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79 lines
5.2 KiB
Markdown
79 lines
5.2 KiB
Markdown
# Block data format (LZSA2)
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Blocks encoded as LZSA2 are composed from consecutive commands. Each command follows this format:
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* token: <XYZ|LL|MMM>
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* optional extra literal length
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* literal values
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* match offset
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* optional extra encoded match length
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**token**
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The token byte is broken down into three parts:
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7 6 5 4 3 2 1 0
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X Y Z L L M M M
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* L: 2-bit literals length (0-2, or 3 if extended). If the number of literals for this command is 0 to 2, the length is encoded in the token and no extra bytes are required. Otherwise, a value of 3 is encoded and extra nibbles or bytes follow as 'optional extra literal length'
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* M: 3-bit encoded match length (0-6, or 7 if extended). Likewise, if the encoded match length for this command is 0 to 6, it is directly stored, otherwise 7 is stored and extra nibbles or bytes follow as 'optional extra encoded match length'. Except for the last command in a block, a command always contains a match, so the encoded match length is the actual match length offset by the minimum, which is 2 bytes. For instance, an actual match length of 5 bytes to be copied, is encoded as 3.
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* XYZ: 3-bit value that indicates how to decode the match offset
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**optional extra literal length**
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If the literals length is 3 or more, the 'L' bits in the token form the value 3, and an extra nibble is read:
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* 0-14: the value is added to the 3 stored in the token, to compose the final literals length.
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* 15: an extra byte follows
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If an extra byte follows, it can have two possible types of value:
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* 3-255: the value is the final literals length. For instance a length of 206 will be stored as 3 in the token + a nibble with the value of 15 + a single byte with the value of 206.
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* 0: a second and third byte follow, forming a little-endian 16-bit value. The final literals value is that 16-bit value. For instance, a literals length of 1027 is stored as 3 in the token, a nibble with the value of 15, then byte values of 0, 3 and 4, as 3 + (4 * 256) = 1027.
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**literal values**
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Literal bytes, whose number is specified by the literals length, follow here. There can be zero literals in a command.
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Important note: for blocks that are part of a stream, the last command in a block ends here, as it always contains literals only. For raw blocks, the last command does contain the match offset and match length, see the note below for EOD detection.
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**match offset**
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The match offset is decoded according to the XYZ bits in the token
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XYZ
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00Z 5-bit offset: read a nibble for offset bits 0-3 and use bit Z of the token as bit 4 of the offset. set bits 5-15 of the offset to 1.
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01Z 9-bit offset: read a byte for offset bits 0-7 and use bit Z for bit 8 of the offset. set bits 9-15 of the offset to 1.
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10Z 13-bit offset: read a byte for offset bits 0-7, read a nibble for offset bits 8-12 and use bit Z for bit 12 of the offset. set bits 13-15 of the offset to 1.
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110 16-bit offset: read a byte for offset bits 0-7, then another byte for offset bits 8-15.
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111 repeat offset: reuse the offset value of the previous match command.
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**important note regarding match offsets: stored as negative values**
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Note that the match offset is negative: it is added to the current decompressed location and not substracted, in order to locate the back-reference to copy. For this reason, as already indicated, unexpressed offset bits are set to 1 instead of 0.
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**optional extra encoded match length**
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If the encoded match length is 7 or more, the 'M' bits in the token form the value 7, and an extra nibble is read:
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* 0-14: the value is added to the 3 stored in the token, and then the minmatch of 2 is added, to compose the final match length.
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* 15: an extra byte follows
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If an extra byte follows here, it can have two possible types of value:
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* 2-255: the final match length is this byte.
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* 0: a second and third byte follow, forming a little-endian 16-bit value. The final encoded match length is that 16-bit value.
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# End Of Data detection for raw blocks
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When the LZSA2 block is part of a stream (see StreamFormat.md), as previously mentioned, the block ends after the literal values of the last command, without a match offset or match length.
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However, in a raw LZSA2 block, the last command does include a 9-bit match offset and a match length. The match length is encoded as a long zero: the 'M' bits in the token form the value 7, then a nibble with the value of 15 is present, then an extra match length byte with the value of 0 ("two match length bytes follow"). Finally, a two-byte zero match length follows, indicating the end of the block. EOD is the only time a zero match length (which normally would indicate a copy of 2 bytes) is encoded as a large 2-byte match value. This allows the EOD test to exist in a rarely used code branch.
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# Reading nibbles
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When the specification indicates that a nibble (4 bit value) must be read:
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* If there are no nibbles ready, read a byte immediately. Return the high 4 bits (bits 4-7) as the nibble and store the low 4 bits for later. Flag that a nibble is ready for next time.
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* If a nibble is ready, return the previously stored low 4 bits (bits 0-3) and flag that no nibble is ready for next time.
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