lzsa/BlockFormat_LZSA2.md

Block data format (LZSA2)

Blocks encoded as LZSA2 are composed from consecutive commands. Each command follows this format:

• token: <XYZ|LL|MMM>
• optional extra literal length
• literal values
• match offset
• optional extra encoded match length

token

The token byte is broken down into three parts:

7 6 5 4 3 2 1 0
X Y Z L L M M M

• 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'
• 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.
• XYZ: 3-bit value that indicates how to decode the match offset

optional extra literal length

If the literals length is 3 or more, the 'L' bits in the token form the value 3, and an extra nibble is read:

• 0-14: the value is added to the 3 stored in the token, to compose the final literals length.
• 15: an extra byte follows

If an extra byte follows, it can have two possible types of value:

• 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.
• 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.

literal values

Literal bytes, whose number is specified by the literals length, follow here. There can be zero literals in a command.

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.

match offset

The match offset is decoded according to the XYZ bits in the token

XYZ
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.
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.
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.
110 16-bit offset: read a byte for offset bits 0-7, then another byte for offset bits 8-15.
111 repeat offset: reuse the offset value of the previous match command.

important note regarding match offsets: stored as negative values

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.

optional extra encoded match length

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:

• 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.
• 15: an extra byte follows

If an extra byte follows here, it can have two possible types of value:

• 2-255: the final match length is this byte.
• 0: a second and third byte follow, forming a little-endian 16-bit value. The final encoded match length is that 16-bit value.

End Of Data detection for raw blocks

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.

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.

Reading nibbles

When the specification indicates that a nibble (4 bit value) must be read:

• 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.
• 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.