BOCSU.java
// © 2016 and later: Unicode, Inc. and others.
// License & terms of use: http://www.unicode.org/copyright.html
/*
*******************************************************************************
* 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;
/**
* Binary Ordered Compression Scheme for Unicode
*
* <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>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>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>Unlike a UTF encoding, BOCU-compressed text is not suitable for random access.
*
* <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>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>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>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>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>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.)
*
* @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
*
* <p>The sum must be <=SLOPE_TAIL_COUNT.
*
* <p>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.
*
* <p>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;
}
}