diff -r 000000000000 -r 6474c204b198 content/canvas/src/WebGLElementArrayCache.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/content/canvas/src/WebGLElementArrayCache.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,86 @@
+/* -*- Mode: C++; tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 4 -*- */
+/* 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 WEBGLELEMENTARRAYCACHE_H
+#define WEBGLELEMENTARRAYCACHE_H
+
+#include "mozilla/MemoryReporting.h"
+#include <stdint.h>
+#include "nscore.h"
+#include "GLDefs.h"
+
+namespace mozilla {
+
+template<typename T>
+struct WebGLElementArrayCacheTree;
+
+/*
+ * WebGLElementArrayCache implements WebGL element array buffer validation for drawElements.
+ *
+ * Its exposes methods meant to be called by WebGL method implementations:
+ *  - Validate, to be called by WebGLContext::DrawElements, is where we use the cache
+ *  - BufferData and BufferSubData, to be called by eponymous WebGL methods, are how
+ *    data is fed into the cache
+ *
+ * Most of the implementation is hidden in the auxilary class template, WebGLElementArrayCacheTree.
+ * Refer to its code for design comments.
+ */
+class WebGLElementArrayCache {
+
+public:
+  bool BufferData(const void* ptr, size_t byteSize);
+  void BufferSubData(size_t pos, const void* ptr, size_t updateByteSize);
+
+  bool Validate(GLenum type, uint32_t maxAllowed, size_t first, size_t count,
+                uint32_t* out_upperBound = nullptr);
+
+  template<typename T>
+  T Element(size_t i) const { return Elements<T>()[i]; }
+
+  WebGLElementArrayCache()
+    : mUntypedData(nullptr)
+    , mByteSize(0)
+    , mUint8Tree(nullptr)
+    , mUint16Tree(nullptr)
+    , mUint32Tree(nullptr)
+  {}
+
+  ~WebGLElementArrayCache();
+
+  size_t SizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf) const;
+
+private:
+
+  template<typename T>
+  bool Validate(uint32_t maxAllowed, size_t first, size_t count,
+                uint32_t* out_upperBound);
+
+  size_t ByteSize() const {
+    return mByteSize;
+  }
+
+  template<typename T>
+  const T* Elements() const { return static_cast<const T*>(mUntypedData); }
+  template<typename T>
+  T* Elements() { return static_cast<T*>(mUntypedData); }
+
+  void InvalidateTrees(size_t firstByte, size_t lastByte);
+
+  template<typename T>
+  friend struct WebGLElementArrayCacheTree;
+  template<typename T>
+  friend struct TreeForType;
+
+  void* mUntypedData;
+  size_t mByteSize;
+  WebGLElementArrayCacheTree<uint8_t>* mUint8Tree;
+  WebGLElementArrayCacheTree<uint16_t>* mUint16Tree;
+  WebGLElementArrayCacheTree<uint32_t>* mUint32Tree;
+};
+
+
+} // end namespace mozilla
+
+#endif // WEBGLELEMENTARRAYCACHE_H