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Apache Commons Compress software defines an API for working with
compression and archive formats. These include: bzip2, gzip, pack200,
lzma, xz, Snappy, traditional Unix Compress, DEFLATE, DEFLATE64, LZ4,
Brotli, Zstandard and ar, cpio, jar, tar, zip, dump, 7z, arj.
/*
* Licensed to the Apache Software Foundation (ASF) under one
* or more contributor license agreements. See the NOTICE file
* distributed with this work for additional information
* regarding copyright ownership. The ASF licenses this file
* to you under the Apache License, Version 2.0 (the
* "License"); you may not use this file except in compliance
* with the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing,
* software distributed under the License is distributed on an
* "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
* KIND, either express or implied. See the License for the
* specific language governing permissions and limitations
* under the License.
*/
package org.apache.commons.compress.compressors.lz77support;
import java.io.IOException;
import java.util.Arrays;
import java.util.Objects;
/**
* Helper class for compression algorithms that use the ideas of LZ77.
*
* Most LZ77 derived algorithms split input data into blocks of
* uncompressed data (called literal blocks) and back-references
* (pairs of offsets and lengths) that state "add length
* bytes that are the same as those already written starting
* offset bytes before the current position. The details
* of how those blocks and back-references are encoded are quite
* different between the algorithms and some algorithms perform
* additional steps (Huffman encoding in the case of DEFLATE for
* example).
*
* This class attempts to extract the core logic - finding
* back-references - so it can be re-used. It follows the algorithm
* explained in section 4 of RFC 1951 (DEFLATE) and currently doesn't
* implement the "lazy match" optimization. The three-byte hash
* function used in this class is the same as the one used by zlib and
* InfoZIP's ZIP implementation of DEFLATE. The whole class is
* strongly inspired by InfoZIP's implementation.
*
* LZ77 is used vaguely here (as well as many other places that
* talk about it :-), LZSS would likely be closer to the truth but
* LZ77 has become the synonym for a whole family of algorithms.
*
* The API consists of a compressor that is fed bytes
* and emits {@link Block}s to a registered callback where the blocks
* represent either {@link LiteralBlock literal blocks}, {@link
* BackReference back-references} or {@link EOD end of data
* markers}. In order to ensure the callback receives all information,
* the {@code #finish} method must be used once all data has been fed
* into the compressor.
*
* Several parameters influence the outcome of the "compression":
*
*
* windowSize- the size of the sliding
* window, must be a power of two - this determines the maximum
* offset a back-reference can take. The compressor maintains a
* buffer of twice of
windowSize - real world values are
* in the area of 32k.
*
* minBackReferenceLength- Minimal length of a back-reference found. A true minimum of 3 is
* hard-coded inside of this implemention but bigger lengths can be
* configured.
*
* maxBackReferenceLength- Maximal length of a back-reference found.
*
* maxOffset- Maximal offset of a back-reference.
*
* maxLiteralLength- Maximal length of a literal block.
*
*
* @see "https://tools.ietf.org/html/rfc1951#section-4"
* @since 1.14
* @NotThreadSafe
*/
public class LZ77Compressor {
/**
* Base class representing blocks the compressor may emit.
*
* This class is not supposed to be subclassed by classes
* outside of Commons Compress so it is considered internal and
* changed that would break subclasses may get introduced with
* future releases.
*/
public static abstract class Block {
/** Enumeration of the block types the compressor may emit. */
public enum BlockType {
LITERAL, BACK_REFERENCE, EOD
}
public abstract BlockType getType();
}
/**
* Represents a literal block of data.
*
* For performance reasons this encapsulates the real data, not
* a copy of it. Don't modify the data and process it inside of
* {@link Callback#accept} immediately as it will get overwritten
* sooner or later.
*/
public static final class LiteralBlock extends Block {
private final byte[] data;
private final int offset, length;
public LiteralBlock(byte[] data, int offset, int length) {
this.data = data;
this.offset = offset;
this.length = length;
}
/**
* The literal data.
*
* This returns a life view of the actual data in order to
* avoid copying, modify the array at your own risk.
