    001/*
    002 * Licensed to the Apache Software Foundation (ASF) under one
    003 * or more contributor license agreements.  See the NOTICE file
    004 * distributed with this work for additional information
    005 * regarding copyright ownership.  The ASF licenses this file
    006 * to you under the Apache License, Version 2.0 (the
    007 * "License"); you may not use this file except in compliance
    008 * with the License.  You may obtain a copy of the License at
    009 *
    010 * http://www.apache.org/licenses/LICENSE-2.0
    011 *
    012 * Unless required by applicable law or agreed to in writing,
    013 * software distributed under the License is distributed on an
    014 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
    015 * KIND, either express or implied.  See the License for the
    016 * specific language governing permissions and limitations
    017 * under the License.
    018 */
    019package org.apache.commons.compress.compressors.lz77support;
    020
    021import java.io.IOException;
    022import java.util.Arrays;
    023import java.util.Objects;
    024
    025/**
    026 * Helper class for compression algorithms that use the ideas of LZ77.
    027 *
    028 * <p>Most LZ77 derived algorithms split input data into blocks of
    029 * uncompressed data (called literal blocks) and back-references
    030 * (pairs of offsets and lengths) that state "add {@code length}
    031 * bytes that are the same as those already written starting
    032 * {@code offset} bytes before the current position. The details
    033 * of how those blocks and back-references are encoded are quite
    034 * different between the algorithms and some algorithms perform
    035 * additional steps (Huffman encoding in the case of DEFLATE for
    036 * example).</p>
    037 *
    038 * <p>This class attempts to extract the core logic - finding
    039 * back-references - so it can be re-used. It follows the algorithm
    040 * explained in section 4 of RFC 1951 (DEFLATE) and currently doesn't
    041 * implement the "lazy match" optimization. The three-byte hash
    042 * function used in this class is the same as the one used by zlib and
    043 * InfoZIP's ZIP implementation of DEFLATE. The whole class is
    044 * strongly inspired by InfoZIP's implementation.</p>
    045 *
    046 * <p>LZ77 is used vaguely here (as well as many other places that
    047 * talk about it :-), LZSS would likely be closer to the truth but
    048 * LZ77 has become the synonym for a whole family of algorithms.</p>
    049 *
    050 * <p>The API consists of a compressor that is fed {@code byte}s
    051 * and emits {@link Block}s to a registered callback where the blocks
    052 * represent either {@link LiteralBlock literal blocks}, {@link
    053 * BackReference back-references} or {@link EOD end of data
    054 * markers}. In order to ensure the callback receives all information,
    055 * the {@code #finish} method must be used once all data has been fed
    056 * into the compressor.</p>
    057 *
    058 * <p>Several parameters influence the outcome of the "compression":</p>
    059 * <dl>
    060 *
    061 *  <dt>{@code windowSize}</dt> <dd>the size of the sliding
    062 *  window, must be a power of two - this determines the maximum
    063 *  offset a back-reference can take. The compressor maintains a
    064 *  buffer of twice of {@code windowSize} - real world values are
    065 *  in the area of 32k.</dd>
    066 *
    067 *  <dt>{@code minBackReferenceLength}</dt>
    068 *  <dd>Minimal length of a back-reference found. A true minimum of 3 is
    069 *  hard-coded inside of this implementation but bigger lengths can be
    070 *  configured.</dd>
    071 *
    072 *  <dt>{@code maxBackReferenceLength}</dt>
    073 *  <dd>Maximal length of a back-reference found.</dd>
    074 *
    075 *  <dt>{@code maxOffset}</dt>
    076 *  <dd>Maximal offset of a back-reference.</dd>
    077 *
    078 *  <dt>{@code maxLiteralLength}</dt>
    079 *  <dd>Maximal length of a literal block.</dd>
    080 * </dl>
    081 *
    082 * @see "https://tools.ietf.org/html/rfc1951#section-4"
    083 * @since 1.14
    084 * @NotThreadSafe
    085 */
    086public class LZ77Compressor {
    087
    088    /**
    089     * Represents a back-reference.
    090     */
    091    public static final class BackReference extends Block {
    092        private final int offset, length;
    093        public BackReference(final int offset, final int length) {
    094            this.offset = offset;
    095            this.length = length;
    096        }
    097        /**
    098         * Provides the length of the back-reference.
