Cache element indices for :nth-child and :nth-last-child selectors.

Source/core/dom/NthIndexCache.h

Index: Source/core/dom/NthIndexCache.h
diff --git a/Source/core/dom/NthIndexCache.h b/Source/core/dom/NthIndexCache.h
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+++ b/Source/core/dom/NthIndexCache.h
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+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef NthIndexCache_h
+#define NthIndexCache_h
+
+#include "core/dom/Element.h"
+#include "core/dom/ElementTraversal.h"
+#include "platform/heap/Handle.h"
+#include "wtf/HashMap.h"
+#include "wtf/OwnPtr.h"
+#include "wtf/RefPtr.h"
+
+namespace blink {
+
+class Document;
+
+class NthIndexCache : NoBaseWillBeGarbageCollected<NthIndexCache> {
+    WTF_MAKE_NONCOPYABLE(NthIndexCache);
+public:
+    static PassOwnPtr<NthIndexCache> create(Document& document)
+    {
+        return adoptPtr(new NthIndexCache(document));
+    }
+
+    unsigned nthChildIndex(Element&);
+    unsigned nthLastChildIndex(Element&);
+
+    void clear();
+
+    DECLARE_TRACE();
+
+private:
+    NthIndexCache(Document& document)
+        : m_document(document) { }
+
+    class NthIndexData : public NoBaseWillBeGarbageCollected<NthIndexData> {
+        WTF_MAKE_NONCOPYABLE(NthIndexData);
+    public:
+        NthIndexData() : m_count(0) { }
+
+        inline unsigned nthIndex(Element&);
+        inline unsigned nthLastIndex(Element&);
+
+        unsigned cacheNthIndices(Element&);
+
+        WillBeHeapHashMap<RawPtrWillBeMember<Element>, unsigned> m_elementIndexMap;
+        unsigned m_count;
+
+        DECLARE_TRACE();
+    };
+
+    NthIndexData& ensureNthIndexDataFor(Node&);
+    inline unsigned nthIndex(Element&);
+
+    using ParentMap = WillBeHeapHashMap<RefPtrWillBeMember<Node>, OwnPtrWillBeMember<NthIndexData>>;
+
+    OwnPtrWillBeMember<ParentMap> m_parentMap;
+    RawPtrWillBeMember<Document> m_document;
+};
+
+inline unsigned NthIndexCache::NthIndexData::nthIndex(Element& element)
+{
+    if (!m_count)
+        return cacheNthIndices(element);
+
+    unsigned index = 0;
+    for (Element* sibling = &element; sibling; sibling = ElementTraversal::previousSibling(*sibling), index++) {
+        auto it = m_elementIndexMap.find(sibling);
+        if (it != m_elementIndexMap.end())
+            return it->value + index;
+    }
+    return index;
+}
+
+inline unsigned NthIndexCache::NthIndexData::nthLastIndex(Element& element)
+{
+    unsigned index = nthIndex(element);
+    return m_count - index + 1;
+}
+
+inline unsigned NthIndexCache::nthChildIndex(Element& element)
+{
+    ASSERT(element.parentNode());
+    return ensureNthIndexDataFor(*element.parentNode()).nthIndex(element);
+}
+
+inline unsigned NthIndexCache::nthLastChildIndex(Element& element)
+{
+    ASSERT(element.parentNode());
+    return ensureNthIndexDataFor(*element.parentNode()).nthLastIndex(element);
+}
+
+} // namespace blink
+
+#endif // NthIndexCache_h