day 5

day5/default.nix

@@ -0,0 +1,23 @@
+{ 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;
+}

utils.nix

@@ -15,4 +15,6 @@ with builtins; rec {
   listRange = l: range 0 ((length l) - 1);
   transpose = l: map (i: map ((flip elemAt) i) l) (listRange (head l));
   invert = f: x: !(f x);
+  elemAtMid = l: elemAt l ((length l) / 2);
+  removeAll = rl: (flip pipe) (map remove rl);
 }