From aaa031e6c6e676eaa7a69e78a2cf7c75686cb9e7 Mon Sep 17 00:00:00 2001
From: Matthew Gretton-Dann <matt@gretton-dann.org.uk>
Date: Thu, 19 Dec 2024 11:53:59 +0000
Subject: [PATCH] Move memoize to the Aoc library.

---
 bin/day2419.ml | 19 ++++---------------
 lib/aoc.ml     |  8 ++++++++
 lib/aoc.mli    |  5 +++++
 3 files changed, 17 insertions(+), 15 deletions(-)

diff --git a/bin/day2419.ml b/bin/day2419.ml
index 543d520..5b7f00f 100644
--- a/bin/day2419.ml
+++ b/bin/day2419.ml
@@ -14,17 +14,6 @@ let towels_of_file fname = Aoc.strings_of_file fname |> towels_of_strings
 (** Memoizing hash table shared between parts 1 and 2. *)
 let memo = Hashtbl.create 1000
 
-(** [memoize memo f value] returns the result of [f value]. The hashtable [memo]
-    is used to cache results, so repeated calls with the same [value] will not
-    call [f] again. *)
-let memoize memo f value =
-  match Hashtbl.find_opt memo value with
-  | Some x -> x
-  | None ->
-      let x = f value in
-      Hashtbl.add memo value x;
-      x
-
 (** [count_hashes memo towels pattern] counts the number of ways of matching
     [pattern] using [towels]. [memo] is a hashtable used for memoizing results.
 *)
@@ -35,7 +24,7 @@ let rec count_matches memo towels pattern =
     | h :: t ->
         let towel_len = String.length h in
         if String.starts_with ~prefix:h pattern then
-          memoize memo
+          Aoc.memoize memo
             (count_matches memo towels)
             (String.sub pattern towel_len (pattern_len - towel_len))
           + count_matched t
@@ -44,12 +33,12 @@ let rec count_matches memo towels pattern =
   if pattern_len = 0 then 1 else count_matched towels
 
 let part1 (towels, patterns) =
-  List.map (count_matches memo towels) patterns
-  |> List.filter (( > ) 0)
+  List.map (Aoc.memoize memo (count_matches memo towels)) patterns
+  |> List.filter (( < ) 0)
   |> List.length
 
 let part2 (towels, patterns) =
-  List.map (memoize memo (count_matches memo towels)) patterns
+  List.map (Aoc.memoize memo (count_matches memo towels)) patterns
   |> List.fold_left ( + ) 0
 
 let _ =
diff --git a/lib/aoc.ml b/lib/aoc.ml
index 02dddd1..62a3885 100644
--- a/lib/aoc.ml
+++ b/lib/aoc.ml
@@ -90,3 +90,11 @@ let pow10 n =
   let rec impl acc = function 0 -> acc | x -> impl (acc * 10) (x - 1) in
   assert (n >= 0);
   impl 1 n
+
+let memoize memo f value =
+  match Hashtbl.find_opt memo value with
+  | Some x -> x
+  | None ->
+      let x = f value in
+      Hashtbl.add memo value x;
+      x
diff --git a/lib/aoc.mli b/lib/aoc.mli
index c99630c..aa194ee 100644
--- a/lib/aoc.mli
+++ b/lib/aoc.mli
@@ -30,6 +30,11 @@ val main : (string -> 'a) -> (('b -> string) * ('a -> 'b)) list -> unit
     [string_of_int]). The second executes the given part. Output is given as if
     done by: [print_string ( prep fname |> snd |> fst )] *)
 
+val memoize : ('a, 'b) Hashtbl.t -> ('a -> 'b) -> 'a -> 'b
+(** [memoize memo f value] returns the result of [f value]. The hashtable [memo]
+    is used to cache results, so repeated calls with the same [value] will not
+    call [f] again. *)
+
 (** Module representing a pair of integers, useful for Set.Make *)
 module IntPair : sig
   type t = int * int