Refactor StyleVariableData to fallback on root map for performance reasons.

third_party/WebKit/Source/core/style/StyleVariableData.cpp

 // 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 "core/style/StyleVariableData.h"
 
 #include "core/style/DataEquivalency.h"
 
 namespace blink {
 
 bool StyleVariableData::operator==(const StyleVariableData& other) const
 {
-    if (m_data.size() != other.m_data.size()) {
+    // It's technically possible for divergent roots to be value-equal,
+    // but unlikely. This equality operator is used for optimization purposes
+    // so it's OK to be occasionally wrong.
+    // TODO(shanestephens): Rename this to something that indicates it may not
+    // always return equality.
+    if (m_root != other.m_root)
         return false;
-    }
+
+    if (m_data.size() != other.m_data.size())
+        return false;
 
     for (const auto& iter : m_data) {
         RefPtr<CSSVariableData> otherData = other.m_data.get(iter.key);
         if (!dataEquivalent(iter.value, otherData))
             return false;
     }
 
     return true;
 }
 
+StyleVariableData::StyleVariableData(StyleVariableData& other)
+{
+    if (!other.m_root) {
+        m_root = &other;
+    } else {
+        m_data = other.m_data;
+        m_root = other.m_root;
+    }
+}
+
+CSSVariableData* StyleVariableData::getVariable(const AtomicString& name) const
+{
+    auto result = m_data.find(name);
+    if (result == m_data.end() && m_root)
+        return m_root->getVariable(name);
+    if (result == m_data.end())
+        return nullptr;
+    return result->value.get();
+}
+
+std::unique_ptr<HashMap<AtomicString, RefPtr<CSSVariableData>>> StyleVariableData::getVariables() const
+{
+    std::unique_ptr<HashMap<AtomicString, RefPtr<CSSVariableData>>> result;
+    if (m_root) {
+        result.reset(new HashMap<AtomicString, RefPtr<CSSVariableData>>(m_root->m_data));
+        for (auto it = m_data.begin(); it != m_data.end(); ++it)
+            result->set(it->key, it->value);
+    } else {
+        result.reset(new HashMap<AtomicString, RefPtr<CSSVariableData>>(m_data));
+    }
+    return result;
+}
+
 } // namespace blink