |
| * |
| * <p>BOCU does not use byte values 0, 1, or 2, but uses all other |
| * byte values for lead and single bytes, so that the middle range of |
| * single bytes is as large as possible.</p> |
| * |
| * <p>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. |
| * (Syn Wee, need an example).</p> |
| * |
| * <p>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. (Syn Wee, |
| * need an example.)</p> |
| * |
| * @author Syn Wee Quek |
| * @since release 2.2, May 3rd 2002 |
| */ |
| public class BOCSU |
| { |
| // public methods ------------------------------------------------------- |
| |
| /** |
| * Encode the code points of a string as |
| * a sequence of byte-encoded differences (slope detection), |
| * preserving lexical order. |
| * |
| * <p>Optimize the difference-taking for runs of Unicode text within |
| * small scripts: |
| * |
| * <p>Most small scripts are allocated within aligned 128-blocks of Unicode |
| * code points. Lexical order is preserved if "prev" is always moved |
| * into the middle of such a block. |
| * |
| * <p>Additionally, "prev" is moved from anywhere in the Unihan |
| * area into the middle of that area. |
| * Note that the identical-level run in a sort key is generated from |
| * NFD text - there are never Hangul characters included. |
| */ |
| public static int writeIdenticalLevelRun(int prev, CharSequence s, int i, int length, ByteArrayWrapper sink) { |
| while (i < length) { |
| // We must have capacity>=SLOPE_MAX_BYTES in case writeDiff() writes that much, |
| // but we do not want to force the sink to allocate |
| // for a large min_capacity because we might actually only write one byte. |
| ensureAppendCapacity(sink, 16, s.length() * 2); |
| byte[] buffer = sink.bytes; |
| int capacity = buffer.length; |
| int p = sink.size; |
| int lastSafe = capacity - SLOPE_MAX_BYTES_; |
| while (i < length && p <= lastSafe) { |
| if (prev < 0x4e00 || prev >= 0xa000) { |
| prev = (prev & ~0x7f) - SLOPE_REACH_NEG_1_; |
| } else { |
| // Unihan U+4e00..U+9fa5: |
| // double-bytes down from the upper end |
| prev = 0x9fff - SLOPE_REACH_POS_2_; |
| } |
| |
| int c = Character.codePointAt(s, i); |
| i += Character.charCount(c); |
| if (c == 0xfffe) { |
| buffer[p++] = 2; // merge separator |
| prev = 0; |
| } else { |
| p = writeDiff(c - prev, buffer, p); |
| prev = c; |
| } |
| } |
| sink.size = p; |
| } |
| return prev; |
| } |
| |
| private static void ensureAppendCapacity(ByteArrayWrapper sink, int minCapacity, int desiredCapacity) { |
| int remainingCapacity = sink.bytes.length - sink.size; |
| if (remainingCapacity >= minCapacity) { return; } |
| if (desiredCapacity < minCapacity) { desiredCapacity = minCapacity; } |
| sink.ensureCapacity(sink.size + desiredCapacity); |
| } |
| |
| // private data members -------------------------------------------------- |
| |
| /** |
| * Do not use byte values 0, 1, 2 because they are separators in sort keys. |
| */ |
| private static final int SLOPE_MIN_ = 3; |
| private static final int SLOPE_MAX_ = 0xff; |
| private static final int SLOPE_MIDDLE_ = 0x81; |
| private static final int SLOPE_TAIL_COUNT_ = SLOPE_MAX_ - SLOPE_MIN_ + 1; |
| private static final int 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. |
| */ |
| private static final int SLOPE_SINGLE_ = 80; |
| private static final int SLOPE_LEAD_2_ = 42; |
| private static final int SLOPE_LEAD_3_ = 3; |
| //private static final int SLOPE_LEAD_4_ = 1; |
| |
| /** |
| * The difference value range for single-byters. |
| */ |
| private static final int SLOPE_REACH_POS_1_ = SLOPE_SINGLE_; |
| private static final int SLOPE_REACH_NEG_1_ = (-SLOPE_SINGLE_); |
| |
| /** |
| * The difference value range for double-byters. |
| */ |
| private static final int SLOPE_REACH_POS_2_ = |
| SLOPE_LEAD_2_ * SLOPE_TAIL_COUNT_ + SLOPE_LEAD_2_ - 1; |
| private static final int SLOPE_REACH_NEG_2_ = (-SLOPE_REACH_POS_2_ - 1); |
| |
| /** |
| * The difference value range for 3-byters. |
| */ |
| private static final int SLOPE_REACH_POS_3_ = SLOPE_LEAD_3_ |
| * SLOPE_TAIL_COUNT_ |
