add memo wrapper for recursive functions

src/utils.ml

@@ -10,6 +10,40 @@ let lines_of_input day =
   IO.(with_in path read_lines_l)
 ;;
 
+(** [f] in [memo f] should be a function that makes recursive calls
+    via the memoized function passed as its first argument. ie:
+    {[
+      let fib_m =
+        memo (fun self x ->
+          match x with
+          | 0 | 1 -> 1
+          | x -> self (x - 1) + self (x - 2))
+      ;;
+    ]} *)
+let memo f =
+  let open Hashtbl in
+  let cache = create 1000 in
+  let rec f_mem k =
+    try find cache k with
+    | Not_found ->
+      let v = f f_mem k in
+      add cache k v;
+      v
+  in
+  f_mem
+;;
+
+let%expect_test "fib memoized" =
+  let fib_m =
+    memo (fun fib_m x ->
+      match x with
+      | 0 | 1 -> 1
+      | x -> fib_m (x - 1) + fib_m (x - 2))
+  in
+  Printf.printf "%i\n" @@ fib_m 100;
+  [%expect {| 1298777728820984005 |}]
+;;
+
 module Parse = struct
   include Angstrom