While the the Section called Compression Streams in Appendix J provide a way of creating gzip and bz2 compatable files on the local filesystem, they do not provide a means for generalized compression over network streams, nor do they provide a means to begin with a non-compressed stream and transition to a compressed one. For this, a compression filter may be applied to any stream resource at any time.
Note: Compression filters do not generate headers and trailers used by commandline utilites such as gzip. They only compress and decompress the payload portions of compressed data streams.
zlib.deflate (compression) and zlib.inflate (decompression) are implementations of the compression methods described in RFC 1951. The deflate filter takes up to three parameters passed as an associative array. level describes the compression strength to use (1-9). Higher numbers will generally yield smaller payloads at the cost of additional processing time. Two special compression levels also exist: 0 (for no compression at all), and -1 (zlib internal default -- currently 6). window is the base-2 log of the compression loopback window size. Higher values (up to 15 -- 32768 bytes) yield better compression at a cost of memory, while lower values (down to 9 -- 512 bytes) yield worse compression in a smaller memory footprint. Default window size is currently 15. memory is a scale indicating how much work memory should be allocated. Valid values range from 1 (minimal allocation) to 9 (maximum allocation). This memory allocation affects speed only and does not impact the size of the generated payload.
Note: Because compression level is the most commonly used parameter, it may be alternatively provided as a simple integer value (rather than an array element).
Example K-7. zlib.deflate and zlib.inflate
Example K-8. zlib.deflate simple
bzip2.compress and bzip2.decompress work in the same manner as the zlib filters described above. The bzip2.compress filter accepts up to two parameters given as elements of an associative array: blocks is an integer value from 1 to 9 specifying the number of 100kbyte blocks of memory to allocate for workspace. work is also an integer value ranging from 0 to 250 indicating how much effort to expend using the normal compression method before falling back on a slower, but more reliable method. Tuning this parameter effects only compression speed. Neither size of compressed output nor memory usage are changed by this setting. A work factor of 0 instructs the bzip library to use an internal default. The bzip2.decompress filter only accepts one parameter, which can be passed as either an ordinary boolean value as the small element of an associative array. small, when set to a TRUE value, instructs the bzip library to perform decompression in a minimal memory footprint at the cost of speed.
Example K-9. bzip2.compress and bzip2.decompress
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