move 2405 to use a hash table.

bin/day2405.ml

@@ -1,19 +1,21 @@
 module IntSet = Set.Make (Int)
-module IntMap = Map.Make (Int)
+module IntHashtbl = Hashtbl.Make (Int)
 
 (** [add_rule a b m] adds the rule that [a] must appear before [b] to the rule
-    map [m]. *)
-let add_rule a b =
-  let update_rule = function
-    | None -> Some (IntSet.singleton b)
-    | Some s -> Some (IntSet.add b s)
-  in
-  IntMap.update a update_rule
+    map [m]. Returns the updated map [m] *)
+let add_rule a b m =
+  match IntHashtbl.find_opt m a with
+  | None ->
+      IntHashtbl.add m a (IntSet.singleton b);
+      m
+  | Some s ->
+      IntHashtbl.replace m a (IntSet.add b s);
+      m
 
 (** [find_rule a b m] returns [true] if the rule map [m] says that [a] should
     appear before [b]. *)
 let find_rule a b m =
-  match IntMap.find_opt a m with
+  match IntHashtbl.find_opt m a with
   | Some s -> ( match IntSet.find_opt b s with Some _ -> true | None -> false)
   | None -> false
 
@@ -51,7 +53,7 @@ let parse_rules =
         | [ a; b ] -> impl (add_rule a b acc) t
         | _ -> failwith "parse_rules.impl")
   in
-  impl IntMap.empty
+  impl (IntHashtbl.create 17)
 
 (** [parse_page_orders lst] parses a list of page orders.  *)
 let parse_page_orders =