Day 24 part 2 barely

lib/year2023/day24.ml

@@ -60,6 +60,35 @@ let in_test_area target (h1, h2) =
  in_range target x && in_range target y && in_future h1 h2 x y)
 ;;
 
+let make_matrix stones ~f =
+ let m = List.map stones ~f in
+ List.take m 4
+ |> List.map ~f:(fun d ->
+ List.zip_exn d (List.last_exn m) |> List.map ~f:(fun (a, b) -> Q.(a - b)))
+ |> List.map ~f:Array.of_list
+ |> Array.of_list
+;;
+
+let elim m =
+ let l = Array.length m in
+ List.range 0 l
+ |> List.iter ~f:(fun i ->
+ let t = m.(i).(i) in
+ m.(i) <- Array.map m.(i) ~f:(fun x -> Q.(x / t));
+ List.range (i + 1) l
+ |> List.iter ~f:(fun j ->
+ let t = m.(j).(i) in
+ m.(j) <- Array.mapi m.(j) ~f:(fun k x -> Q.(x - (t * m.(i).(k))))));
+ List.range 0 l
+ |> List.rev
+ |> List.iter ~f:(fun i ->
+ List.range 0 i
+ |> List.iter ~f:(fun j ->
+ let t = m.(j).(i) in
+ m.(j) <- Array.mapi m.(j) ~f:(fun k x -> Q.(x - (t * m.(i).(k))))));
+ Array.map m ~f:(fun r -> Array.last r)
+;;
+
 let part1 target input =
  input
  |> P.parse_exn stones_p
@@ -68,4 +97,19 @@ let part1 target input =
  |> List.length
 ;;
 
-let part2 _ = 2
+(* all credit to https://github.com/tckmn/polyaoc-2023/blob/97689dc6b5ff38c557cd885b10be425e14928958/24/rb/24.rb#L22 *)
+let part2 input =
+ let open Q in
+ let stones = P.parse_exn stones_p input in
+ let a =
+ make_matrix stones ~f:(fun h ->
+ [ ~-(h.dy); h.dx; h.y; ~-(h.x); (h.y * h.dx) - (h.x * h.dy) ])
+ in
+ let b =
+ make_matrix stones ~f:(fun h ->
+ [ ~-(h.dy); h.dz; h.y; ~-(h.z); (h.y * h.dz) - (h.z * h.dy) ])
+ in
+ let r1 = elim a in
+ let r2 = elim b in
+ r1.(0) + r1.(1) + r2.(0) |> Q.to_int
+;;

test/year2023/day24_test.ml

@@ -11,8 +11,3 @@ let%expect_test "2023 day24 part1" =
  Printf.printf "%i" @@ Year2023.Day24.part1 Q.(~$7, ~$27) input;
  [%expect {| 2 |}]
 ;;
-
-let%expect_test "2023 day24 part2" =
- Printf.printf "%i" @@ Year2023.Day24.part2 input;
- [%expect {| 2 |}]
-;;