From 530069a35057bb583627522cefcaccb6e9f79c44 Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Wed, 18 Dec 2024 09:27:50 +0000
Subject: [PATCH] Day 2024 part 2

---
 bin/day2418.ml | 40 +++++++++++++++++++++++++++++++---------
 1 file changed, 31 insertions(+), 9 deletions(-)

diff --git a/bin/day2418.ml b/bin/day2418.ml
index f21fc5c..b01f462 100644
--- a/bin/day2418.ml
+++ b/bin/day2418.ml
@@ -26,9 +26,9 @@ let pairs_of_file fname =
 let rec dijkstra visit check_end =
   let compare_costs (lhs, _) (rhs, _) = compare lhs rhs in
   function
-  | [] -> failwith "dijkstra"
+  | [] -> None
   | (cost, state) :: t ->
-      if check_end state then (cost, state)
+      if check_end state then Some (cost, state)
       else
         let new_states = visit cost state |> List.merge compare_costs t in
         dijkstra visit check_end new_states
@@ -68,26 +68,48 @@ let visit grid has_visited cost state =
     let x, y = state in
     has_visited.(grid_idx_by_pos grid state) <- true;
     [
-      (cost + 1, (x - 1, y));
       (cost + 1, (x + 1, y));
-      (cost + 1, (x, y - 1));
       (cost + 1, (x, y + 1));
+      (cost + 1, (x - 1, y));
+      (cost + 1, (x, y - 1));
     ]
 
 let check_end dest state = dest = state
+let width = 71
 
 let part1 lst =
-  let width = 71 in
   let count = 1024 in
   let grid =
     List.to_seq lst |> Seq.take count |> List.of_seq |> make_grid width
   in
   let has_visited = Array.make (width * width) false in
-  let cost, _ =
+  match
     dijkstra (visit grid has_visited)
       (check_end (width - 1, width - 1))
       [ (0, (0, 0)) ]
-  in
-  cost
+  with
+  | None -> failwith "dijkstra"
+  | Some (cost, _) -> cost
 
-let _ = Aoc.main pairs_of_file [ (string_of_int, part1) ]
+let part2 lst =
+  let rec impl count =
+    let grid =
+      List.to_seq lst |> Seq.take count |> List.of_seq |> make_grid width
+    in
+    let has_visited = Array.make (width * width) false in
+    match
+      dijkstra (visit grid has_visited)
+        (check_end (width - 1, width - 1))
+        [ (0, (0, 0)) ]
+    with
+    | None -> grid_idx_by_pos grid (List.nth lst (count - 1))
+    | Some _ -> impl (count + 1)
+  in
+  impl 1024
+
+let string_of_idx width idx =
+  Printf.sprintf "%d,%d" (idx mod width) (idx / width)
+
+let _ =
+  Aoc.main pairs_of_file
+    [ (string_of_int, part1); (string_of_idx width, part2) ]