Early Hammer
mfp explanation video
Early hammer (or4ng33xp0) src
smb3rngchk (or4ng33xp0)
or4ng33xp0 made a tool to check the rng results every level. This is how the frames were identified to create an optimal TAS.
The RNG algorithm (LSFR) was reverse engineered and reimplemnted in c, and the minimum amount of other game logic was reimplemented as well to try to simulate the frames.
NBote: this does not count lag frames either, so the runner is expected to figure out how to avoid that.