Code tidy up for 2024 day 21.

lib/aoc.ml

@@ -99,3 +99,5 @@ let memoize memo f value =
       let x = f value in
       Hashtbl.add memo value x;
       x
+
+let rec apply_n n fn arg = if n <= 0 then arg else apply_n (n - 1) fn (fn arg)

lib/aoc.mli

@@ -35,6 +35,10 @@ val memoize : ('a, 'b) Hashtbl.t -> ('a -> 'b) -> 'a -> 'b
     is used to cache results, so repeated calls with the same [value] will not
     call [f] again. *)
 
+val apply_n : int -> ('a -> 'a) -> 'a -> 'a
+(** [apply_n n fn arg] is equivalent to [(fn (fn ... (fn (fn arg))))] where [fn]
+    is called [n] times.*)
+
 (** Module representing a pair of integers, useful for Set.Make *)
 module IntPair : sig
   type t = int * int