Package org.apache.poi.util

Class LZWDecompresser

java.lang.Object
org.apache.poi.util.LZWDecompresser
Direct Known Subclasses:
CompressedRTF, HDGFLZW
public abstract class LZWDecompresser extends 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
    Description
    static final int
    DICT_MASK
    the mask for calculating / wrapping dictionary offsets
    static final int
    DICT_SIZE
    the size of our dictionary

  • Constructor Summary

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

  • Method Summary

    Modifier and Type
    Method
    Description
    protected 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
    getMaxRecordLength()
     
    protected abstract int
    populateDictionary(byte[] dict)
    Populates the dictionary, and returns where in it to begin writing new codes.
    static void
    setMaxRecordLength(int length)
     

    Methods inherited from class java.lang.Object

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

  • Field Details

    • DICT_SIZE

      public static final int DICT_SIZE
      the size of our dictionary
      See Also:

    • DICT_MASK

      public static final int DICT_MASK
      the mask for calculating / wrapping dictionary offsets
      See Also:

  • Constructor Details

    • LZWDecompresser

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

  • Method Details

    • setMaxRecordLength

      public static void setMaxRecordLength(int length)
      Parameters:
      length - the max record length allowed for LZWDecompresser

    • getMaxRecordLength

      public static int getMaxRecordLength()
      Returns:
      the max record length allowed for LZWDecompresser

    • 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(InputStream src) throws IOException
      Decompresses the given input stream, returning the array of bytes of the decompressed input.
      Throws:
      IOException

    • decompress

      public void decompress(InputStream src, OutputStream res) throws 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:
      IOException