From 01e008256c4de5a6144ed821f95879d180040558 Mon Sep 17 00:00:00 2001
From: Simon Cruanes <simon.cruanes.2007@m4x.org>
Date: Mon, 4 Mar 2013 17:30:34 +0100
Subject: [PATCH] hashset.union/inter take an optional argument for the
 resulting set

---
 hashset.ml  | 19 +++++++++++++------
 hashset.mli |  8 +++++---
 2 files changed, 18 insertions(+), 9 deletions(-)

diff --git a/hashset.ml b/hashset.ml
index bdbc058d..e7dd8b8d 100644
--- a/hashset.ml
+++ b/hashset.ml
@@ -55,10 +55,17 @@ let to_seq set =
 let of_seq set seq =
   Sequence.iter (fun x -> add set x) seq
 
-let union s1 s2 =
-  of_seq s1 (to_seq s2)
+let union ?into (s1 : 'a t) (s2 : 'a t) =
+  let into = match into with
+  | Some s -> of_seq s (to_seq s1); s
+  | None -> copy s1 in
+  of_seq into (to_seq s2);
+  into
 
-let inter s1 s2 =
-  let set = copy s1 in
-  filter (fun x -> mem s2 x) set;
-  set
+let inter ?into (s1 : 'a t) (s2 : 'a t) =
+  let into = match into with
+  | Some s -> s
+  | None -> empty ~eq:s1.PHashtbl.eq ~hash:s1.PHashtbl.hash (cardinal s1) in
+  (* add to [into] elements of [s1] that also belong to [s2] *)
+  of_seq into (Sequence.filter (fun x -> mem s2 x) (to_seq s1));
+  into
diff --git a/hashset.mli b/hashset.mli
index c56c5ce5..f6821655 100644
--- a/hashset.mli
+++ b/hashset.mli
@@ -34,7 +34,7 @@ val empty : ?max_load:float -> ?eq:('a -> 'a -> bool) ->
 
 val copy : 'a t -> 'a t
 
-val clear : 'a t -> 'a t
+val clear : 'a t -> unit
 
 val cardinal : 'a t -> int
 
@@ -55,6 +55,8 @@ val to_seq : 'a t -> 'a Sequence.t
 
 val of_seq : 'a t -> 'a Sequence.t -> unit
 
-val union : 'a t -> 'a t -> 'a t
+val union : ?into:'a t -> 'a t -> 'a t -> 'a t
+  (** Set union. The result is stored in [into] *)
 
-val inter : 'a t -> 'a t -> 'a t
+val inter : ?into:'a t -> 'a t -> 'a t -> 'a t
+  (** Set intersection. The result is stored in [into] *)