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

fun sortAsc (l: int list) : int list = ListMergeSort.sort (fn (a, b) => a > b) l

fun sum (l: int list) : int = foldl (fn (a, b) => a + b) 0 l

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

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)
)

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

val totalDistance = sum distances