    099         * @return the length
    100         */
    101        public int getLength() {
    102            return length;
    103        }
    104        /**
    105         * Provides the offset of the back-reference.
    106         * @return the offset
    107         */
    108        public int getOffset() {
    109            return offset;
    110        }
    111        @Override
    112        public BlockType getType() {
    113            return BlockType.BACK_REFERENCE;
    114        }
    115        @Override
    116        public String toString() {
    117            return "BackReference with offset " + offset + " and length " + length;
    118        }
    119    }
    120
    121    /**
    122     * Base class representing blocks the compressor may emit.
    123     *
    124     * <p>This class is not supposed to be subclassed by classes
    125     * outside of Commons Compress so it is considered internal and
    126     * changed that would break subclasses may get introduced with
    127     * future releases.</p>
    128     */
    129    public static abstract class Block {
    130        /** Enumeration of the block types the compressor may emit. */
    131        public enum BlockType {
    132            LITERAL, BACK_REFERENCE, EOD
    133        }
    134        public abstract BlockType getType();
    135    }
    136
    137    /**
    138     * Callback invoked while the compressor processes data.
    139     *
    140     * <p>The callback is invoked on the same thread that receives the
    141     * bytes to compress and may be invoked multiple times during the
    142     * execution of {@link #compress} or {@link #finish}.</p>
    143     */
    144    public interface Callback {
    145        /**
    146         * Consumes a block.
    147         * @param b the block to consume
    148         * @throws IOException in case of an error
    149         */
    150        void accept(Block b) throws IOException;
    151    }
    152
    153    /** A simple "we are done" marker. */
    154    public static final class EOD extends Block {
    155        @Override
    156        public BlockType getType() {
    157            return BlockType.EOD;
    158        }
    159    }
    160
    161    /**
    162     * Represents a literal block of data.
    163     *
    164     * <p>For performance reasons this encapsulates the real data, not
    165     * a copy of it. Don't modify the data and process it inside of
    166     * {@link Callback#accept} immediately as it will get overwritten
    167     * sooner or later.</p>
    168     */
    169    public static final class LiteralBlock extends Block {
    170        private final byte[] data;
    171        private final int offset, length;
    172        public LiteralBlock(final byte[] data, final int offset, final int length) {
    173            this.data = data;
    174            this.offset = offset;
    175            this.length = length;
    176        }
    177        /**
    178         * The literal data.
    179         *
    180         * <p>This returns a life view of the actual data in order to
    181         * avoid copying, modify the array at your own risk.</p>
    182         * @return the data
    183         */
    184        public byte[] getData() {
    185            return data;
    186        }
    187        /**
    188         * Length of literal block.
    189         * @return the length
    190         */
    191        public int getLength() {
    192            return length;
    193        }
    194        /**
    195         * Offset into data where the literal block starts.
    196         * @return the offset
    197         */
    198        public int getOffset() {
    199            return offset;
    200        }
    201        @Override
    202        public BlockType getType() {
    203            return BlockType.LITERAL;
    204        }
    205        @Override
    206        public String toString() {
    207            return "LiteralBlock starting at " + offset + " with length " + length;
    208        }
    209    }
    210
    211    private static final Block THE_EOD = new EOD();
    212
    213    static final int NUMBER_OF_BYTES_IN_HASH = 3;
    214    private static final int NO_MATCH = -1;
    215
    216    // we use a 15 bit hash code as calculated in updateHash
    217    private static final int HASH_SIZE = 1 << 15;
    218    private static final int HASH_MASK = HASH_SIZE - 1;
    219
    220    private static final int H_SHIFT = 5;
    221    private final Parameters params;
    222    private final Callback callback;
    223
    224    // the sliding window, twice as big as "windowSize" parameter
    225    private final byte[] window;
    226
    227    // the head of hash-chain - indexed by hash-code, points to the
    228    // location inside of window of the latest sequence of bytes with
    229    // the given hash.
    230    private final int[] head;
    231    // for each window-location points to the latest earlier location
    232    // with the same hash. Only stores values for the latest
    233    // "windowSize" elements, the index is "window location modulo
    234    // windowSize".
