org.apache.poi.util.LZWDecompresser Maven / Gradle / Ivy
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this work for additional information regarding copyright ownership.
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(the "License"); you may not use this file except in compliance with
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http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
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package org.apache.poi.util;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import org.apache.commons.io.output.UnsynchronizedByteArrayOutputStream;
/**
* This class provides common functionality for the
* various LZW implementations in the different file
* formats.
* It's currently used by HDGF and HMEF.
*
* Two good resources on LZW are:
* http://en.wikipedia.org/wiki/LZW
* http://marknelson.us/1989/10/01/lzw-data-compression/
*/
public abstract class LZWDecompresser {
/** the size of our dictionary */
public static final int DICT_SIZE = 0x1000;
/** the mask for calculating / wrapping dictionary offsets */
public static final int DICT_MASK = 0xFFF;
//arbitrarily selected; may need to increase
private static final int DEFAULT_MAX_RECORD_LENGTH = 1_000_000;
private static int MAX_RECORD_LENGTH = DEFAULT_MAX_RECORD_LENGTH;
/**
* Does the mask bit mean it's compressed or uncompressed?
*/
private final boolean maskMeansCompressed;
/**
* How much to append to the code length in the stream
* to get the real code length? Normally 2 or 3
*/
private final int codeLengthIncrease;
/**
* Does the 12 bits of the position get stored in
* Little Endian or Big Endian form?
* This controls whether a pos+length of 0x12 0x34
* becomes a position of 0x123 or 0x312
*/
private final boolean positionIsBigEndian;
/**
* @param length the max record length allowed for LZWDecompresser
*/
public static void setMaxRecordLength(int length) {
MAX_RECORD_LENGTH = length;
}
/**
* @return the max record length allowed for LZWDecompresser
*/
public static int getMaxRecordLength() {
return MAX_RECORD_LENGTH;
}
protected LZWDecompresser(boolean maskMeansCompressed,
int codeLengthIncrease, boolean positionIsBigEndian) {
this.maskMeansCompressed = maskMeansCompressed;
this.codeLengthIncrease = codeLengthIncrease;
this.positionIsBigEndian = positionIsBigEndian;
}
/**
* Populates the dictionary, and returns where in it
* to begin writing new codes.
* Generally, if the dictionary is pre-populated, then new
* codes should be placed at the end of that block.
* Equally, if the dictionary is left with all zeros, then
* usually the new codes can go in at the start.
*/
protected abstract int populateDictionary(byte[] dict);
/**
* Adjusts the position offset if needed when looking
* something up in the dictionary.
*/
protected abstract int adjustDictionaryOffset(int offset);
/**
* Decompresses the given input stream, returning the array of bytes
* of the decompressed input.
*/
public byte[] decompress(InputStream src) throws IOException {
UnsynchronizedByteArrayOutputStream res = new UnsynchronizedByteArrayOutputStream();
decompress(src, res);
return res.toByteArray();
}
/**
* Perform a streaming decompression of the input.
* Works by:
* 1) Reading a flag byte, the 8 bits of which tell you if the
* following 8 codes are compressed our un-compressed
* 2) Consider the 8 bits in turn
* 3) If the bit is set, the next code is un-compressed, so
* add it to the dictionary and output it
* 4) If the bit isn't set, then read in the length and start
* position in the dictionary, and output the bytes there
* 5) Loop until we've done all 8 bits, then read in the next
* flag byte
*/
public void decompress(InputStream src, OutputStream res) throws IOException {
// How far through the output we've got
// (This is normally used &4095, so it nicely wraps)
// The initial value is set when populating the dictionary
int pos;
// The flag byte is treated as its 8 individual
// bits, which tell us if the following 8 codes
// are compressed or un-compressed
int flag;
// The mask, between 1 and 255, which is used when
// processing each bit of the flag byte in turn
int mask;
// We use 12 bit codes:
// * 0-255 are real bytes
// * 256-4095 are the substring codes
// Java handily initialises our buffer / dictionary
// to all zeros
final byte[] buffer = new byte[DICT_SIZE];
pos = populateDictionary(buffer);
// These are bytes as looked up in the dictionary
// It needs to be signed, as it'll get passed on to
// the output stream
final byte[] dataB = IOUtils.safelyAllocate(16L + codeLengthIncrease, MAX_RECORD_LENGTH);
// This is an unsigned byte read from the stream
// It needs to be unsigned, so that bit stuff works
int dataI;
// The compressed code sequence is held over 2 bytes
int dataIPt1, dataIPt2;
// How long a code sequence is, and where in the
// dictionary to start at
int len, pntr;
while ((flag = src.read()) != -1) {
// Compare each bit in our flag byte in turn:
for (mask = 1; mask < 0x100; mask <<= 1) {
// Is this a new code (un-compressed), or
// the use of existing codes (compressed)?
boolean isMaskSet = (flag & mask) > 0;
if (isMaskSet ^ maskMeansCompressed) {
// Retrieve the un-compressed code
if ((dataI = src.read()) != -1) {
// Save the byte into the dictionary
buffer[pos++ & DICT_MASK] = (byte) dataI;
// And output the byte
res.write(dataI);
}
} else {
// We have a compressed sequence
// Grab the next 16 bits of data
dataIPt1 = src.read();
dataIPt2 = src.read();
if (dataIPt1 == -1 || dataIPt2 == -1) break;
// Build up how long the code sequence is, and
// what position of the code to start at
// (The position is the usually the first 12 bits,
// and the length is usually the last 4 bits)
len = (dataIPt2 & 0x0F) + codeLengthIncrease;
if (positionIsBigEndian) {
pntr = (dataIPt1 << 4) + (dataIPt2 >>> 4);
} else {
pntr = dataIPt1 + ((dataIPt2 & 0xF0) << 4);
}
// Adjust the pointer as needed
pntr = adjustDictionaryOffset(pntr);
// Loop over the codes, outputting what they correspond to
for (int i = 0; i < len; i++) {
dataB[i] = buffer[(pntr + i) & DICT_MASK];
buffer[(pos + i) & DICT_MASK] = dataB[i];
}
res.write(dataB, 0, len);
// Record how far along the stream we have moved
pos += len;
}
}
}
}
}