2024 day 20 part 2.

bin/day2420.ml

@@ -0,0 +1,93 @@
+module IntMap = Map.Make (Int)
+
+let update_value = function None -> Some 1 | Some x -> Some (x + 1)
+
+let populate_grid grid start =
+  let costs = Array.make (Aoc.Grid.length grid) max_int in
+  let rec step acc cost = function
+    | [] -> acc
+    | (x, y) :: t ->
+        if
+          Aoc.Grid.pos_is_valid grid (x, y)
+          && Aoc.Grid.get_by_pos grid (x, y) <> '#'
+          && costs.(Aoc.Grid.idx_of_pos grid (x, y)) = max_int
+        then begin
+          costs.(Aoc.Grid.idx_of_pos grid (x, y)) <- cost;
+          if Aoc.Grid.get_by_pos grid (x, y) = 'E' then step acc cost t
+          else
+            step
+              ((x - 1, y) :: (x + 1, y) :: (x, y - 1) :: (x, y + 1) :: acc)
+              cost t
+        end
+        else step acc cost t
+  in
+  let rec dfs cost lst =
+    let next_step = step [] cost lst in
+    if next_step = [] then costs else dfs (cost + 1) next_step
+  in
+  let costs = dfs 0 [ start ] in
+  costs
+
+let manhattan_distance (x, y) (x', y') = abs (x - x') + abs (y - y')
+
+let within_distance (x, y) distance =
+  let rec impl' acc y' x' =
+    if manhattan_distance (x, y) (x', y') > distance then acc
+    else impl' ((x', y') :: acc) y' (x' + 1)
+  in
+  let rec impl acc y' =
+    if y' - y > distance then acc
+    else impl (impl' acc y' (x - (distance - abs (y - y')))) (y' + 1)
+  in
+  impl [] (y - distance)
+
+let find_cost2 map depth_first grid idx length =
+  let saving idx' =
+    let cost = depth_first.(idx) in
+    let cost' = depth_first.(idx') in
+    let saving =
+      cost' - cost
+      - manhattan_distance
+          (Aoc.Grid.pos_of_idx grid idx)
+          (Aoc.Grid.pos_of_idx grid idx')
+    in
+    saving
+  in
+  let rec impl acc = function
+    | [] -> acc
+    | h :: t -> impl (IntMap.update (saving h) update_value acc) t
+  in
+  within_distance (Aoc.Grid.pos_of_idx grid idx) length
+  |> List.filter (Aoc.Grid.pos_is_valid grid)
+  |> List.map (Aoc.Grid.idx_of_pos grid)
+  |> List.filter (fun x -> Aoc.Grid.get_by_idx grid x <> '#')
+  |> List.filter (fun x -> depth_first.(x) - depth_first.(idx) >= 0)
+  |> impl map
+
+let find_cost_reductions2 grid start cheat_length =
+  let depth_first = populate_grid grid start in
+  let rec impl acc idx =
+    if idx >= Aoc.Grid.length grid then acc
+    else
+      let acc = find_cost2 acc depth_first grid idx cheat_length in
+      impl acc (idx + 1)
+  in
+  impl IntMap.empty 0
+
+let part2 cheat_length (grid, start) =
+  let map = find_cost_reductions2 grid start cheat_length in
+  let map = IntMap.filter (fun k _ -> k >= 100) map in
+  IntMap.fold (fun _ v a -> v + a) map 0
+
+let find_start grid =
+  match Aoc.Grid.idx_from_opt grid 0 'S' with
+  | None -> failwith "find_start"
+  | Some x -> Aoc.Grid.pos_of_idx grid x
+
+let data_of_file fname =
+  let grid = Aoc.Grid.of_file fname in
+  let start = find_start grid in
+  (grid, start)
+
+let _ =
+  Aoc.main data_of_file [ (string_of_int, part2 2); (string_of_int, part2 20) ]

bin/dune

@@ -18,7 +18,8 @@
   day2416
   day2417
   day2418
-  day2419)
+  day2419
+  day2420)
  (names
   day2401
   day2402
@@ -38,5 +39,6 @@
   day2416
   day2417
   day2418
-  day2419)
+  day2419
+  day2420)
  (libraries str aoc))

lib/aoc.ml

@@ -66,14 +66,15 @@ module Grid = struct
 
   let idx_from_opt grid = String.index_from_opt grid.grid
 
-  let update_pos grid pos c =
-    let idx = idx_of_pos grid pos in
+  let update_idx grid idx c =
     let builder = Buffer.create (length grid) in
     Buffer.add_string builder (String.sub grid.grid 0 idx);
     Buffer.add_char builder c;
     Buffer.add_string builder
       (String.sub grid.grid (idx + 1) (length grid - idx - 1));
     { grid with grid = Buffer.contents builder }
+
+  let update_pos grid pos c = update_idx grid (idx_of_pos grid pos) c
 end
 
 let log10i i =

lib/aoc.mli

@@ -143,4 +143,8 @@ module Grid : sig
   val update_pos : t -> int * int -> char -> t
   (** [Grid.update_pos grid pos c] returns a grid with the character at position
       [pos] changed to [c]. *)
+
+  val update_idx : t -> int -> char -> t
+  (** [Grid.update_pos grid idx c] returns a grid with the character at index
+      [idx] changed to [c]. *)
 end