fun sortAsc (l : int list) : int list = ListMergeSort.sort (op >) l

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

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

val file = TextIO.openIn "day_1.txt"
val input = TextIO.inputAll file;
TextIO.closeIn file;

val lines = String.tokens (fn (char) => char = #"\n") input

val pairs = map
  (fn (line) =>
    let val substr = Substring.extract (line, 0, NONE)
    in [
      substrToInt (Substring.takel Char.isDigit substr),
      substrToInt (Substring.taker Char.isDigit substr)
    ] end
  )
  lines

val lists = (
  sortAsc (map (fn (pair) => hd pair) pairs),
  sortAsc (map (fn (pair) => List.last pair) pairs)
)

(* Part 1 *)

val distances = ListPair.map (fn (n1, n2) => abs(n1 - n2)) lists

val totalDistance = sum distances

(* Part 2 *)

val similarities = ListPair.map
  (fn (n1, n2) =>
    let val frequency = List.length (List.filter (fn (n) => n = n1) (#2 lists))
    in n1 * frequency end
  )
  lists

val similarityScore = sum similarities