    235    private final int[] prev;
    236    // bit mask used when indexing into prev
    237    private final int wMask;
    238    private boolean initialized;
    239    // the position inside of window that shall be encoded right now
    240    private int currentPosition;
    241    // the number of bytes available to compress including the one at
    242    // currentPosition
    243    private int lookahead;
    244    // the hash of the three bytes stating at the current position
    245    private int insertHash;
    246
    247    // the position inside the window where the current literal
    248    // block starts (in case we are inside a literal block).
    249    private int blockStart;
    250
    251    // position of the current match
    252    private int matchStart = NO_MATCH;
    253
    254    // number of missed insertString calls for the up to three last
    255    // bytes of the last match that can only be performed once more
    256    // data has been read
    257    private int missedInserts;
    258
    259    /**
    260     * Initializes a compressor with parameters and a callback.
    261     * @param params the parameters
    262     * @param callback the callback
    263     * @throws NullPointerException if either parameter is {@code null}
    264     */
    265    public LZ77Compressor(final Parameters params, final Callback callback) {
    266        Objects.requireNonNull(params, "params");
    267        Objects.requireNonNull(callback, "callback");
    268
    269        this.params = params;
    270        this.callback = callback;
    271
    272        final int wSize = params.getWindowSize();
    273        window = new byte[wSize * 2];
    274        wMask = wSize - 1;
    275        head = new int[HASH_SIZE];
    276        Arrays.fill(head, NO_MATCH);
    277        prev = new int[wSize];
    278    }
    279
    280    private void catchUpMissedInserts() {
    281        while (missedInserts > 0) {
    282            insertString(currentPosition - missedInserts--);
    283        }
    284    }
    285
    286    private void compress() throws IOException {
    287        final int minMatch = params.getMinBackReferenceLength();
    288        final boolean lazy = params.getLazyMatching();
    289        final int lazyThreshold = params.getLazyMatchingThreshold();
    290
    291        while (lookahead >= minMatch) {
    292            catchUpMissedInserts();
    293            int matchLength = 0;
    294            final int hashHead = insertString(currentPosition);
    295            if (hashHead != NO_MATCH && hashHead - currentPosition <= params.getMaxOffset()) {
    296                // sets matchStart as a side effect
    297                matchLength = longestMatch(hashHead);
    298
    299                if (lazy && matchLength <= lazyThreshold && lookahead > minMatch) {
    300                    // try to find a longer match using the next position
    301                    matchLength = longestMatchForNextPosition(matchLength);
    302                }
    303            }
    304            if (matchLength >= minMatch) {
    305                if (blockStart != currentPosition) {
    306                    // emit preceding literal block
    307                    flushLiteralBlock();
    308                    blockStart = NO_MATCH;
    309                }
    310                flushBackReference(matchLength);
    311                insertStringsInMatch(matchLength);
    312                lookahead -= matchLength;
    313                currentPosition += matchLength;
    314                blockStart = currentPosition;
    315            } else {
    316                // no match, append to current or start a new literal
    317                lookahead--;
    318                currentPosition++;
    319                if (currentPosition - blockStart >= params.getMaxLiteralLength()) {
    320                    flushLiteralBlock();
    321                    blockStart = currentPosition;
    322                }
    323            }
    324        }
    325    }
    326    /**
    327     * Feeds bytes into the compressor which in turn may emit zero or
    328     * more blocks to the callback during the execution of this
    329     * method.
    330     * @param data the data to compress - must not be null
    331     * @throws IOException if the callback throws an exception
    332     */
    333    public void compress(final byte[] data) throws IOException {
    334        compress(data, 0, data.length);
    335    }
    336    /**
    337     * Feeds bytes into the compressor which in turn may emit zero or
    338     * more blocks to the callback during the execution of this
    339     * method.
