bz2 format

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BZ2 Header format
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.magic:16                       = 'BZ' signature/magic number
.version:8                      = 'h' for Bzip2 ('H'uffman coding), '0' for Bzip1 (deprecated)
.hundred_k_blocksize:8          = '1'..'9' block-size 100 kB-900 kB

.compressed_magic:48            = 0x314159265359 (BCD (pi))
.crc:32                         = checksum for this block
.randomised:1                   = 0=>normal, 1=>randomised (deprecated)
.origPtr:24                     = starting pointer into BWT for after untransform
.huffman_used_map:16            = bitmap, of ranges of 16 bytes, present/not present
.huffman_used_bitmaps:0..256    = bitmap, of symbols used, present/not present (multiples of 16)
.huffman_groups:3               = 2..6 number of different Huffman tables in use
.selectors_used:15              = number of times that the Huffman tables are swapped (each 50 bytes)
*.selector_list:1..6            = zero-terminated bit runs (0..62) of MTF'ed Huffman table (*selectors_used)
.start_huffman_length:5         = 0..20 starting bit length for Huffman deltas
*.delta_bit_length:1..40        = 0=>next symbol; 1=>alter length
                                                { 1=>decrement length;  0=>increment length } (*(symbols+2)*groups)
.contents:2..∞                  = Huffman encoded data stream until end of block

.eos_magic:48                   = 0x177245385090 (BCD sqrt(pi))
.crc:32                         = checksum for whole stream
.padding:0..7                   = align to whole byte

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