Byte-aligned, efficient lossless packer that is optimized for fast decompression on 8-bit micros
LZSA is a collection of byte-aligned compression formats that are 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.
Check out The Hollow by Darklite and Offense, winner of the Solskogen 2019 wild compo, that uses LZSA on Z80.
Gabba by Stardust ranked 2nd in the ZX Spectrum demo compo at CAFe demoparty 2019 and also used LZSA on Z80.
Myst Demake for the Apple II by Vince Weaver, uses LZSA on 6502.
The 8 bit guy's Commander X16 ROM uses LZSA on 6502 as well.
RomWBW uses LZSA on Z80 for a variety of hobbyist computers.
The popular rasm assembler for Z80 features LZSA-compressed data sections.
The desolate game port to the ZX Spectrum uses LZSA compression on Z80.
The Lowtech demo for the Apple II+ and IIe, by Wiz/Imphobia, compresses data with LZSA.
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.
The compression formats give the user choices that range from decompressing faster than LZ4 on 8-bit systems with better compression, to compressing as well as ZX7 with much better decompression speed. LZSA1 is designed to replace LZ4 and LZSA2 to replace ZX7, in 8-bit scenarios.
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 LZSA2 676681 52,49%
Performance over well-known compression corpus files:Uncompressed LZ4_HC -19 -B4 -BD LZSA1 LZSA2 Canterbury 2810784 935827 (33,29%) 850792 (30,27%) 770877 (27,43%) Silesia 211938580 77299725 (36,47%) 73706340 (34,78%) 68928564 (32,52%) Calgary 3251493 1248780 (38,40%) 1192123 (36,67%) 1110290 (34,15%) Large 11159482 3771025 (33,79%) 3648393 (32,69%) 3519480 (31,54%) enwik9 1000000000 371841591 (37,18%) 355360043 (35,54%) 334900611 (33,49%)
As an example of LZSA1's simplicity, a size-optimized decompressor on Z80 has been implemented in 67 bytes.
The compressor is approximately 2X slower than LZ4_HC but compresses better while maintaining similar decompression speeds and decompressor simplicity.
The main differences between LZSA1 and the LZ4 compression format are:
As for LZSA2: * 5-bit, 9-bit, 13-bit and 16-bit match offsets, using nibble encoding * Rep-matches * Shorter encoding of lengths, also using nibbles * A minmatch of 2 bytes * No (slow) bit-packing. LZSA2 uses byte alignment in the hot path, and nibbles.
8-bit assembly code:
Decompression code is provided for common 8-bit CPUs such as Z80 and 6502. However, if you would like to write your own, or understand the encoding, LZSA compresses data to a format that is fast and simple to decompress on 8-bit CPUs. It is encoded in either a stream of blocks, or as a single raw block, depending on command-line settings. The encoding is deliberately designed to avoid complicated operations on 8-bits (such as 16-bit math).