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diff --git a/Source/WebCore/css/CSSVariableData.cpp b/Source/WebCore/css/CSSVariableData.cpp
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+++ b/Source/WebCore/css/CSSVariableData.cpp
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+// Copyright 2015 The Chromium Authors. All rights reserved.
+// 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:
+//
+//    * 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.
+
+#include "config.h"
+#include "CSSVariableData.h"
+
+#include "CSSCustomPropertyValue.h"
+#include "CSSParser.h"
+#include "CSSParserTokenRange.h"
+#include <wtf/text/AtomicStringHash.h>
+#include <wtf/text/StringBuilder.h>
+#include <wtf/text/StringView.h>
+
+namespace WebCore {
+
+template<typename CharacterType> void CSSVariableData::updateTokens(const CSSParserTokenRange& range)
+{
+    const CharacterType* currentOffset = m_backingString.characters<CharacterType>();
+    for (const CSSParserToken& token : range) {
+        if (token.hasStringBacking()) {
+            unsigned length = token.value().length();
+            StringView string(currentOffset, length);
+            m_tokens.append(token.copyWithUpdatedString(string));
+            currentOffset += length;
+        } else
+            m_tokens.append(token);
+    }
+    ASSERT(currentOffset == m_backingString.characters<CharacterType>() + m_backingString.length());
+}
+
+bool CSSVariableData::operator==(const CSSVariableData& other) const
+{
+    return tokens() == other.tokens();
+}
+
+void CSSVariableData::consumeAndUpdateTokens(const CSSParserTokenRange& range)
+{
+    StringBuilder stringBuilder;
+    CSSParserTokenRange localRange = range;
+
+    while (!localRange.atEnd()) {
+        CSSParserToken token = localRange.consume();
+        if (token.hasStringBacking())
+            stringBuilder.append(token.value());
+    }
+    m_backingString = stringBuilder.toString();
+    if (m_backingString.is8Bit())
+        updateTokens<LChar>(range);
+    else
+        updateTokens<UChar>(range);
+}
+
+CSSVariableData::CSSVariableData(const CSSParserTokenRange& range, bool needsVariableResolution)
+    : m_needsVariableResolution(needsVariableResolution)
+{
+    ASSERT(!range.atEnd());
+    consumeAndUpdateTokens(range);
+}
+
+bool CSSVariableData::checkVariablesForCycles(const AtomicString& name, CustomPropertyValueMap& customProperties, HashSet<AtomicString>& seenProperties, HashSet<AtomicString>& invalidProperties) const
+{
+    if (invalidProperties.contains(name))
+        return false;
+    
+    HashSet<AtomicString> newSeenProperties = seenProperties;
+    newSeenProperties.add(name);
+    
+    bool valid = checkVariablesForCyclesWithRange(m_tokens, customProperties, newSeenProperties, invalidProperties);
+    if (!valid)
+        invalidProperties.add(name);
+    
+    return valid;
+}
+    
+bool CSSVariableData::checkVariablesForCyclesWithRange(CSSParserTokenRange range, CustomPropertyValueMap& customProperties, HashSet<AtomicString>& seenProperties, HashSet<AtomicString>& invalidProperties) const
+{
+    while (!range.atEnd()) {
+        if (range.peek().functionId() == CSSValueVar) {
+            CSSParserTokenRange block = range.consumeBlock();
+            
+            block.consumeWhitespace();
+            ASSERT(block.peek().type() == IdentToken);
+            AtomicString variableName = block.consumeIncludingWhitespace().value().toAtomicString();
+            ASSERT(block.atEnd() || block.peek().type() == CommaToken);
+            if (seenProperties.contains(variableName))
+                return false;
+
+            RefPtr<CSSCustomPropertyValue> value = customProperties.get(variableName);
+            if (value && value->containsVariables() && !value->checkVariablesForCycles(variableName, customProperties, seenProperties, invalidProperties))
+                return false;
+
+            if (range.peek().type() == CommaToken) {
+                // Fallback.
+                range.consume();
+                return checkVariablesForCyclesWithRange(block, customProperties, seenProperties, invalidProperties);
+            }
+        } else
+            range.consume();
+    }
+    return true;
+}
+
+bool CSSVariableData::resolveVariableFallback(const CustomPropertyValueMap& customProperties, CSSParserTokenRange range, Vector<CSSParserToken>& result) const
+{
+    if (range.atEnd())
+        return false;
+    ASSERT(range.peek().type() == CommaToken);
+    range.consume();
+    return resolveTokenRange(customProperties, range, result);
+}
+    
+bool CSSVariableData::resolveVariableReference(const CustomPropertyValueMap& customProperties, CSSParserTokenRange range, Vector<CSSParserToken>& result) const
+{
+    range.consumeWhitespace();
+    ASSERT(range.peek().type() == IdentToken);
+    AtomicString variableName = range.consumeIncludingWhitespace().value().toAtomicString();
+    ASSERT(range.atEnd() || (range.peek().type() == CommaToken));
+    
+    RefPtr<CSSCustomPropertyValue> property = customProperties.get(variableName);
+    if (!property || !property->value())
+        return resolveVariableFallback(customProperties, range, result);
+    
+    if (property->containsVariables()) {
+        // FIXME: Avoid doing this work more than once.
+        RefPtr<CSSVariableData> resolvedData = property->value()->resolveVariableReferences(customProperties);
+        if (!resolvedData)
+            return false;
+        result.appendVector(resolvedData->tokens());
+    } else
+        result.appendVector(property->value()->tokens());
+    
+    return true;
+}
+
+RefPtr<CSSVariableData> CSSVariableData::resolveVariableReferences(const CustomPropertyValueMap& customProperties) const
+{
+    Vector<CSSParserToken> resolvedTokens;
+    CSSParserTokenRange range = m_tokens;
+    if (!resolveTokenRange(customProperties, range, resolvedTokens))
+        return nullptr;
+    return CSSVariableData::createResolved(resolvedTokens, *this);
+}
+    
+bool CSSVariableData::resolveTokenRange(const CustomPropertyValueMap& customProperties, CSSParserTokenRange range, Vector<CSSParserToken>& result) const
+{
+    bool success = true;
+    while (!range.atEnd()) {
+        if (range.peek().functionId() == CSSValueVar)
+            success &= resolveVariableReference(customProperties, range.consumeBlock(), result);
+        else
+            result.append(range.consume());
+    }
+    return success;
+}
+
+} // namespace WebCore