2024 day 12 part 2

bin/day2412.ml

@@ -1,9 +1,40 @@
-let find_regions grid =
+let get_by_pos_opt grid pos =
+  if Aoc.Grid.pos_is_valid grid pos then Some (Aoc.Grid.get_by_pos grid pos)
+  else None
+
+let perimeter grid (x, y) =
+  let region = Aoc.Grid.get_by_pos grid (x, y) in
+  0
+  + (if get_by_pos_opt grid (x - 1, y) = Some region then 1 else 0)
+  + (if get_by_pos_opt grid (x + 1, y) = Some region then 1 else 0)
+  + (if get_by_pos_opt grid (x, y + 1) = Some region then 1 else 0)
+  + if get_by_pos_opt grid (x, y - 1) = Some region then 1 else 0
+
+let is_corner grid (x, y) (dx, dy) =
+  let region = Aoc.Grid.get_by_pos grid (x, y) in
+  if
+    get_by_pos_opt grid (x + dx, y) <> Some region
+    && get_by_pos_opt grid (x, y + dy) <> Some region
+  then true
+  else if
+    get_by_pos_opt grid (x + dx, y) = Some region
+    && get_by_pos_opt grid (x, y + dy) = Some region
+    && get_by_pos_opt grid (x + dx, y + dy) <> Some region
+  then true
+  else false
+
+let corners grid pos =
+  (if is_corner grid pos (-1, -1) then 1 else 0)
+  + (if is_corner grid pos (-1, 1) then 1 else 0)
+  + (if is_corner grid pos (1, 1) then 1 else 0)
+  + if is_corner grid pos (1, -1) then 1 else 0
+
+let find_regions calc grid =
   let working = Array.make (Aoc.Grid.length grid) ~-1 in
-  let add_pos region pos perimeter lst =
+  let add_pos region pos lst =
-    if Aoc.Grid.pos_is_valid grid pos && Aoc.Grid.get_by_pos grid pos = region
+    if get_by_pos_opt grid pos = Some region then
-    then (Aoc.Grid.idx_of_pos grid pos :: lst, perimeter - 1)
+      Aoc.Grid.idx_of_pos grid pos :: lst
-    else (lst, perimeter)
+    else lst
   in
   let rec scan_pos perimeter area id = function
     | [] -> (perimeter, area)
@@ -12,12 +43,12 @@ let find_regions grid =
         working.(idx) <- id;
         let x, y = Aoc.Grid.pos_of_idx grid idx in
         let area = succ area in
-        let perimeter = perimeter + 4 in
+        let perimeter = perimeter + calc grid (x, y) in
         let region = Aoc.Grid.get_by_idx grid idx in
-        let t, perimeter = add_pos region (x - 1, y) perimeter t in
+        let t = add_pos region (x - 1, y) t in
-        let t, perimeter = add_pos region (x + 1, y) perimeter t in
+        let t = add_pos region (x + 1, y) t in
-        let t, perimeter = add_pos region (x, y - 1) perimeter t in
+        let t = add_pos region (x, y - 1) t in
-        let t, perimeter = add_pos region (x, y + 1) perimeter t in
+        let t = add_pos region (x, y + 1) t in
         scan_pos perimeter area id t
   in
   let rec impl acc idx id =
@@ -29,7 +60,9 @@ let find_regions grid =
   in
   impl [] 0 0
 
-let part1 grid =
+let part calc grid =
-  find_regions grid |> List.fold_left (fun acc (p, a) -> acc + (p * a)) 0
+  find_regions calc grid |> List.fold_left (fun acc (p, a) -> acc + (p * a)) 0
 
-let _ = Aoc.main Aoc.Grid.of_file [ (string_of_int, part1) ]
+let _ =
+  Aoc.main Aoc.Grid.of_file
+    [ (string_of_int, part perimeter); (string_of_int, part corners) ]