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Bzip2 compression |
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bzip2 is a free and open source lossless data compression algorithm and program developed by Julian Seward. Seward made the first public release of bzip2, version 0.15, in July 1996. The compressor's stability and popularity grew over the next several years, and Seward released version 1.05 in March 2008. In most cases, bzip2's absolute compression efficiency is surpassed by PPM algorithms. bzip2 typically compresses files to within 10% to 15% of the best available techniques (the PPM family of statistical compressors), whilst being around twice as fast at compression and six times faster at decompression. bzip2 compresses most files more effectively than the more traditional gzip or Zip, but is slower. In this manner it is fairly similar to other recent-generation compression algorithms. |
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Like gzip, bzip2 is only a data compressor. It is not an archiver like CAB or ZIP; the program itself has no facilities for multiple files, encryption or archive-splitting, but, in the UNIX tradition, relies instead on separate external utilities such as tar and GnuPG for these tasks. bzip2 compresses well. So it packs more stuff into overfull disk drives, distribution CDs, backup tapes, USB sticks, etc. And/or it reduces customer download times, long distance network traffic, etc. It's not the world's fastest compressor, but it's still fast enough to be very useful. bzip2 compresses files using the Burrows-Wheeler block-sorting text compression algorithm, and Huffman coding. Compression is generally considerably better than that achieved by more conventional LZ77/LZ78-based compressors, and approaches the performance of the PPM family of statistical compressors. The plaintext input blocks are generally all the same size, which can be selected between 100 and 900 kB using a command-line argument. Compression blocks are delimited by a 48-bit magic number derived from the binary-coded decimal representation of π, 0x314159265359, with the end-of-stream similarly delimited by a value representing sqrt(π), 0x177245385090. bzip2 is known to be quite slow at compressing, leading users to opt for alternatives such as gzip when time is an issue. This problem is asymmetric, as decompression is relatively fast. Motivated by the large CPU time required for compression, a modified version was created in 2003 that supported multi-threading, giving significant speed improvements on multi-CPU and multi-core computers. As of January 2008, this functionality has not been incorporated into the main project. A .bz2 stream consists of a 4-byte header, followed by zero or more compressed blocks, immediately followed by an end-of-stream marker containing a 32-bit CRC for the plaintext whole stream processed. The compressed blocks are bit-aligned and no padding occurs. |
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WinTar completely supports bzip2 compression method. When reading archives, WinTar can automatically detect archive formats and its compression method, including gzip method. When writing archive files, WinTar requires a compression method before archiving starts. It also can be specified a None compression method which means the files will be written into arhive, but no compression apply to it. See Using WinTar... |
Related compression methods
See also:bzip2 home |
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