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diff --git a/Source/WebCore/dom/DocumentOrderedMap.h b/Source/WebCore/dom/DocumentOrderedMap.h
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+/*
+ * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010 Apple Inc. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are
+ * met:
+ *
+ *     * Redistributions of source code must retain the above copyright
+ * notice, this list of conditions and the following disclaimer.
+ *     * Redistributions in binary form must reproduce the above
+ * copyright notice, this list of conditions and the following disclaimer
+ * in the documentation and/or other materials provided with the
+ * distribution.
+ *     * Neither the name of Google Inc. nor the names of its
+ * contributors may be used to endorse or promote products derived from
+ * this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+ * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+ * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+ * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+ * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+ * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+ * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#ifndef DocumentOrderedMap_h
+#define DocumentOrderedMap_h
+
+#include <wtf/HashCountedSet.h>
+#include <wtf/HashMap.h>
+#include <wtf/text/AtomicStringImpl.h>
+
+namespace WebCore {
+
+class Document;
+class Element;
+
+class DocumentOrderedMap {
+public:
+    void add(AtomicStringImpl*, Element*);
+    void remove(AtomicStringImpl*, Element*);
+    void clear();
+
+    bool contains(AtomicStringImpl*) const;
+    bool containsMultiple(AtomicStringImpl*) const;
+    // concrete instantiations of the get<>() method template
+    Element* getElementById(AtomicStringImpl*, const Document*) const;
+    Element* getElementByMapName(AtomicStringImpl*, const Document*) const;
+    Element* getElementByLowercasedMapName(AtomicStringImpl*, const Document*) const;
+
+    void checkConsistency() const;
+
+private:
+    template<bool keyMatches(AtomicStringImpl*, Element*)> Element* get(AtomicStringImpl*, const Document*) const;
+
+    typedef HashMap<AtomicStringImpl*, Element*> Map;
+
+    // We maintain the invariant that m_duplicateCounts is the count of all elements with a given key
+    // excluding the one referenced in m_map, if any. This means it one less than the total count
+    // when the first node with a given key is cached, otherwise the same as the total count.
+    mutable Map m_map;
+    mutable HashCountedSet<AtomicStringImpl*> m_duplicateCounts;
+};
+
+inline bool DocumentOrderedMap::contains(AtomicStringImpl* id) const
+{
+    return m_map.contains(id) || m_duplicateCounts.contains(id);
+}
+
+inline bool DocumentOrderedMap::containsMultiple(AtomicStringImpl* id) const
+{
+    return m_duplicateCounts.contains(id);
+}
+
+} // namespace WebCore
+
+#endif // DocumentOrderedMap_h
+