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-rw-r--r--gfx/src/PingPongRegion.h63
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diff --git a/gfx/src/PingPongRegion.h b/gfx/src/PingPongRegion.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef PingPongRegion_h__
+#define PingPongRegion_h__
+
+/* This class uses a pair of regions and swaps between them while
+ * accumulating to avoid the heap allocations associated with
+ * modifying a region in place.
+ *
+ * It is sizeof(T)*2 + sizeof(T*) and can use end up using
+ * approximately double the amount of memory as using single
+ * region so use it sparingly.
+ */
+
+template <typename T>
+class PingPongRegion
+{
+ typedef typename T::RectType RectType;
+public:
+ PingPongRegion()
+ {
+ rgn = &rgn1;
+ }
+
+ void SubOut(const RectType& aOther)
+ {
+ T* nextRgn = nextRegion();
+ nextRgn->Sub(*rgn, aOther);
+ rgn = nextRgn;
+ }
+
+ void OrWith(const RectType& aOther)
+ {
+ T* nextRgn = nextRegion();
+ nextRgn->Or(*rgn, aOther);
+ rgn = nextRgn;
+ }
+
+ T& Region()
+ {
+ return *rgn;
+ }
+
+private:
+
+ T* nextRegion()
+ {
+ if (rgn == &rgn1) {
+ return &rgn2;
+ } else {
+ return &rgn1;
+ }
+ }
+
+ T* rgn;
+ T rgn1;
+ T rgn2;
+};
+
+#endif