PX8 has been replaced by PX9: https://www.lexaloffle.com/bbs/?tid=34058
But I'll leave this here for reference, and for existing projects using PX8.
This is a library mostly for compressing graphics and maps, but can also be adapted to compress sfx. It is designed for data-heavy carts and requires around 450 tokens for decompression, although this can be reduced if needed by removing remap(), hard-coding parameters, and/or removing predicted spans at the cost of compression performance. If someone wants a smaller/weaker version, let me know!
To use it, compress a 2D rectangle to an address in memory, and supply a function for fetching the source values (normally SGET or MGET). For map data, you probably want to set p.cbits to something like 4 first.
0x2000 is the address of the top half of the map, so this will compress the top half of the sprite sheet (128 sprites) and write the compresed data over the map data.
DECOMP() takes the memory address of the compressed data, the top left corner of where to decompress to, and functions for getting and setting decompressed values. So to decompress this data at 0x2000 back to the screen, starting 32 pixels down:
Only the decompression section of the code is needed once you have compressed data stored on a cart. A typical workflow would be to make a utility cartridge that grabs data from multiple source cartridges (using RELOAD() with a 4th parameter to indicate where to read from), and then CSTORE them to a single cartridge (again, using CSTORE()'s 4th external cart parameter).
Here's an example that stores 2-byte lengths at the start of each compressed block, to allow seeking out the start of any given gfx. I've commented the cstore and reload lines so that it will work if you paste it at the end of the main PX8 cart example: