{ lib, utils, ... }:
with lib;
with builtins;
with utils;
let
  input_parts = reSplit "\n\n" (readFile ./input);
  orders = map (reSplit "\\|") (reSplit "\n" (head input_parts));
  records = map (reSplit ",") (init (reSplit "\n" (last input_parts)));

  # defines mapping of number -> [ blocked numbers that can't come after. Sorted for readability. ]
  blocking = mapAttrs (_: v: map head v |> naturalSort |> unique ) (groupBy last orders);

  isValid = r: all (i: let p = elemAt r i; in !hasAttr p blocking || (intersectLists (drop (i + 1) r ) blocking."${p}") == [] ) (listRange r);

  sortRecord = r: if r == [] then [] else (let
    h = head r;
    intersect = if !hasAttr h blocking then [] else intersectLists (tail r ) blocking."${h}";
    # Idea: check first element, and pull all "bad" elements ahead. If there is no "bad" elements, take the head and go down.
  in if intersect == [] then [h] ++ sortRecord (tail r) else sortRecord (intersect ++ [ h ] ++ (removeAll intersect (tail r))) );
in {
  part1 = filter isValid records |> map elemAtMid |> listSumWith toInt;
  part2 = filter (invert isValid) records |> map sortRecord |> map elemAtMid |> listSumWith toInt;
}