    340     * @param data the data to compress - must not be null
    341     * @param off the start offset of the data
    342     * @param len the number of bytes to compress
    343     * @throws IOException if the callback throws an exception
    344     */
    345    public void compress(final byte[] data, int off, int len) throws IOException {
    346        final int wSize = params.getWindowSize();
    347        while (len > wSize) { // chop into windowSize sized chunks
    348            doCompress(data, off, wSize);
    349            off += wSize;
    350            len -= wSize;
    351        }
    352        if (len > 0) {
    353            doCompress(data, off, len);
    354        }
    355    }
    356
    357    // performs the actual algorithm with the pre-condition len <= windowSize
    358    private void doCompress(final byte[] data, final int off, final int len) throws IOException {
    359        final int spaceLeft = window.length - currentPosition - lookahead;
    360        if (len > spaceLeft) {
    361            slide();
    362        }
    363        System.arraycopy(data, off, window, currentPosition + lookahead, len);
    364        lookahead += len;
    365        if (!initialized && lookahead >= params.getMinBackReferenceLength()) {
    366            initialize();
    367        }
    368        if (initialized) {
    369            compress();
    370        }
    371    }
    372
    373    /**
    374     * Tells the compressor to process all remaining data and signal
    375     * end of data to the callback.
    376     *
    377     * <p>The compressor will in turn emit at least one block ({@link
    378     * EOD}) but potentially multiple blocks to the callback during
    379     * the execution of this method.</p>
    380     * @throws IOException if the callback throws an exception
    381     */
    382    public void finish() throws IOException {
    383        if (blockStart != currentPosition || lookahead > 0) {
    384            currentPosition += lookahead;
    385            flushLiteralBlock();
    386        }
    387        callback.accept(THE_EOD);
    388    }
    389
    390    private void flushBackReference(final int matchLength) throws IOException {
    391        callback.accept(new BackReference(currentPosition - matchStart, matchLength));
    392    }
    393
    394    private void flushLiteralBlock() throws IOException {
    395        callback.accept(new LiteralBlock(window, blockStart, currentPosition - blockStart));
    396    }
    397
    398    private void initialize() {
    399        for (int i = 0; i < NUMBER_OF_BYTES_IN_HASH - 1; i++) {
    400            insertHash = nextHash(insertHash, window[i]);
    401        }
    402        initialized = true;
    403    }
    404
    405    /**
    406     * Inserts the current three byte sequence into the dictionary and
    407     * returns the previous head of the hash-chain.
    408     *
    409     * <p>Updates {@code insertHash} and {@code prev} as a
    410     * side effect.</p>
    411     */
    412    private int insertString(final int pos) {
    413        insertHash = nextHash(insertHash, window[pos - 1 + NUMBER_OF_BYTES_IN_HASH]);
    414        final int hashHead = head[insertHash];
    415        prev[pos & wMask] = hashHead;
    416        head[insertHash] = pos;
    417        return hashHead;
    418    }
    419
    420    private void insertStringsInMatch(final int matchLength) {
    421        // inserts strings contained in current match
    422        // insertString inserts the byte 2 bytes after position, which may not yet be available -> missedInserts
    423        final int stop = Math.min(matchLength - 1, lookahead - NUMBER_OF_BYTES_IN_HASH);
    424        // currentPosition has been inserted already
    425        for (int i = 1; i <= stop; i++) {
    426            insertString(currentPosition + i);
    427        }
    428        missedInserts = matchLength - stop - 1;
    429    }
    430
    431    /**
    432     * Searches the hash chain for real matches and returns the length
    433     * of the longest match (0 if none were found) that isn't too far
    434     * away (WRT maxOffset).
