fun numberAt (i : int) = fn (s : string) =>
  if i >= String.size s then NONE
  else SOME (Substring.takel Char.isDigit (Substring.extract (s, i, NONE)))

fun substrToInt (n : substring) = valOf (Int.fromString (Substring.string n))

fun sum (l : int list) : int = foldl (op +) 0 l

val file = TextIO.openIn "day_3.txt"
val input = TextIO.inputAll file;
TextIO.closeIn file;

val instructions = List.tabulate ((size input), (fn i =>
  let
    val n1 = numberAt (i + 4) input
    val n2 = Option.mapPartial (fn n => numberAt (i + 5 + Substring.size n) input) n1
  in
    if isSome n1 andalso isSome n2 andalso
      String.substring (input, i, 4) = "mul(" andalso
      String.sub (input, i + 4 + Substring.size (valOf n1)) = #"," andalso
      String.sub (input, i + 5 + Substring.size (valOf n1) + Substring.size (valOf n2)) = #")"
    then SOME (n1, n2) else NONE
end))

val instructions = List.mapPartial
  (fn (num) => case num of SOME (SOME x, SOME y) => SOME (substrToInt x, substrToInt y) | _ => NONE)
  instructions

val result = sum (map (op *) instructions)