CCRandom: Add sample_without_replacement

src/core/CCRandom.ml

@@ -78,6 +78,20 @@ let replicate n g st =
     if n = 0 then acc else aux (g st :: acc) (n-1)
   in aux [] n
 
+(* Sample without replacement using rejection sampling. *)
+let sample_without_replacement (type elt)  ?(compare=compare) k (rng:elt t) st=
+  let module S = Set.Make(struct type t=elt let compare = compare end) in
+  let rec aux s k =
+    if k <= 0 then
+      S.elements s
+    else
+      let x = rng st in
+      if S.mem x s then
+        aux s k
+      else
+        aux (S.add x s) (k-1) in
+  aux S.empty k
+
 let list_seq l st = List.map (fun f -> f st) l
 
 exception SplitFail

src/core/CCRandom.mli

@@ -76,6 +76,14 @@ val replicate : int -> 'a t -> 'a list t
 (** [replicate n g] makes a list of [n] elements which are all generated
     randomly using [g] *)
 
+val sample_without_replacement:
+  ?compare:('a -> 'a -> int) -> int -> 'a t -> 'a list t
+(** [sample_without_replacement n g] makes a list of [n] elements which are all
+    generated randomly using [g] with the added constraint that none of the generated
+    random values are equal
+    @since 0.15
+ *)
+
 val list_seq : 'a t list -> 'a list t
 (** Build random lists from lists of random generators
     @since 0.4 *)