* @return the data
*/
public byte[] getData() {
return data;
}
/**
* Offset into data where the literal block starts.
* @return the offset
*/
public int getOffset() {
return offset;
}
/**
* Length of literal block.
* @return the length
*/
public int getLength() {
return length;
}
@Override
public BlockType getType() {
return BlockType.LITERAL;
}
@Override
public String toString() {
return "LiteralBlock starting at " + offset + " with length " + length;
}
}
/**
* Represents a back-reference.
*/
public static final class BackReference extends Block {
private final int offset, length;
public BackReference(int offset, int length) {
this.offset = offset;
this.length = length;
}
/**
* Provides the offset of the back-reference.
* @return the offset
*/
public int getOffset() {
return offset;
}
/**
* Provides the length of the back-reference.
* @return the length
*/
public int getLength() {
return length;
}
@Override
public BlockType getType() {
return BlockType.BACK_REFERENCE;
}
@Override
public String toString() {
return "BackReference with offset " + offset + " and length " + length;
}
}
/** A simple "we are done" marker. */
public static final class EOD extends Block {
@Override
public BlockType getType() {
return BlockType.EOD;
}
}
private static final Block THE_EOD = new EOD();
/**
* Callback invoked while the compressor processes data.
*
* The callback is invoked on the same thread that receives the
* bytes to compress and may be invoked multiple times during the
* execution of {@link #compress} or {@link #finish}.
*/
public interface Callback {
/**
* Consumes a block.
* @param b the block to consume
* @throws IOException in case of an error
*/
void accept(Block b) throws IOException;
}
static final int NUMBER_OF_BYTES_IN_HASH = 3;
private static final int NO_MATCH = -1;
private final Parameters params;
private final Callback callback;
// the sliding window, twice as big as "windowSize" parameter
private final byte[] window;
// the head of hash-chain - indexed by hash-code, points to the
// location inside of window of the latest sequence of bytes with
// the given hash.
private final int[] head;
// for each window-location points to the latest earlier location
// with the same hash. Only stores values for the latest
// "windowSize" elements, the index is "window location modulo
// windowSize".
private final int[] prev;
// bit mask used when indexing into prev
private final int wMask;
private boolean initialized = false;
// the position inside of window that shall be encoded right now
private int currentPosition;
// the number of bytes available to compress including the one at
// currentPosition
private int lookahead = 0;
// the hash of the three bytes stating at the current position
private int insertHash = 0;
// the position inside of the window where the current literal
// block starts (in case we are inside of a literal block).
private int blockStart = 0;
// position of the current match
private int matchStart = NO_MATCH;
// number of missed insertString calls for the up to three last
// bytes of the last match that can only be performed once more
// data has been read
private int missedInserts = 0;
/**
* Initializes a compressor with parameters and a callback.
* @param params the parameters
* @param callback the callback
* @throws NullPointerException if either parameter is null
*/
public LZ77Compressor(Parameters params, Callback callback) {
Objects.requireNonNull(params, "params");
Objects.requireNonNull(callback, "callback");
this.params = params;
this.callback = callback;
final int wSize = params.getWindowSize();
window = new byte[wSize * 2];
wMask = wSize - 1;
head = new int[HASH_SIZE];
Arrays.fill(head, NO_MATCH);
prev = new int[wSize];
}
/**
* Feeds bytes into the compressor which in turn may emit zero or
* more blocks to the callback during the execution of this
* method.
* @param data the data to compress - must not be null
* @throws IOException if the callback throws an exception
*/
public void compress(byte[] data) throws IOException {
compress(data, 0, data.length);
}
/**
* Feeds bytes into the compressor which in turn may emit zero or
* more blocks to the callback during the execution of this
* method.
* @param data the data to compress - must not be null
* @param off the start offset of the data
* @param len the number of bytes to compress
* @throws IOException if the callback throws an exception
*/
public void compress(byte[] data, int off, int len) throws IOException {
final int wSize = params.getWindowSize();
while (len > wSize) { // chop into windowSize sized chunks
doCompress(data, off, wSize);
off += wSize;
len -= wSize;
}
if (len > 0) {
doCompress(data, off, len);
}
}
/**
* Tells the compressor to process all remaining data and signal
* end of data to the callback.
