finished day 12, thx dijkstra !!

src/day12.ml

@@ -0,0 +1,87 @@
+open Containers
+
+let infinity = 1000000000
+
+type node =
+  { char : char
+  ; mutable visited : bool
+  ; mutable distance : int
+  ; mutable previous : (Vec2.t * node) option
+  }
+[@@deriving show]
+
+let parse_grid start_anywhere =
+  let s = ref @@ Vec2.of_tuple (0, 0) in
+  let e = ref @@ Vec2.of_tuple (0, 0) in
+  let unvisited = ref [] in
+  let grid =
+    Utils.lines_of_input 12
+    |> Vec2.parse_grid (fun char ->
+      { char; visited = false; distance = infinity; previous = None })
+  in
+  Vec2.iter_grid grid (fun pos ->
+    let n = Vec2.at_e grid pos in
+    match n.char with
+    | 'E' -> e := pos
+    | 'S' ->
+      n.distance <- 0;
+      s := pos
+    | 'a' when start_anywhere ->
+      n.distance <- 0;
+      unvisited := pos :: !unvisited
+    | _ -> ());
+  grid, !e, !s, !unvisited
+;;
+
+let reachable f t =
+  let height c =
+    match c with
+    | 'S' -> 'a'
+    | 'E' -> 'z'
+    | c -> c
+  in
+  Char.compare (height f) (height t) >= -1
+;;
+
+let rec path grid goal current unvisited =
+  let current_node = Vec2.at_e grid current in
+  let neighbors = Vec2.directions |> List.map (fun dir -> Vec2.(current + dir)) in
+  neighbors
+  |> List.iter (fun neighbor ->
+    match Vec2.at grid neighbor with
+    | Some node when (not node.visited) && reachable current_node.char node.char ->
+      let new_distance = current_node.distance + 1 in
+      if new_distance < node.distance
+      then (
+        node.distance <- new_distance;
+        node.previous <- Some (current, current_node));
+      if Option.is_none @@ List.find_opt (fun x -> Vec2.(x = neighbor)) !unvisited
+      then unvisited := neighbor :: !unvisited
+    | _ -> ());
+  current_node.visited <- true;
+  unvisited
+    := List.sort
+         (fun a b ->
+           let a = Vec2.at_e grid a in
+           let b = Vec2.at_e grid b in
+           a.distance - b.distance)
+         !unvisited;
+  match !unvisited with
+  | shortest :: _ when Vec2.(shortest = goal) -> Vec2.at grid shortest
+  | shortest :: rest -> path grid goal shortest @@ ref rest
+  | [] -> None
+;;
+
+let%expect_test "Day 12.1" =
+  let grid, e, s, unvisited = parse_grid false in
+  let solution = Option.get_exn_or "coulnd't find" @@ path grid e s @@ ref unvisited in
+  Printf.printf "%d" solution.distance;
+  [%expect {| 497 |}]
+;;
+
+let%expect_test "Day 12.2" =
+  let grid, e, s, unvisited = parse_grid true in
+  let solution = Option.get_exn_or "coulnd't find" @@ path grid e s @@ ref unvisited in
+  Printf.printf "%d" solution.distance;
+  [%expect {| 492 |}]
+;;

src/vec2.ml

@@ -27,12 +27,16 @@ let at (grid : 'a grid) (point : t) =
   else None
 ;;
 
+let at_e (grid : 'a grid) (point : t) =
+  Option.get_exn_or "out of bounds!" @@ at grid point
+;;
+
 let find (grid : 'a grid) pred =
   let open Option.Infix in
-  let* idx, _ = Array.find_idx pred grid.items in
+  let* idx, item = Array.find_idx pred grid.items in
   let y = idx / grid.width in
   let x = idx mod grid.width in
-  Some (x, y)
+  Some ((x, y), item)
 ;;
 
 let parse_grid parse_char lines : 'a grid =

src/vec2.mli

@@ -12,6 +12,7 @@ val ( + ) : t -> t -> t
 val ( - ) : t -> t -> t
 val ( = ) : t -> t -> bool
 val at : 'a grid -> t -> 'a option
+val at_e : 'a grid -> t -> 'a
 val parse_grid : (char -> 'a) -> string list -> 'a grid
-val find : 'a grid -> ('a -> bool) -> t option
+val find : 'a grid -> ('a -> bool) -> (t * 'a) option
 val iter_grid : 'a grid -> (t -> unit) -> unit