This is a BeebAsm 6502 port of https://xxl.atari.pl/zx5-decompressor/ which in itself is a port of https://github.com/einar-saukas/ZX5
ZX5 is an experimental data compressor derived from ZX0, similarly targeted for low-end platforms, including 8-bit computers like the ZX Spectrum.
Compared to ZX0 format that supports 3 block types (including copy from last offset), ZX5 extends this concept adding 2 more block types: copy from second-to-last and third-to-last offset. This additional complexity sometimes gives ZX5 a slightly better compression than ZX0 in some cases. However ZX5 decompressor is a little larger, moreover ZX5 compressor is very much slower and consumes too much memory therefore it's not very practical. For this reason, it's highly recommended to use ZX0 instead. Although if you desperately need better compression for a very specific file, then it's worth to try ZX5 anyway...
The ZX5 data compression format and algorithm was designed and implemented by Einar Saukas, based on previous work on ZX0.
The experimental C compressor is available under the "BSD-3" license. In practice, this is relevant only if you want to modify its source code and/or incorporate the compressor within your own products. Otherwise, if you just execute it to compress files, you can simply ignore these conditions.
The decompressors can be used freely within your own programs (either for the ZX Spectrum or any other platform), even for commercial releases. The only condition is that you must indicate somehow in your documentation that you have used ZX5.