| * SLOPE_TAIL_COUNT_ |
| + (SLOPE_LEAD_3_ - 1) |
| * SLOPE_TAIL_COUNT_ + |
| (SLOPE_TAIL_COUNT_ - 1); |
| private static final int SLOPE_REACH_NEG_3_ = (-SLOPE_REACH_POS_3_ - 1); |
| |
| /** |
| * The lead byte start values. |
| */ |
| private static final int SLOPE_START_POS_2_ = SLOPE_MIDDLE_ |
| + SLOPE_SINGLE_ + 1; |
| private static final int SLOPE_START_POS_3_ = SLOPE_START_POS_2_ |
| + SLOPE_LEAD_2_; |
| private static final int SLOPE_START_NEG_2_ = SLOPE_MIDDLE_ + |
| SLOPE_REACH_NEG_1_; |
| private static final int SLOPE_START_NEG_3_ = SLOPE_START_NEG_2_ |
| - SLOPE_LEAD_2_; |
| |
| // private constructor --------------------------------------------------- |
| |
| /** |
| * Constructor private to prevent initialization |
| */ |
| ///CLOVER:OFF |
| private BOCSU() |
| { |
| } |
| ///CLOVER:ON |
| |
| // private methods ------------------------------------------------------- |
| |
| /** |
| * Integer division and modulo with negative numerators |
| * yields negative modulo results and quotients that are one more than |
| * what we need here. |
| * @param number which operations are to be performed on |
| * @param factor the factor to use for division |
| * @return (result of division) << 32 | modulo |
| */ |
| private static final long getNegDivMod(int number, int factor) |
| { |
| int modulo = number % factor; |
| long result = number / factor; |
| if (modulo < 0) { |
| -- result; |
| modulo += factor; |
| } |
| return (result << 32) | modulo; |
| } |
| |
| /** |
| * Encode one difference value -0x10ffff..+0x10ffff in 1..4 bytes, |
| * preserving lexical order |
| * @param diff |
| * @param buffer byte buffer to append to |
| * @param offset to the byte buffer to start appending |
| * @return end offset where the appending stops |
| */ |
| private static final int writeDiff(int diff, byte buffer[], int offset) |
| { |
| if (diff >= SLOPE_REACH_NEG_1_) { |
| if (diff <= SLOPE_REACH_POS_1_) { |
| buffer[offset ++] = (byte)(SLOPE_MIDDLE_ + diff); |
| } |
| else if (diff <= SLOPE_REACH_POS_2_) { |
| buffer[offset ++] = (byte)(SLOPE_START_POS_2_ |
| + (diff / SLOPE_TAIL_COUNT_)); |
| buffer[offset ++] = (byte)(SLOPE_MIN_ + |
| (diff % SLOPE_TAIL_COUNT_)); |
| } |
| else if (diff <= SLOPE_REACH_POS_3_) { |
| buffer[offset + 2] = (byte)(SLOPE_MIN_ |
| + (diff % SLOPE_TAIL_COUNT_)); |
| diff /= SLOPE_TAIL_COUNT_; |
| buffer[offset + 1] = (byte)(SLOPE_MIN_ |
| + (diff % SLOPE_TAIL_COUNT_)); |
| buffer[offset] = (byte)(SLOPE_START_POS_3_ |
| + (diff / SLOPE_TAIL_COUNT_)); |
| offset += 3; |
| } |
| else { |
| buffer[offset + 3] = (byte)(SLOPE_MIN_ |
| + diff % SLOPE_TAIL_COUNT_); |
| diff /= SLOPE_TAIL_COUNT_; |
| buffer[offset + 2] = (byte)(SLOPE_MIN_ |
| + diff % SLOPE_TAIL_COUNT_); |
| diff /= SLOPE_TAIL_COUNT_; |
| buffer[offset + 1] = (byte)(SLOPE_MIN_ |
| + diff % SLOPE_TAIL_COUNT_); |
| buffer[offset] = (byte)SLOPE_MAX_; |
| offset += 4; |
| } |
| } |
| else { |
| long division = getNegDivMod(diff, SLOPE_TAIL_COUNT_); |
| int modulo = (int)division; |
| if (diff >= SLOPE_REACH_NEG_2_) { |
| diff = (int)(division >> 32); |
| buffer[offset ++] = (byte)(SLOPE_START_NEG_2_ + diff); |
| buffer[offset ++] = (byte)(SLOPE_MIN_ + modulo); |
| } |
| else if (diff >= SLOPE_REACH_NEG_3_) { |
| buffer[offset + 2] = (byte)(SLOPE_MIN_ + modulo); |
| diff = (int)(division >> 32); |
| division = getNegDivMod(diff, SLOPE_TAIL_COUNT_); |
| modulo = (int)division; |
| diff = (int)(division >> 32); |
| buffer[offset + 1] = (byte)(SLOPE_MIN_ + modulo); |
| buffer[offset] = (byte)(SLOPE_START_NEG_3_ + diff); |
| offset += 3; |
| } |
| else { |
| buffer[offset + 3] = (byte)(SLOPE_MIN_ + modulo); |
| diff = (int)(division >> 32); |
| division = getNegDivMod(diff, SLOPE_TAIL_COUNT_); |
| modulo = (int)division; |
| diff = (int)(division >> 32); |
| buffer[offset + 2] = (byte)(SLOPE_MIN_ + modulo); |
| division = getNegDivMod(diff, SLOPE_TAIL_COUNT_); |
| modulo = (int)division; |
| buffer[offset + 1] = (byte)(SLOPE_MIN_ + modulo); |
| buffer[offset] = SLOPE_MIN_; |
| offset += 4; |
| } |
| } |
| return offset; |
| } |
| } |
