(author here) The mutation is only for performance critical code. I'm first trying to match C/Rust performance in my code, and then transform it to more idiomatic functional code (which flambda2 in OxCaml can optimise).

It's too difficult right now to directly jump to the functional version since I don't understand the flambda2 compiler well enough to predict whta optimisations will work! OxCaml is stabilising more this year so that should get easier in time.