day 11 memo ftw

2024/days/Day11.ml

@@ -1 +1,44 @@
-let day11 _input = ("TODO", "TODO")
+open Utils
+
+let parse =
+  let open Angstrom in
+  Result.get_ok
+  % parse_string ~consume:All (sep_by (char ' ') digits <* char '\n')
+
+let step m =
+  let open Iter in
+  let memo = Hashtbl.create 2048 in
+
+  let rec count depth stone =
+    if depth >= m then 1
+    else
+      let count_next = count (depth + 1) in
+
+      let cached = Hashtbl.find_opt memo (stone, depth) in
+      match cached with
+      | Some res -> res
+      | None ->
+          let res =
+            match stone with
+            | 0 -> count_next 1
+            | s ->
+                let digits = log10 s in
+                if digits mod 2 = 0 then
+                  let p = pows.(digits / 2) in
+                  let left = s / p in
+                  let right = s - (left * p) in
+
+                  count_next left + count_next right
+                else count_next (s * 2024)
+          in
+          Hashtbl.add memo (stone, depth) res;
+          res
+  in
+  fun stones -> of_list stones |> map (count 0) |> sum
+
+let part1 = step 25
+let part2 = step 75
+
+let day11 input =
+  let stones = parse input in
+  (string_of_int @@ part1 stones, string_of_int @@ part2 stones)

2024/lib/Aoc.ml

@@ -54,6 +54,7 @@ let days =
     (8, Day08.day08);
     (9, Day09.day09);
     (10, Day10.day10);
+    (11, Day11.day11);
   ]
 
 let run_day ctx (day, fn) =