I recently implemented a good portion of x86(-64) decoder for some side project [1] and kinda surprised how it got even more complicated in recent days. Sandpile.org [2] was really useful for my purpose.

[1] Namely, a version of Fabian Giesen's disfilter for x86-64, for yet another side project which is still not in public: https://gist.github.com/lifthrasiir/df47509caac2f065032ef72e...

[2] https://sandpile.org/