gitendo / lz4gb Goto Github PK

I just started doing GameBoy dev coming from NES dev. On my last project I used the lz4 code that came with cc65 and it worked brilliantly. My first project for GB was to implement lz4, and was pretty proud that I got it down to 120 bytes (much smaller than the 6502 version that ships with cc65). Then I actually bothered to Google if somebody had already done this, and found that yours was even smaller. (Gah!) After incorporating your optimizations I was able to get my version down to 77 bytes and still read vanilla lz4 streams. That version is here:
https://gist.github.com/slembcke/229ca5e17f78ecac1ade9c8efd5ff485

As I understand it, the modifications you made to the lz4 format were to use a 0 offset as a terminator. (Something I had also noticed all my files had. Related to this maybe? https://github.com/lz4/lz4/blob/dev/doc/lz4_Block_format.md#parsing-restrictions), and swapping the endian of the back ref offset (which was clever!).

With further format modifications, I think it's possible to get it even smaller. Specifically, moving the offset distance before the additional count increment bytes means that for both the literals and back ref, you could add the additional count bytes and copy in a single function call as well as remove some RAM accesses. My (untested) function is 64 bytes long and uses a single HRAM byte. I might try modifying your compressor to do this tomorrow.

At first I wasn't so sure about the GB CPU, but this is turning out to be kinda fun!