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let
  pkgs = import <nixpkgs> { };
  input = builtins.readFile ./input;
  lines = builtins.filter (i: i != "") (pkgs.lib.strings.splitString "\n" input);

  removeElem =
    idx: list:
    (if idx != 0 then (pkgs.lib.sublist 0 idx list) else [ ])
    ++ (
      if idx != (builtins.length list) then
        (pkgs.lib.sublist (idx + 1) ((builtins.length list) - idx) list)
      else
        [ ]
    );

  reports = builtins.map (
    l:
    let
      split = pkgs.lib.strings.splitString " " l;
    in
    builtins.map (s: pkgs.lib.toInt s) split
  ) lines;
  safe =
    report:
    let
      range = pkgs.lib.range 0 ((builtins.length report) - 1);
      asc = builtins.sort (p: q: p > q) report;
      desc = builtins.sort (p: q: p < q) report;
      tolerable = builtins.foldl' (
        s: idx:
        let
          elem = builtins.elemAt report idx;
          prev = if idx != 0 then builtins.elemAt report (idx - 1) else 0;
          diff = if elem < prev then prev - elem else elem - prev;
        in
        if idx != 0 then s && (0 < diff && diff < 4) else true
      ) true range;
    in
    (report == asc || report == desc) && tolerable;
  part1 = builtins.foldl' (sum: report: if (safe report) then sum + 1 else sum) 0 reports;
  permutations = builtins.map (
    r:
    let
      range = pkgs.lib.range 0 ((builtins.length r) - 1);
    in
    builtins.map (idx: removeElem idx r) range
  ) reports;
  part2 = builtins.foldl' (
    sum: muts:
    let
      isSafe = builtins.foldl' (s: mut: s || (safe mut)) false muts;
    in
    if isSafe then sum + 1 else sum
  ) 0 permutations;
in
[
  part1
  part2
]