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diff --git a/dom/smil/nsSMILTimeValue.h b/dom/smil/nsSMILTimeValue.h
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+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim: set ts=8 sts=2 et sw=2 tw=80: */
+/* 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/. */
+
+#ifndef NS_SMILTIMEVALUE_H_
+#define NS_SMILTIMEVALUE_H_
+
+#include "nsSMILTypes.h"
+#include "nsDebug.h"
+
+/*----------------------------------------------------------------------
+ * nsSMILTimeValue class
+ *
+ * A tri-state time value.
+ *
+ * First a quick overview of the SMIL time data types:
+ *
+ * nsSMILTime          -- a timestamp in milliseconds.
+ * nsSMILTimeValue     -- (this class) a timestamp that can take the additional
+ *                        states 'indefinite' and 'unresolved'
+ * nsSMILInstanceTime  -- an nsSMILTimeValue used for constructing intervals. It
+ *                        contains additional fields to govern reset behavior
+ *                        and track timing dependencies (e.g. syncbase timing).
+ * nsSMILInterval      -- a pair of nsSMILInstanceTimes that defines a begin and
+ *                        an end time for animation.
+ * nsSMILTimeValueSpec -- a component of a begin or end attribute, such as the
+ *                        '5s' or 'a.end+2m' in begin="5s; a.end+2m". Acts as
+ *                        a broker between an nsSMILTimedElement and its
+ *                        nsSMILInstanceTimes by generating new instance times
+ *                        and handling changes to existing times.
+ *
+ * Objects of this class may be in one of three states:
+ *
+ * 1) The time is resolved and has a definite millisecond value
+ * 2) The time is resolved and indefinite
+ * 3) The time is unresolved
+ *
+ * In summary:
+ *
+ * State      | GetMillis       | IsDefinite | IsIndefinite | IsResolved
+ * -----------+-----------------+------------+--------------+------------
+ * Definite   | nsSMILTimeValue | true       | false        | true
+ * -----------+-----------------+------------+--------------+------------
+ * Indefinite | --              | false      | true         | true
+ * -----------+-----------------+------------+--------------+------------
+ * Unresolved | --              | false      | false        | false
+ *
+ */
+
+class nsSMILTimeValue
+{
+public:
+  // Creates an unresolved time value
+  nsSMILTimeValue()
+  : mMilliseconds(kUnresolvedMillis),
+    mState(STATE_UNRESOLVED)
+  { }
+
+  // Creates a resolved time value
+  explicit nsSMILTimeValue(nsSMILTime aMillis)
+  : mMilliseconds(aMillis),
+    mState(STATE_DEFINITE)
+  { }
+
+  // Named constructor to create an indefinite time value
+  static nsSMILTimeValue Indefinite()
+  {
+    nsSMILTimeValue value;
+    value.SetIndefinite();
+    return value;
+  }
+
+  bool IsIndefinite() const { return mState == STATE_INDEFINITE; }
+  void SetIndefinite()
+  {
+    mState = STATE_INDEFINITE;
+    mMilliseconds = kUnresolvedMillis;
+  }
+
+  bool IsResolved() const { return mState != STATE_UNRESOLVED; }
+  void SetUnresolved()
+  {
+    mState = STATE_UNRESOLVED;
+    mMilliseconds = kUnresolvedMillis;
+  }
+
+  bool IsDefinite() const { return mState == STATE_DEFINITE; }
+  nsSMILTime GetMillis() const
+  {
+    MOZ_ASSERT(mState == STATE_DEFINITE,
+               "GetMillis() called for unresolved or indefinite time");
+
+    return mState == STATE_DEFINITE ? mMilliseconds : kUnresolvedMillis;
+  }
+
+  void SetMillis(nsSMILTime aMillis)
+  {
+    mState = STATE_DEFINITE;
+    mMilliseconds = aMillis;
+  }
+
+  int8_t CompareTo(const nsSMILTimeValue& aOther) const;
+
+  bool operator==(const nsSMILTimeValue& aOther) const
+  { return CompareTo(aOther) == 0; }
+
+  bool operator!=(const nsSMILTimeValue& aOther) const
+  { return CompareTo(aOther) != 0; }
+
+  bool operator<(const nsSMILTimeValue& aOther) const
+  { return CompareTo(aOther) < 0; }
+
+  bool operator>(const nsSMILTimeValue& aOther) const
+  { return CompareTo(aOther) > 0; }
+
+  bool operator<=(const nsSMILTimeValue& aOther) const
+  { return CompareTo(aOther) <= 0; }
+
+  bool operator>=(const nsSMILTimeValue& aOther) const
+  { return CompareTo(aOther) >= 0; }
+
+private:
+  static nsSMILTime kUnresolvedMillis;
+
+  nsSMILTime mMilliseconds;
+  enum {
+    STATE_DEFINITE,
+    STATE_INDEFINITE,
+    STATE_UNRESOLVED
+  } mState;
+};
+
+#endif // NS_SMILTIMEVALUE_H_