Bzip2

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File Format
Name bzip2
Ontology
Extension(s) .bz2
MIME Type(s) application/x-bzip2
PRONOM x-fmt/268
Wikidata ID Q27866052
Released 1997

bzip2 is a data compression algorithm and compressed file format. It was developed by Julian Seward.

Contents

Identification

A bzip2 file starts with the byte pattern 42 5a 68 ?? 31 41 59 26 53 59.

The first three bytes are ASCII "BZh". (For signature "BZ0", refer to the original bzip format.) The "h" has been said to stand for "Huffman coding", but confirmation is needed.

The byte at offset 3 is a code for the block size. Its possible values range from 0x31 to 0x39 (ASCII "1" to "9").

The bytes at offset 4-9 are derived from the digits of the mathematical constant π (BCD-encoded).

The end-of-file marker uses magic number (hex) 17 72 45 38 50 90, derived from the square root of π. However, it is not byte-aligned. The result is that one of the following byte sequences appears beginning 10 bytes from the end of the file:

b9 22 9c 28 48
dc 91 4e 14 24
ee 48 a7 0a 12
77 24 53 85 09
bb 92 29 c2 84
5d c9 14 e1 42
2e e4 8a 70 a1
17 72 45 38 50

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