add CCHashtbl.get_or_add

src/core/CCHashtbl.ml

@@ -103,6 +103,26 @@ let update tbl ~f ~k =
   assert_equal None (get tbl 1);
 *)
 
+let get_or_add tbl ~f ~k =
+  try Hashtbl.find tbl k
+  with Not_found ->
+    let v = f k in
+    Hashtbl.add tbl k v;
+    v
+
+(*$R
+  let tbl = Hashtbl.create 32 in
+  let v1 = get_or_add tbl ~k:1 ~f:(fun _ -> "1") in
+  assert_equal "1" v1;
+  assert_equal (Some "1") (get tbl 1);
+  let v2 = get_or_add tbl ~k:2 ~f:(fun _ ->"2") in
+  assert_equal "2" v2;
+  assert_equal (Some "2") (get tbl 2);
+  assert_equal "2" (get_or_add tbl ~k:2 ~f:(fun _ -> assert false));
+  assert_equal 2 (Hashtbl.length tbl);
+  ()
+*)
+
 let print pp_k pp_v fmt m =
   Format.fprintf fmt "@[<hov2>tbl {@,";
   let first = ref true in
@@ -188,7 +208,10 @@ module type S = sig
   (** List of bindings (order unspecified)  *)
 
   val of_list : (key * 'a) list -> 'a t
-  (** From the given list of bindings, added in order *)
+  (** Build a table from the given list of bindings [k_i -> v_i],
+      added in order using {!add}. If a key occurs several times,
+      it will be added several times, and the visible binding
+      will be the last one. *)
 
   val update : 'a t -> f:(key -> 'a option -> 'a option) -> k:key -> unit
   (** [update tbl ~f ~k] updates key [k] by calling [f k (Some v)] if
@@ -198,6 +221,13 @@ module type S = sig
       using {!Hashtbl.replace}
       @since 0.14 *)
 
+  val get_or_add : 'a t -> f:(key -> 'a) -> k:key -> 'a
+  (** [get_or_add tbl ~k ~f] finds and returns the binding of [k]
+      in [tbl], if it exists. If it does not exist, then [f k]
+      is called to obtain a new binding [v]; [k -> v] is added
+      to [tbl] and [v] is returned.
+      @since NEXT_RELEASE *)
+
   val print : key printer -> 'a printer -> 'a t printer
   (** Printer for tables
       @since 0.13 *)
@@ -277,6 +307,13 @@ module Make(X : Hashtbl.HashedType)
       | Some _, Some v' -> replace tbl k v'
       | Some _, None -> remove tbl k
 
+  let get_or_add tbl ~f ~k =
+    try find tbl k
+    with Not_found ->
+      let v = f k in
+      add tbl k v;
+      v
+
   let to_seq tbl k = iter (fun key v -> k (key,v)) tbl
 
   let add_seq tbl seq = seq (fun (k,v) -> add tbl k v)

src/core/CCHashtbl.mli

@@ -92,6 +92,13 @@ val update : ('a, 'b) Hashtbl.t -> f:('a -> 'b option -> 'b option) -> k:'a -> u
     using {!Hashtbl.replace}
     @since 0.14 *)
 
+val get_or_add : ('a, 'b) Hashtbl.t -> f:('a -> 'b) -> k:'a -> 'b
+(** [get_or_add tbl ~k ~f] finds and returns the binding of [k]
+    in [tbl], if it exists. If it does not exist, then [f k]
+    is called to obtain a new binding [v]; [k -> v] is added
+    to [tbl] and [v] is returned.
+    @since NEXT_RELEASE *)
+
 val print : 'a printer -> 'b printer -> ('a, 'b) Hashtbl.t printer
 (** Printer for table
     @since 0.13 *)
@@ -181,6 +188,13 @@ module type S = sig
       using {!Hashtbl.replace}
       @since 0.14 *)
 
+  val get_or_add : 'a t -> f:(key -> 'a) -> k:key -> 'a
+  (** [get_or_add tbl ~k ~f] finds and returns the binding of [k]
+      in [tbl], if it exists. If it does not exist, then [f k]
+      is called to obtain a new binding [v]; [k -> v] is added
+      to [tbl] and [v] is returned.
+      @since NEXT_RELEASE *)
+
   val print : key printer -> 'a printer -> 'a t printer
   (** Printer for tables
       @since 0.13 *)