An alternative approach, especially if one does not have knowledge of reverse engineering and assembly, might be to identify what compression algorithm is used and usually you can find compression/decompression tools already written by other rom hackers.
From my experience, the majority of the time you see a non-standard compression algorithm, it's going to be an LZ variant, possibly with a Huffman layer. Arithmetic/range coding and other algorithms tend to be rarer.
Hi! We're also working on Heiretsu. Funnily enough, it uses the same compression algorithm that this game does (they're made by the same developer using the same SDK). I've actually done a ton of work making tooling for the game and we're basically ready to go, just lacking in translator manpower so we're completing Chokuretsu first. If you want to check out some of what we've written, you can see some of what we've done on our GitHub! https://github.com/haroohie-club/HeiretsuTranslationUtility
1. I reverse engineered the graphics format for the game Sopwith 2 so that I could change the sprites. I wanted to change the airplane into a flying ox: the ox was an obstacle in the game that you could crash into.
