{ pkgs, lib }:

with lib;

let
  input = pipe (fileContents ./input.txt) [
    (splitString "\n")
    (map (split "[[:space:]]+"))
    (map (filter (x: x != [])))
    (map (map toInt))
  ];

  # - The levels are either all increasing or all decreasing.
  # - Any two adjacent levels differ by at least one and at most three.
  safeCondition = xs: let
    zipped = zipLists xs (tail xs);
  in (all ({ fst, snd }: let x = fst - snd; in  1 <= x && x <=  3) zipped)
  || (all ({ fst, snd }: let x = fst - snd; in -1 >= x && x >= -3) zipped);

  answer1 = lib.pipe input [
    (filter safeCondition)
    length
    toString
  ];

  answer2 = lib.pipe input [
    (filter (xs: let
      variants = genList (i: (take i xs) ++ (drop (i + 1) xs)) (length xs);
    in any safeCondition variants))
    length
    toString
  ];
in
  pkgs.writeText "answers" ''
  Task1:
    ${answer1}

  Task2:
    ${answer2}
''