    435     *
    436     * <p>Sets matchStart to the index of the start position of the
    437     * longest match as a side effect.</p>
    438     */
    439    private int longestMatch(int matchHead) {
    440        final int minLength = params.getMinBackReferenceLength();
    441        int longestMatchLength = minLength - 1;
    442        final int maxPossibleLength = Math.min(params.getMaxBackReferenceLength(), lookahead);
    443        final int minIndex = Math.max(0, currentPosition - params.getMaxOffset());
    444        final int niceBackReferenceLength = Math.min(maxPossibleLength, params.getNiceBackReferenceLength());
    445        final int maxCandidates = params.getMaxCandidates();
    446        for (int candidates = 0; candidates < maxCandidates && matchHead >= minIndex; candidates++) {
    447            int currentLength = 0;
    448            for (int i = 0; i < maxPossibleLength; i++) {
    449                if (window[matchHead + i] != window[currentPosition + i]) {
    450                    break;
    451                }
    452                currentLength++;
    453            }
    454            if (currentLength > longestMatchLength) {
    455                longestMatchLength = currentLength;
    456                matchStart = matchHead;
    457                if (currentLength >= niceBackReferenceLength) {
    458                    // no need to search any further
    459                    break;
    460                }
    461            }
    462            matchHead = prev[matchHead & wMask];
    463        }
    464        return longestMatchLength; // < minLength if no matches have been found, will be ignored in compress()
    465    }
    466
    467    private int longestMatchForNextPosition(final int prevMatchLength) {
    468        // save a bunch of values to restore them if the next match isn't better than the current one
    469        final int prevMatchStart = matchStart;
    470        final int prevInsertHash = insertHash;
    471
    472        lookahead--;
    473        currentPosition++;
    474        final int hashHead = insertString(currentPosition);
    475        final int prevHashHead = prev[currentPosition & wMask];
    476        int matchLength = longestMatch(hashHead);
    477
    478        if (matchLength <= prevMatchLength) {
    479            // use the first match, as the next one isn't any better
    480            matchLength = prevMatchLength;
    481            matchStart = prevMatchStart;
    482
    483            // restore modified values
    484            head[insertHash] = prevHashHead;
    485            insertHash = prevInsertHash;
    486            currentPosition--;
    487            lookahead++;
    488        }
    489        return matchLength;
    490    }
    491
    492    /**
    493     * Assumes we are calculating the hash for three consecutive bytes
    494     * as a rolling hash, i.e. for bytes ABCD if H is the hash of ABC
    495     * the new hash for BCD is nextHash(H, D).
    496     *
    497     * <p>The hash is shifted by five bits on each update so all
    498     * effects of A have been swapped after the third update.</p>
    499     */
    500    private int nextHash(final int oldHash, final byte nextByte) {
    501        final int nextVal = nextByte & 0xFF;
    502        return ((oldHash << H_SHIFT) ^ nextVal) & HASH_MASK;
    503    }
    504
    505    /**
    506     * Adds some initial data to fill the window with.
    507     *
    508     * <p>This is used if the stream has been cut into blocks and
    509     * back-references of one block may refer to data of the previous
    510     * block(s). One such example is the LZ4 frame format using block
    511     * dependency.</p>
    512     *
    513     * @param data the data to fill the window with.
    514     * @throws IllegalStateException if the compressor has already started to accept data
    515     */
    516    public void prefill(final byte[] data) {
    517        if (currentPosition != 0 || lookahead != 0) {
    518            throw new IllegalStateException("The compressor has already started to accept data, can't prefill anymore");
    519        }
    520
    521        // don't need more than windowSize for back-references
    522        final int len = Math.min(params.getWindowSize(), data.length);
    523        System.arraycopy(data, data.length - len, window, 0, len);
    524
    525        if (len >= NUMBER_OF_BYTES_IN_HASH) {
    526            initialize();
    527            final int stop = len - NUMBER_OF_BYTES_IN_HASH + 1;
    528            for (int i = 0; i < stop; i++) {
    529                insertString(i);
    530            }
    531            missedInserts = NUMBER_OF_BYTES_IN_HASH - 1;
    532        } else { // not enough data to hash anything
    533            missedInserts = len;
    534        }
    535        blockStart = currentPosition = len;
    536    }
    537
    538    private void slide() throws IOException {
    539        final int wSize = params.getWindowSize();
    540        if (blockStart != currentPosition && blockStart < wSize) {
    541            flushLiteralBlock();
    542            blockStart = currentPosition;
    543        }
    544        System.arraycopy(window, wSize, window, 0, wSize);
    545        currentPosition -= wSize;
    546        matchStart -= wSize;
    547        blockStart -= wSize;
    548        for (int i = 0; i < HASH_SIZE; i++) {
    549            final int h = head[i];
    550            head[i] = h >= wSize ? h - wSize : NO_MATCH;
    551        }
    552        for (int i = 0; i < wSize; i++) {
    553            final int p = prev[i];
    554            prev[i] = p >= wSize ? p - wSize : NO_MATCH;
    555        }
    556    }
    557}