sm.erl

-module(sm).
-export([multi_step/1,
         multi_steps/1

        ]).

-type instr() :: {push, integer()} | {pop, pos_integer()}.
-type state() :: {non_neg_integer(), [instr()], [integer()]}.

-spec step(state()) -> state() | halt | err.
step({PC, Is, S}) ->
  case nth(PC, Is) of
    none -> err;
    {push, X} -> {Is, [X|S]};
    {pop, N} ->
      case rotate(N, S) of
        err -> err;
        S1 -> {Is, S1}
      end
  end;
step({[], _}) -> halt.

nth(0, [X|_]) -> X;
nth(N, [_|Xs]) -> nth(N-1, Xs);
nth(_, []) -> none.


-spec rotate(pos_integer(), list()) -> list() | err.
rotate(N, Xs) -> rotate_(N, [], Xs).

rotate_(0, Ys, [_|Xs]) -> Ys ++ Xs;
rotate_(N, Ys, [X|Xs]) -> rotate_(N-1, [X|Ys], Xs);
rotate_(_, _, _) -> err.

-spec multi_step(state()) -> [integer()] | err.
multi_step(St = {_, _, S}) ->
  case step(St) of
    err -> err;
    halt -> S;
    St2 -> multi_step(St2)
  end.

-spec multi_steps(state()) -> ok.
multi_steps(St) ->
  case multi_step(St) of
    err -> ok;
    _ -> throw(ok)
  end.