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

Source/core/dom/NthIndexCache.cpp

Index: Source/core/dom/NthIndexCache.cpp
diff --git a/Source/core/dom/NthIndexCache.cpp b/Source/core/dom/NthIndexCache.cpp
new file mode 100644
index 0000000000000000000000000000000000000000..1cb353d9d7d62ee8467a06048501c12a5e37ace9
--- /dev/null
+++ b/Source/core/dom/NthIndexCache.cpp
@@ -0,0 +1,62 @@
+// 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.
+
+#include "config.h"
+#include "core/dom/NthIndexCache.h"
+
+#include "core/dom/Document.h"
+
+namespace blink {
+
+void NthIndexCache::clear()
+{
+    if (m_parentMap)
+        m_parentMap->clear();
+}
+
+NthIndexCache::NthIndexData& NthIndexCache::ensureNthIndexDataFor(Node& parent)
+{
+    if (!m_parentMap)
+        m_parentMap = adoptPtrWillBeNoop(new ParentMap());
+
+    ParentMap::AddResult addResult = m_parentMap->add(&parent, nullptr);
+    if (addResult.isNewEntry)
+        addResult.storedValue->value = adoptPtrWillBeNoop(new NthIndexData());
+
+    ASSERT(addResult.storedValue->value);
+    return *addResult.storedValue->value;
+}
+
+unsigned NthIndexCache::NthIndexData::cacheNthIndices(Element& element)
+{
+    unsigned index = 0;
+    const unsigned spread = 3;
+    unsigned count = 0;
+    for (Element* sibling = ElementTraversal::firstChild(*element.parentNode()); sibling; sibling = ElementTraversal::nextSibling(*sibling)) {
+        if (!(++count % spread))
+            m_elementIndexMap.add(sibling, count);
+        if (sibling == &element)
+            index = count;
+    }
+    ASSERT(count && index);
+    m_count = count;
+    return index;
+}
+
+DEFINE_TRACE(NthIndexCache)
+{
+#if ENABLE(OILPAN)
+    visitor->trace(m_parentMap);
+    visitor->trace(m_document);
+#endif
+}
+
+DEFINE_TRACE(NthIndexCache::NthIndexData)
+{
+#if ENABLE(OILPAN)
+    visitor->trace(m_elementIndexMap);
+#endif
+}
+
+} // namespace blink