*
* The compressor will in turn emit at least one block ({@link
* EOD}) but potentially multiple blocks to the callback during
* the execution of this method.
* @throws IOException if the callback throws an exception
*/
public void finish() throws IOException {
if (blockStart != currentPosition || lookahead > 0) {
currentPosition += lookahead;
flushLiteralBlock();
}
callback.accept(THE_EOD);
}
/**
* Adds some initial data to fill the window with.
*
* This is used if the stream has been cut into blocks and
* back-references of one block may refer to data of the previous
* block(s). One such example is the LZ4 frame format using block
* dependency.
*
* @param data the data to fill the window with.
* @throws IllegalStateException if the compressor has already started to accept data
*/
public void prefill(byte[] data) {
if (currentPosition != 0 || lookahead != 0) {
throw new IllegalStateException("The compressor has already started to accept data, can't prefill anymore");
}
// don't need more than windowSize for back-references
final int len = Math.min(params.getWindowSize(), data.length);
System.arraycopy(data, data.length - len, window, 0, len);
if (len >= NUMBER_OF_BYTES_IN_HASH) {
initialize();
final int stop = len - NUMBER_OF_BYTES_IN_HASH + 1;
for (int i = 0; i < stop; i++) {
insertString(i);
}
missedInserts = NUMBER_OF_BYTES_IN_HASH - 1;
} else { // not enough data to hash anything
missedInserts = len;
}
blockStart = currentPosition = len;
}
// we use a 15 bit hashcode as calculated in updateHash
private static final int HASH_SIZE = 1 << 15;
private static final int HASH_MASK = HASH_SIZE - 1;
private static final int H_SHIFT = 5;
/**
* Assumes we are calculating the hash for three consecutive bytes
* as a rolling hash, i.e. for bytes ABCD if H is the hash of ABC
* the new hash for BCD is nextHash(H, D).
*
* The hash is shifted by five bits on each update so all
* effects of A have been swapped after the third update.
*/
private int nextHash(int oldHash, byte nextByte) {
final int nextVal = nextByte & 0xFF;
return ((oldHash << H_SHIFT) ^ nextVal) & HASH_MASK;
}
// performs the actual algorithm with the pre-condition len <= windowSize
private void doCompress(byte[] data, int off, int len) throws IOException {
int spaceLeft = window.length - currentPosition - lookahead;
if (len > spaceLeft) {
slide();
}
System.arraycopy(data, off, window, currentPosition + lookahead, len);
lookahead += len;
if (!initialized && lookahead >= params.getMinBackReferenceLength()) {
initialize();
}
if (initialized) {
compress();
}
}
private void slide() throws IOException {
final int wSize = params.getWindowSize();
if (blockStart != currentPosition && blockStart < wSize) {
flushLiteralBlock();
blockStart = currentPosition;
}
System.arraycopy(window, wSize, window, 0, wSize);
currentPosition -= wSize;
matchStart -= wSize;
blockStart -= wSize;
for (int i = 0; i < HASH_SIZE; i++) {
int h = head[i];
head[i] = h >= wSize ? h - wSize : NO_MATCH;
}
for (int i = 0; i < wSize; i++) {
int p = prev[i];
prev[i] = p >= wSize ? p - wSize : NO_MATCH;
}
}
private void initialize() {
for (int i = 0; i < NUMBER_OF_BYTES_IN_HASH - 1; i++) {
insertHash = nextHash(insertHash, window[i]);
}
initialized = true;
}
private void compress() throws IOException {
final int minMatch = params.getMinBackReferenceLength();
final boolean lazy = params.getLazyMatching();
final int lazyThreshold = params.getLazyMatchingThreshold();
while (lookahead >= minMatch) {
catchUpMissedInserts();
int matchLength = 0;
int hashHead = insertString(currentPosition);
if (hashHead != NO_MATCH && hashHead - currentPosition <= params.getMaxOffset()) {
// sets matchStart as a side effect
matchLength = longestMatch(hashHead);
if (lazy && matchLength <= lazyThreshold && lookahead > minMatch) {
// try to find a longer match using the next position
matchLength = longestMatchForNextPosition(matchLength);
}
}
if (matchLength >= minMatch) {
if (blockStart != currentPosition) {
// emit preceeding literal block
flushLiteralBlock();
blockStart = NO_MATCH;
}
flushBackReference(matchLength);
insertStringsInMatch(matchLength);
lookahead -= matchLength;
currentPosition += matchLength;
blockStart = currentPosition;
} else {
// no match, append to current or start a new literal
lookahead--;
currentPosition++;
if (currentPosition - blockStart >= params.getMaxLiteralLength()) {
flushLiteralBlock();
blockStart = currentPosition;
}
}
}
}
/**
* Inserts the current three byte sequence into the dictionary and
* returns the previous head of the hash-chain.
