I don't get it. There's a straw man of mutating state mid-step, which no sane simulation would do. var new_state = current_state.clone(); run_tick( current

I don't get it. There's a straw man of mutating state mid-step, which no sane simulation would do. var new_state = current_state.clone(); run_tick( current_state, new_state ); current_state = new_state; Get your infinite undo with copy-on-write semantics for your objects & pushing current_state onto an undo list before replacing it with new_state. I get that a purely functional simulation would have these free.