org.apache.poi.util

Class LZWDecompresser

java.lang.Object

org.apache.poi.util.LZWDecompresser

Direct Known Subclasses:

CompressedRTF, HDGFLZW

public abstract class LZWDecompresser
extends java.lang.Object

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/

Field Summary

Fields

Modifier and Type	Field and Description
static int	DICT_MASK the mask for calculating / wrapping dictionary offsets
static int	DICT_SIZE the size of our dictionary

Constructor Summary

Constructors

Modifier	Constructor and Description
protected	LZWDecompresser(boolean maskMeansCompressed, int codeLengthIncrease, boolean positionIsBigEndian)

Method Summary

Modifier and Type	Method and Description
protected abstract int	adjustDictionaryOffset(int offset) Adjusts the position offset if needed when looking something up in the dictionary.
byte[]	decompress(java.io.InputStream src) Decompresses the given input stream, returning the array of bytes of the decompressed input.
void	decompress(java.io.InputStream src, java.io.OutputStream res) Perform a streaming decompression of the input.
protected abstract int	populateDictionary(byte[] dict) Populates the dictionary, and returns where in it to begin writing new codes.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Field Detail

DICT_SIZE

public static final int DICT_SIZE

the size of our dictionary

See Also:

Constant Field Values

DICT_MASK

public static final int DICT_MASK

the mask for calculating / wrapping dictionary offsets

See Also:

Constant Field Values

Constructor Detail

LZWDecompresser

protected LZWDecompresser(boolean maskMeansCompressed,
                          int codeLengthIncrease,
                          boolean positionIsBigEndian)

Method Detail

populateDictionary

protected abstract int populateDictionary(byte[] dict)

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.

adjustDictionaryOffset

protected abstract int adjustDictionaryOffset(int offset)

Adjusts the position offset if needed when looking something up in the dictionary.

decompress

public byte[] decompress(java.io.InputStream src)
                  throws java.io.IOException

Decompresses the given input stream, returning the array of bytes of the decompressed input.

Throws:

java.io.IOException

decompress

public void decompress(java.io.InputStream src,
                       java.io.OutputStream res)
                throws java.io.IOException

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

Throws:

java.io.IOException