*
* Updates insertHash and prev as a
* side effect.
*/
private int insertString(int pos) {
insertHash = nextHash(insertHash, window[pos - 1 + NUMBER_OF_BYTES_IN_HASH]);
int hashHead = head[insertHash];
prev[pos & wMask] = hashHead;
head[insertHash] = pos;
return hashHead;
}
private int longestMatchForNextPosition(final int prevMatchLength) {
// save a bunch of values to restore them if the next match isn't better than the current one
final int prevMatchStart = matchStart;
final int prevInsertHash = insertHash;
lookahead--;
currentPosition++;
int hashHead = insertString(currentPosition);
final int prevHashHead = prev[currentPosition & wMask];
int matchLength = longestMatch(hashHead);
if (matchLength <= prevMatchLength) {
// use the first match, as the next one isn't any better
matchLength = prevMatchLength;
matchStart = prevMatchStart;
// restore modified values
head[insertHash] = prevHashHead;
insertHash = prevInsertHash;
currentPosition--;
lookahead++;
}
return matchLength;
}
private void insertStringsInMatch(int matchLength) {
// inserts strings contained in current match
// insertString inserts the byte 2 bytes after position, which may not yet be available -> missedInserts
final int stop = Math.min(matchLength - 1, lookahead - NUMBER_OF_BYTES_IN_HASH);
// currentPosition has been inserted already
for (int i = 1; i <= stop; i++) {
insertString(currentPosition + i);
}
missedInserts = matchLength - stop - 1;
}
private void catchUpMissedInserts() {
while (missedInserts > 0) {
insertString(currentPosition - missedInserts--);
}
}
private void flushBackReference(int matchLength) throws IOException {
callback.accept(new BackReference(currentPosition - matchStart, matchLength));
}
private void flushLiteralBlock() throws IOException {
callback.accept(new LiteralBlock(window, blockStart, currentPosition - blockStart));
}
/**
* Searches the hash chain for real matches and returns the length
* of the longest match (0 if none were found) that isn't too far
* away (WRT maxOffset).
*
* Sets matchStart to the index of the start position of the
* longest match as a side effect.
*/
private int longestMatch(int matchHead) {
final int minLength = params.getMinBackReferenceLength();
int longestMatchLength = minLength - 1;
final int maxPossibleLength = Math.min(params.getMaxBackReferenceLength(), lookahead);
final int minIndex = Math.max(0, currentPosition - params.getMaxOffset());
final int niceBackReferenceLength = Math.min(maxPossibleLength, params.getNiceBackReferenceLength());
final int maxCandidates = params.getMaxCandidates();
for (int candidates = 0; candidates < maxCandidates && matchHead >= minIndex; candidates++) {
int currentLength = 0;
for (int i = 0; i < maxPossibleLength; i++) {
if (window[matchHead + i] != window[currentPosition + i]) {
break;
}
currentLength++;
}
if (currentLength > longestMatchLength) {
longestMatchLength = currentLength;
matchStart = matchHead;
if (currentLength >= niceBackReferenceLength) {
// no need to search any further
break;
}
}
matchHead = prev[matchHead & wMask];
}
return longestMatchLength; // < minLength if no matches have been found, will be ignored in compress()
}
}