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diff --git a/Source/WebCore/style/StyleSharingResolver.h b/Source/WebCore/style/StyleSharingResolver.h
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+++ b/Source/WebCore/style/StyleSharingResolver.h
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+/*
+ * Copyright (C) 2016 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:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. 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.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``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 APPLE INC. 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.
+ */
+
+#pragma once
+
+#include <wtf/HashMap.h>
+
+namespace WebCore {
+
+class Document;
+class DocumentRuleSets;
+class Element;
+class Node;
+class RenderStyle;
+class RuleSet;
+class SelectorFilter;
+class SpaceSplitString;
+class StyledElement;
+
+namespace Style {
+
+class Update;
+
+class SharingResolver {
+public:
+    SharingResolver(const Document&, const DocumentRuleSets&, const SelectorFilter&);
+
+    std::unique_ptr<RenderStyle> resolve(const Element&, const Update&);
+
+private:
+    struct Context;
+
+    StyledElement* findSibling(const Context&, Node*, unsigned& count) const;
+    Node* locateCousinList(const Element* parent) const;
+    bool canShareStyleWithElement(const Context&, const StyledElement& candidateElement) const;
+    bool styleSharingCandidateMatchesRuleSet(const StyledElement&, const RuleSet*) const;
+    bool sharingCandidateHasIdenticalStyleAffectingAttributes(const Context&, const StyledElement& sharingCandidate) const;
+    bool classNamesAffectedByRules(const SpaceSplitString& classNames) const;
+
+    const Document& m_document;
+    const DocumentRuleSets& m_ruleSets;
+    const SelectorFilter& m_selectorFilter;
+
+    HashMap<const Element*, const Element*> m_elementsSharingStyle;
+};
+
+}
+}