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lzsa/StreamFormat.md
Emmanuel Marty 07104538b7
Fix #46
2020-05-06 19:36:39 +02:00

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Stream format

The stream format is composed of:

  • a header
  • one or more frames
  • a footer

Header format

The 3-bytes LZSA header contains a signature and a traits byte:

0    1                2
0x7b 0x9e             7 6 5 4 3 2 1
                      V V V Z Z Z Z
<--- signature --->   <- traits ->

Trait bits:

  • V: 3 bit code that indicates which block data encoding is used. 0 is LZSA1 and 1 is LZSA2.
  • Z: these bits in the traits are set to 0 for LZSA1 and LZSA2.

Frame format

Each frame contains a 3-bytes length followed by block data that expands to up to 64 Kb of decompressed data. The block data is encoded either as LZSA1 or LZSA2 depending on the V bits of the traits byte in the header.

0    1    2
DSZ0 DSZ1 U|DSZ2
  • DSZ0 (length byte 0) contains bits 0-7 of the block data size
  • DSZ1 (length byte 1) contains bits 8-15 of the block data size
  • DSZ2 (bit 0 of length byte 2) contains bit 16 of the block data size
  • U (bit 7 of length byte 2) is set if the block data is uncompressed, and clear if the block data is compressed.
  • Bits 1..6 of length byte 2 are currently undefined and must be set to 0.

Footer format

The stream ends with the EOD frame: the 3 length bytes are set to 0x00, 0x00, 0x00, and no block data follows.