add CCVector.ensure_with

src/core/CCVector.ml

@@ -117,13 +117,12 @@ let _grow v x =
       _resize v size
     )
 
-(* resize so that capacity is at least size. Use a doubling-size
-  strategy so that calling many times [ensure] will
+(* v is not empty; ensure it has at least [size] slots.
+
+  Use a doubling-size strategy so that calling many times [ensure] will
   behave well *)
-let ensure v size =
-  if Array.length v.vec = 0
-    then ()
-  else if size > Sys.max_array_length
+let ensure_not_empty_ v size =
+  if size > Sys.max_array_length
     then failwith "vec.ensure: size too big"
   else (
     let n = ref (max 16 (Array.length v.vec)) in
@@ -131,6 +130,16 @@ let ensure v size =
     _resize v !n
   )
 
+let ensure_with ~init v size =
+  if Array.length v.vec = 0
+  then v.vec <- Array.make size init
+  else ensure_not_empty_ v size
+
+let ensure v size =
+  if Array.length v.vec = 0
+  then ()
+  else ensure_not_empty_  v size
+
 let clear v =
   v.size <- 0
 

src/core/CCVector.mli

@@ -72,9 +72,16 @@ val init : int -> (int -> 'a) -> ('a, 'mut) t
 val clear : ('a, rw) t -> unit
 (** clear the content of the vector *)
 
+val ensure_with : init:'a -> ('a, rw) t -> int -> unit
+(** Hint to the vector that it should have at least the given capacity.
+    @param init if [capacity v = 0], used as a filler
+      element for the underlying array (see {!create_with})
+    @since NEXT_RELEASE *)
+
 val ensure : ('a, rw) t -> int -> unit
 (** Hint to the vector that it should have at least the given capacity.
-    Just a hint, will not be enforced if the vector is empty. *)
+    Just a hint, will not be enforced if the vector is empty and [init]
+      is not provided. *)
 
 val is_empty : ('a, _) t -> bool
 (** is the vector empty? *)