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let
  pkgs = import <nixpkgs> { };
  input = builtins.readFile ./input;

  mulRe = ''(mul\([[:digit:]]{1,3},[[:digit:]]{1,3}\))'';
  mulMatch = mulInput: builtins.split mulRe mulInput;
  mulMatches = mulMatch input;
  onlyLists = list: builtins.filter (i: builtins.isList i) list;
  muls = onlyLists mulMatches;
  pairs =
    list:
    builtins.map (
      m:
      let
        re = ''mul\(([[:digit:]]{1,3}),([[:digit:]]{1,3})\)'';
        str = builtins.match re (builtins.elemAt m 0);
      in
      builtins.map (s: pkgs.lib.toInt s) str
    ) list;
  mulPairs = pairs muls;
  sumPairs =
    pairs:
    builtins.foldl' (
      sum: pair:
      let
        left = builtins.elemAt pair 0;
        right = builtins.elemAt pair 1;
      in
      sum + (left * right)
    ) 0 pairs;
  part1 = sumPairs mulPairs;
  cursedInput = builtins.replaceStrings [ "don't" ] [ "Z" ] input;
  firstDoRe = ''^([^d]+)Z\(\)'';
  firstDoReMatches = builtins.split firstDoRe cursedInput;

  doRe = ''do\(\)([^Z]+)Z\(\)'';
  doReMatches = builtins.split doRe cursedInput;
  doMuls =
    matches:
    let
      subInputs = builtins.map (l: builtins.elemAt l 0) (onlyLists matches);
      subMatches = builtins.map (si: mulMatch si) subInputs;
      subMuls = builtins.map (sm: onlyLists sm) subMatches;
      subPairs = builtins.map (sm: pairs sm) subMuls;
      subSums = builtins.map (sp: sumPairs sp) subPairs;
    in
    builtins.foldl' (sum: ss: sum + ss) 0 subSums;

  part2 =
    let
      sum1 = doMuls firstDoReMatches;
      sum2 = doMuls doReMatches;
    in
    sum1 + sum2;

in
[
  part1
  part2
]