fun isDecreasing (l: int list) : bool =
    ListPair.all (fn (a, b) => b < a andalso b >= a - 3) (l, tl l)

fun isIncreasing (l: int list) : bool =
    ListPair.all (fn (a, b) => b > a andalso b <= a + 3) (l, tl l)

val file = TextIO.openIn "day_2.txt"
val input = TextIO.inputAll file;
TextIO.closeIn file;

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

val reports = map
    (fn (line) =>
        let val numbers = String.tokens Char.isSpace line
        in map (fn (n) => valOf (Int.fromString n)) numbers end
    )
    lines

val safeReports = List.filter (fn (report) => isDecreasing report orelse isIncreasing report) reports

val safeAmount = List.length safeReports