1 files changed, 63 insertions, 0 deletions
diff --git a/gfx/src/PingPongRegion.h b/gfx/src/PingPongRegion.h
new file mode 100644
index 000000000..d3bdcae1b
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+++ b/gfx/src/PingPongRegion.h
@@ -0,0 +1,63 @@
+/* -*- 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