stateTransition.ml

let nextState rotationStep =
  match rotationStep with
  | Reversing ({
      validCount;
      front = firstElem :: restFront;
      frontReversed;
      rear = lastElem :: restRear;
      rearReversed;
    }) -> Reversing ({
      validCount = validCount + 1;
      front = restFront;
      frontReversed = firstElem :: frontReversed;
      rear = restRear;
      rearReversed = lastElem :: rearReversed;
    })
  | Reversing ({
      validCount;
      front = [];
      frontReversed;
      (* Invariant: restRear must be empty *)
      rear = lastElem :: restRear;
      rearReversed;
    }) -> Appending ({
      validCount;
      front = frontReversed;
      rear = lastElem :: rearReversed;
    })
  | Appending ({validCount = 0; front; rear}) -> Done rear
  (* Transfer one element of front to rear *)
  | Appending ({validCount; front = elem :: restFront; rear}) ->
      Appending ({
        validCount = validCount - 1;
        front = restFront;
        rear = elem :: rear;
      })
  | rotationStep -> rotationStep (* No-op *)
;;