lzsa/README.md

LZSA is a byte-aligned compression format that is specifically engineered for very fast decompression on 8-bit systems. It can compress files of any size by using blocks of a maximum size of 64 Kb with block-interdependent compression and up to 64 Kb of back-references for matches.

The LZSA compression tool uses an aggressive optimal packing strategy to try to find the sequence of commands that gives the smallest packed file that decompresses to the original while maintaining the maximum possible decompression speed.

Compression ratio comparison between LZSA and other optimal packers, for a workload composed of ZX Spectrum and C64 files:

                     Bytes            Ratio            Decompression speed vs. LZ4
ZX7                  687133           53,30%           47,73%
LZ5 1.4.1            727107           56,40%           75%
LZSA                 736169           57,11% <------   90%
Lizard -29           776122           60,21%           Not measured
LZ4_HC -19 -B4 -BD   781049           60,59%           100%
Uncompressed         1289127          100%             N/A

Performance over well-known compression corpus files:

                     Uncompressed     LZ4_HC -19 -B4 -BD    LZSA
Canterbury           2810784          935827 (33,29%)       855044 (30,42%)
Silesia              211938580        77299725 (36,47%)     73707039 (34,78%)
Calgary              3251493          1248780 (38,40%)      1196448 (36,80%)
Large                11159482         3771025 (33,79%)      3648420 (32,69%)
enwik9               1000000000       371841591 (37,18%)    355360717 (35,54%)

As an example of LZSA's simplicity, a size-optimized decompressor on Z80 has been implemented in 69 bytes.

The compressor is approximately 2X slower than LZ4_HC but compresses better while maintaining similar decompression speeds and decompressor simplicity.

The main differences with the LZ4 compression format are:

• The use of short (8-bit) match offsets where possible. The match-finder and optimizer cooperate to try and use the shortest match offsets possible.
• Shorter encoding of lengths. As blocks are maximum 64 Kb in size, lengths can only be up to 64 Kb.
• As a result of the smaller commands due to the possibly shorter match offsets, a minimum match size of 3 bytes instead of 4. The use of small matches is driven by the optimizer, and used where they provide gains.

Inspirations:

• LZ4 by Yann Collet.
• LZ5/Lizard by Przemyslaw Skibinski and Yann Collet.
• The suffix array intervals in Wimlib by Eric Biggers.

License:

• The LZSA code is available under the Zlib license.
• The compressor (shrink.c) is available under the CC0 license due to using portions of code from Eric Bigger's Wimlib in the suffix array-based matchfinder.

Stream format

The stream format is composed of:

• a header
• one or more frames
• a footer

Header format

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

0    1                2
0x7b 0x9e             0x00
<--- signature --->   <- traits ->

The traits are set to 0x00 for this version of the format.

Frame format

Each frame contains a 3-bytes length followed by block data that expands to up to 64 Kb of decompressed data.

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.

Block data format

LZSA blocks 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

token

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 values

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

Important note: the last command in a block ends here, as it always contains literals only.

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.

Footer format

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