type key = Plus | Minus | Times | Div | Equals | Digit of int | Store | Recall | Clear | Off ;;

type state = {
    mutable lastComputation:int;
    mutable lastKeyActivated:bool;
    mutable lastOperator:key;
    mutable valuePrinted:int;
    mutable memory:int
} ;;

exception Invalid_key ;;
exception Key_off ;;

let translation c = match c with
  '+' -> Plus
| '-' -> Minus
| '*' -> Times
| '/' -> Div
| '=' -> Equals
| 'C' | 'c' -> Clear
| 'M' -> Store
| 'm' -> Recall
| 'O' | 'o' -> Off
| '0'..'9' as c -> Digit ((Char.code c) - (Char.code '0'))
| _ -> raise Invalid_key ;;

let transition state key = match key with
  Clear -> state.valuePrinted <- 0
| Digit n -> state.valuePrinted <- (if state.lastKeyActivated then state.valuePrinted * 10 + n else n) ;
             state.lastKeyActivated <- true
| Store -> state.lastKeyActivated <- false ;
           state.memory <- state.valuePrinted
| Recall -> state.lastKeyActivated <- false ;
            state.valuePrinted <- state.memory
| Off -> raise Key_off
| _ -> let lastComputation = match state.lastOperator with
    Plus -> state.lastComputation + state.valuePrinted
  | Minus -> state.lastComputation - state.valuePrinted
  | Times -> state.lastComputation * state.valuePrinted
  | Div -> state.lastComputation / state.valuePrinted
  | Equals -> state.valuePrinted
  | _ -> failwith "transitional: imposible match"
    in
  state.lastComputation <- lastComputation ;
  state.lastKeyActivated <- false ;
  state.lastOperator <- key ;
  state.valuePrinted <- lastComputation ;;

let go () =
  let state = {
    lastComputation = 0 ;
    lastKeyActivated = false ;
    lastOperator = Times ;
    valuePrinted = 0 ;
    memory = 0
  }
  in try
    while true do
      try
    let input = translation (input_char stdin)
    in
    transition state input ;
    print_newline () ;
    print_string "result: " ;
    print_int state.valuePrinted ;
    print_newline () ;
      with
    Invalid_key -> ()
    done
  with
    Key_off -> () ;;