Package org.apache.poi.util
Class LZWDecompresser
java.lang.Object
org.apache.poi.util.LZWDecompresser
- Direct Known Subclasses:
CompressedRTF
,HDGFLZW
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/
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Constructor Summary
ConstructorsModifierConstructorDescriptionprotected
LZWDecompresser
(boolean maskMeansCompressed, int codeLengthIncrease, boolean positionIsBigEndian) -
Method Summary
Modifier and TypeMethodDescriptionprotected abstract int
adjustDictionaryOffset
(int offset) Adjusts the position offset if needed when looking something up in the dictionary.byte[]
decompress
(InputStream src) Decompresses the given input stream, returning the array of bytes of the decompressed input.void
decompress
(InputStream src, OutputStream res) Perform a streaming decompression of the input.static int
fromByte
(byte b) Given a java byte, turn it into an integer between 0 and 255 (i.e.static byte
fromInt
(int b) Given an integer, turn it into a java byte, handling the wrapping.protected abstract int
populateDictionary
(byte[] dict) Populates the dictionary, and returns where in it to begin writing new codes.
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Constructor Details
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LZWDecompresser
protected LZWDecompresser(boolean maskMeansCompressed, int codeLengthIncrease, boolean positionIsBigEndian)
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Method Details
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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
Decompresses the given input stream, returning the array of bytes of the decompressed input.- Throws:
IOException
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decompress
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:
IOException
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fromInt
public static byte fromInt(int b) Given an integer, turn it into a java byte, handling the wrapping. This is a convenience method -
fromByte
public static int fromByte(byte b) Given a java byte, turn it into an integer between 0 and 255 (i.e. handle the unwrapping). This is a convenience method
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