From bfd65557bf8a39063b3b4ce7f59dca03ea3b2554 Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Mon, 16 Dec 2024 11:33:17 +0000
Subject: [PATCH] 2024 day 16 part 2

Quite slow takes ~11 mins to run.
---
 bin/day2416.ml | 84 +++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 63 insertions(+), 21 deletions(-)

diff --git a/bin/day2416.ml b/bin/day2416.ml
index 3e335c1..d9c14b6 100644
--- a/bin/day2416.ml
+++ b/bin/day2416.ml
@@ -5,15 +5,15 @@ let find grid c =
 
 let input_of_file fname =
   let grid = Aoc.Grid.of_file fname in
-  let start_state = (find grid 'S', (1, 0)) in
-  (grid, start_state)
+  let start_pos = find grid 'S' in
+  (grid, start_pos)
 
 let rec dijkstra visit check_end states =
   let compare_costs (lhs, _) (rhs, _) = compare lhs rhs in
   match states with
-  | [] -> failwith "dijkstra"
+  | [] -> None
   | (cost, state) :: t ->
-      if check_end state then cost
+      if check_end state then Some (cost, state, t)
       else
         let new_states =
           visit cost state |> List.sort compare_costs
@@ -21,7 +21,7 @@ let rec dijkstra visit check_end states =
         in
         dijkstra visit check_end new_states
 
-let visited_idx grid (p, (dx, dy)) =
+let visited_idx grid ((dx, dy), p) =
   let add =
     match (dx, dy) with
     | 1, 0 -> 0
@@ -32,27 +32,69 @@ let visited_idx grid (p, (dx, dy)) =
   in
   (Aoc.Grid.idx_of_pos grid p * 4) + add
 
-let has_visited grid visited_grid state = visited_grid.(visited_idx grid state)
-
-let mark_visited grid visited_grid state =
-  visited_grid.(visited_idx grid state) <- true
-
-let visit grid visited_grid cost ((((x, y) as p), (dx, dy)) as state) =
-  if has_visited grid visited_grid state then []
+let visit grid visited_grid cost state =
+  let (dx, dy), ((x, y) as p) = List.hd state in
+  let has_visited = visited_grid.(visited_idx grid (List.hd state)) in
+  if has_visited then []
   else if Aoc.Grid.get_by_pos grid p = '#' then []
   else (
-    Printf.printf "%d %d %d %d %d\n" cost x y dx dy;
-    mark_visited grid visited_grid state;
+    visited_grid.(visited_idx grid (List.hd state)) <- true;
     [
-      (cost + 1, ((x + dx, y + dy), (dx, dy)));
-      (cost + 1000, (p, (-dy, dx)));
-      (cost + 1000, (p, (dy, -dx)));
+      (cost + 1, ((dx, dy), (x + dx, y + dy)) :: state);
+      (cost + 1000, ((-dy, dx), p) :: state);
+      (cost + 1000, ((dy, -dx), p) :: state);
     ])
 
-let check_end grid (p, _) = Aoc.Grid.get_by_pos grid p = 'E'
+let visit_max grid visited_grid max_cost cost state =
+  let has_visited = visited_grid.(visited_idx grid (List.hd state)) < cost in
+  let (dx, dy), ((x, y) as p) = List.hd state in
+  if has_visited then []
+  else if cost > max_cost then []
+  else if Aoc.Grid.get_by_pos grid p = '#' then []
+  else (
+    (*Printf.printf "%d (%d, %d)\n" cost x y;
+    flush stdout;*)
+    visited_grid.(visited_idx grid (List.hd state)) <- cost;
+    [
+      (cost + 1, ((dx, dy), (x + dx, y + dy)) :: state);
+      (cost + 1000, ((-dy, dx), p) :: state);
+      (cost + 1000, ((dy, -dx), p) :: state);
+    ])
 
-let part (grid, start_state) =
+let[@warning "-32"] print_point (x, y) = Printf.printf "(%d, %d)" x y
+
+let check_end grid state =
+  let _, p = List.hd state in
+  Aoc.Grid.get_by_pos grid p = 'E'
+
+let check_end2 grid _ state = check_end grid state
+
+let part1 (grid, start_pos) =
   let visited_grid = Array.make (Aoc.Grid.length grid * 4) false in
-  dijkstra (visit grid visited_grid) (check_end grid) [ (0, start_state) ]
+  match
+    dijkstra (visit grid visited_grid) (check_end grid)
+      [ (0, [ ((1, 0), start_pos) ]) ]
+  with
+  | None -> failwith "part"
+  | Some (cost, _, _) -> cost
 
-let _ = Aoc.main input_of_file [ (string_of_int, part) ]
+let part2 (grid, start_pos) =
+  let cost = part1 (grid, start_pos) in
+  let visited_grid = Array.make (Aoc.Grid.length grid * 4) max_int in
+  let rec impl acc lst =
+    match
+      dijkstra
+        (visit_max grid visited_grid cost)
+        (check_end2 grid visited_grid)
+        lst
+    with
+    | None -> acc
+    | Some (_, states, remainder) ->
+        List.iter (fun x -> visited_grid.(visited_idx grid x) <- 0) states;
+        impl (states :: acc) remainder
+  in
+  impl [] [ (0, [ ((1, 0), start_pos) ]) ]
+  |> List.concat |> List.map snd |> List.sort_uniq compare |> List.length
+
+let _ =
+  Aoc.main input_of_file [ (string_of_int, part1); (string_of_int, part2) ]