Block data format (LZSA1)
Blocks encoded as LZSA1 are composed from consecutive commands. Each command follows this format:
- token: <O|LLL|MMMM>
- optional extra literal length
- literal values
- match offset low
- optional match offset high
- optional extra encoded match length
The token byte is broken down into three parts:
7 6 5 4 3 2 1 0 O L L L M M M M
- L: 3-bit literals length (0-6, or 7 if extended). If the number of literals for this command is 0 to 6, the length is encoded in the token and no extra bytes are required. Otherwise, a value of 7 is encoded and extra bytes follow as 'optional extra literal length'
- M: 4-bit encoded match length (0-14, or 15 if extended). Likewise, if the encoded match length for this command is 0 to 14, it is directly stored, otherwise 15 is stored and extra 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 3 bytes. For instance, an actual match length of 10 bytes to be copied, is encoded as 7.
- O: set for a 2-bytes match offset, clear for a 1-byte match offset
optional extra literal length
If the literals length is 7 or more, the 'L' bits in the token form the value 7, and an extra byte follows here, with three possible types of value:
- 0-248: the value is added to the 7 stored in the token, to compose the final literals length. For instance a length of 206 will be stored as 7 in the token + a single byte with the value of 199, as 7 + 199 = 206.
- 250: a second byte follows. The final literals value is 256 + the second byte. For instance, a literals length of 499 is encoded as 7 in the token, a byte with the value of 250, and a final byte with the value of 243, as 256 + 243 = 499.
- 249: 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 1024 is stored as 7 in the token, then byte values of 249, 0 and 4, as (4 * 256) = 1024.
The extension byte values are chosen so that all three cases can be detected on 8-bit CPUs with a simple addition and overflow check.
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 low
The low 8 bits of the match offset follows.
optional match offset high
If the 'O' bit (bit 7) is set in the token, the high 8 bits of the match offset follow, otherwise they are understood to be all set to 1. For instance, a short offset of 0x70 is interpreted as 0xff70.
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.
optional extra encoded match length
If the encoded match length is 15 or more, the 'M' bits in the token form the value 15, and an extra byte follows here, with three possible types of value.
- 0-237: the value is added to the 15 stored in the token. The final value is 3 + 15 + this byte.
- 239: a second byte follows. The final match length is 256 + the second byte.
- 238: a second and third byte follow, forming a little-endian 16-bit value. The final encoded match length is that 16-bit value.
Again, the extension byte values are chosen so that all cases can be detected with a simple addition and overflow check on 8-bit CPUs.
End Of Data detection for raw blocks
When the LZSA1 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 LZSA1 block, the last command does include a 1-byte match offset (set to zero) and a match length. The match length is encoded as a long zero: the 'M' bits in the token form the value 15, then an extra match length byte is present, with the value 238 ("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 3 bytes) is encoded as a large 2-byte match value. This allows the EOD test to exist in a rarely used code branch.