- Author comments
- Franz Inc. Open Source Info
Deflation is a means of compressing an octet sequence that combines the LZ77 algorithm for marking common substrings and Huffman coding to take advantage of the different frequencies of occurence of byte sequences in the file. This algorithm may not be as easy to understand or as efficient as the LZW compression algorithm but Deflate does have the important advantage in that it is not patented. Thus Deflate widely used. Presently it's the most common compression method used by Windows Zip programs (e.g. Winzip) and in the Unix gzip program. Java jar files, being just zip files, also use this compression method.
John Foderaro, Franz Inc.
If you have any doubt that Lisp is a better language for writing bit level algorithms you need only compare the source for this version of the Inflate algorithm with the C version found in programs such as gzip. The C code is essentially unreadable.
All Common Lisps, but tested on Allegro Common Lisp.
None for Allegro Common Lisp. For non-Allegro Common Lisps the acl-compat package is required, which provides the if* macro.
Load the inflate source file:
open a stream p to a file containing the compressed data. If this file has a gzip header then:
(skip-gzip-header p) (make-instance 'inflate-stream :input-handle p)
will return a stream which can be read to recover the uncompressed data.
inflates the input-stream into the output-stream. Both streams should be of type unsigned-byte 8. Closing the inflate-stream will not close stream p. That must be done separately.
If the input stream is positioned on the header of a gzip'ed file then the header is skipped. If the input-stream is not positioned on the gzip header then nothing is done.
The zlib source code is licensed under the terms of the Lisp Lesser GNU Public License, known as the LLGPL. The LLGPL consists of a preamble and the LGPL. Where these conflict, the preamble takes precedence. This project is referenced in the preamble as the LIBRARY.
At present this module only contains the code to uncompress a deflated byte sequence. We may write code to do deflation in the future but at present have no pressing need for this functionality.
We will be adding a simple-streams interface soon. With this interface you can layer the decompressor onto a simple-stream and decompress while you read bytes or characters from the stream.
See the forms at the end of the inflate.cl file that is part of this project for examples and tests.
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