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diff --git a/layout/style/CSSVariableResolver.h b/layout/style/CSSVariableResolver.h
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+/* vim: set shiftwidth=2 tabstop=8 autoindent cindent expandtab: */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/* object that resolves CSS variables using specified and inherited variable
+ * values
+ */
+
+#ifndef mozilla_CSSVariableResolver_h
+#define mozilla_CSSVariableResolver_h
+
+#include "mozilla/DebugOnly.h"
+#include "nsCSSParser.h"
+#include "nsCSSScanner.h"
+#include "nsDataHashtable.h"
+#include "nsTArray.h"
+
+namespace mozilla {
+
+class CSSVariableDeclarations;
+class CSSVariableValues;
+class EnumerateVariableReferencesData;
+
+class CSSVariableResolver
+{
+  friend class CSSVariableDeclarations;
+  friend class CSSVariableValues;
+  friend class EnumerateVariableReferencesData;
+public:
+  /**
+   * Creates a new CSSVariableResolver that will output a set of resolved,
+   * computed variables into aOutput.
+   */
+  explicit CSSVariableResolver(CSSVariableValues* aOutput)
+    : mOutput(aOutput)
+#ifdef DEBUG
+    , mResolved(false)
+#endif
+  {
+    MOZ_ASSERT(aOutput);
+  }
+
+  /**
+   * Resolves the set of inherited variables from aInherited and the
+   * set of specified variables from aSpecified.  The resolved variables
+   * are written into mOutput.
+   */
+  void Resolve(const CSSVariableValues* aInherited,
+               const CSSVariableDeclarations* aSpecified);
+
+private:
+  struct Variable
+  {
+    Variable(const nsAString& aVariableName,
+             nsString aValue,
+             nsCSSTokenSerializationType aFirstToken,
+             nsCSSTokenSerializationType aLastToken,
+             bool aWasInherited)
+      : mVariableName(aVariableName)
+      , mValue(aValue)
+      , mFirstToken(aFirstToken)
+      , mLastToken(aLastToken)
+      , mWasInherited(aWasInherited)
+      , mResolved(false)
+      , mReferencesNonExistentVariable(false)
+      , mInStack(false)
+      , mIndex(0)
+      , mLowLink(0) { }
+
+    nsString mVariableName;
+    nsString mValue;
+    nsCSSTokenSerializationType mFirstToken;
+    nsCSSTokenSerializationType mLastToken;
+
+    // Whether this variable came from the set of inherited variables.
+    bool mWasInherited;
+
+    // Whether this variable has been resolved yet.
+    bool mResolved;
+
+    // Whether this variables includes any references to non-existent variables.
+    bool mReferencesNonExistentVariable;
+
+    // Bookkeeping for the cycle remover algorithm.
+    bool mInStack;
+    size_t mIndex;
+    size_t mLowLink;
+  };
+
+  /**
+   * Adds or modifies an existing entry in the set of variables to be resolved.
+   * This is intended to be called by the AddVariablesToResolver functions on
+   * the CSSVariableDeclarations and CSSVariableValues objects passed in to
+   * Resolve.
+   *
+   * @param aName The variable name (not including any "--" prefix that would
+   *   be part of the custom property name) whose value is to be set.
+   * @param aValue The variable value.
+   * @param aFirstToken The type of token at the start of the variable value.
+   * @param aLastToken The type of token at the en of the variable value.
+   * @param aWasInherited Whether this variable came from the set of inherited
+   *   variables.
+   */
+  void Put(const nsAString& aVariableName,
+           nsString aValue,
+           nsCSSTokenSerializationType aFirstToken,
+           nsCSSTokenSerializationType aLastToken,
+           bool aWasInherited);
+
+  // Helper functions for Resolve.
+  void RemoveCycles(size_t aID);
+  void ResolveVariable(size_t aID);
+
+  // A mapping of variable names to an ID that indexes into mVariables
+  // and mReferences.
+  nsDataHashtable<nsStringHashKey, size_t> mVariableIDs;
+
+  // The set of variables.
+  nsTArray<Variable> mVariables;
+
+  // The list of variables that each variable references.
+  nsTArray<nsTArray<size_t> > mReferences;
+
+  // The next index to assign to a variable found during the cycle removing
+  // algorithm's traversal of the variable reference graph.
+  size_t mNextIndex;
+
+  // Stack of variable IDs that we push to as we traverse the variable reference
+  // graph while looking for cycles.  Variable::mInStack reflects whether a
+  // given variable has its ID in mStack.
+  nsTArray<size_t> mStack;
+
+  // CSS parser to use for parsing property values with variable references.
+  nsCSSParser mParser;
+
+  // The object to output the resolved variables into.
+  CSSVariableValues* mOutput;
+
+#ifdef DEBUG
+  // Whether Resolve has been called.
+  bool mResolved;
+#endif
+};
+
+} // namespace mozilla
+
+#endif