{ pkgs, lib }:

with lib;

let
  input = pipe (fileContents ./input.txt) [
    (splitString "\n")
    (map (split "[[:space:]]+"))
    (map (x: {
      fst = elemAt x 0;
      snd = elemAt x 2;
    }))
    (x: {
      fst = catAttrs "fst" x;
      snd = catAttrs "snd" x;
    })
  ];

  abs = x: if x < 0 then -x else x;

  answer1 = lib.pipe input [
    (mapAttrs (_: xs: sort lessThan (map toInt xs)))
    ({ fst, snd }: zipListsWith (x: y: abs (x - y)) fst snd)
    (foldl add 0)
    toString
  ];

  similarity = left: right: let
    rightCount = foldl (acc: x: acc // { ${x} = (acc.${x} or 0) + 1; }) { } right;
  in foldl add 0 (map (x: (toInt x) * rightCount.${x} or 0) left);

  answer2 = lib.pipe input [
    ({ fst, snd }: similarity fst snd)
    toString
  ];
in
  pkgs.writeText "answers" ''
  Task1:
    ${answer1}

  Task2:
    ${answer2}
''