Eric Waters

NAME

Compress::LZW::Progressive - Progressive LZW-like compression

SYNOPSIS

  use Compress::LZW::Progressive;

  my $codec = Compress::LZW::Progressive->new();

  my $compressed = $codec->compress($data);

DESCRIPTION

This module implements a progressive LZW-like compression technique. The progressive nature means that, in general, the more times you call $codec->compress(), the more efficient the codec will get (more highly compressed the data will be).

CUSTOM LZW

The codec is LZW-like because it has the following differences with Compress::LZW:

- Compressor can request the decompressor to delete a certain number of least frequently used codes - Stream will have a end_segment codeword at the end of a segment - Number of bits used is predefined, and cannot change.

USAGE

new (...)

    Creates a new codec for compressing and/or decompressing

    • bits => $num_bits (default 16)

      Number of bits to use in dictionary entries. Generally this will be between 12-16. The greater the number of bits, the more dictionary entries can be held (2^^$num_bits entries), and therefore the larger memory requirements necessary on compression and decompression. Additionally, the more bits used, the easier it is for the decompressor to decompress.

    • debug => $boolean (default 0)

      If true, will print debug information to STDOUT.

reset ($which)

    Reset the state of the compressor/decompressor. If $which is either 'compress' or 'decompress', it'll only reset that part. Otherwise, it'll reset both.

compress ($str)

decompress ($str)

    Compress or decompress the string given and return the (de)compressed data.

stats ($show_phrases)

    Prints efficiency statistics: how compressed the data is, how many code words used, how many Kb the dictionary is occupying in memory. If $show_phrases is true, it'll also spit out phrase length statistics in the dictionary.

TO DO

For more efficiency, the codec should support outputting codes over less than two bytes. For example, a 12 bit compressed segment would be better expressed using 1.5 bytes per code, since you're not going to be using 4 bits of each output code using "pack 'S*'".

KNOWN BUGS

The LZW algorithim implemented here is not compatible with any other LZW implementation. It is a slight varient from that implemented in Compress::LZW, but don't expect it to work with any other LZW compressed data.

COPYRIGHT

Copyright (c) 2006 Eric Waters and XMission LLC (http://www.xmission.com/). All rights reserved. This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.

The full text of the license can be found in the LICENSE file included with this module.

AUTHOR

Eric Waters <ewaters@uarc.com>