Package org.apache.poi.util

Class LZWDecompresser

  • 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 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 Description
      protected LZWDecompresser​(boolean maskMeansCompressed, int codeLengthIncrease, boolean positionIsBigEndian)  

    • Method Summary

      All Methods Static Methods Instance Methods Abstract Methods Concrete Methods 
      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​(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.
      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 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

      • 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​(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