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diff --git a/dom/smil/SMILTimeValue.h b/dom/smil/SMILTimeValue.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 DOM_SMIL_SMILTIMEVALUE_H_
+#define DOM_SMIL_SMILTIMEVALUE_H_
+
+#include "mozilla/SMILTypes.h"
+#include "nsDebug.h"
+
+namespace mozilla {
+
+/*----------------------------------------------------------------------
+ * SMILTimeValue class
+ *
+ * A tri-state time value.
+ *
+ * First a quick overview of the SMIL time data types:
+ *
+ * SMILTime          -- a timestamp in milliseconds.
+ * SMILTimeValue     -- (this class) a timestamp that can take the additional
+ *                      states 'indefinite' and 'unresolved'
+ * SMILInstanceTime  -- an SMILTimeValue used for constructing intervals. It
+ *                      contains additional fields to govern reset behavior
+ *                      and track timing dependencies (e.g. syncbase timing).
+ * SMILInterval      -- a pair of SMILInstanceTimes that defines a begin and
+ *                      an end time for animation.
+ * SMILTimeValueSpec -- 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 SMILTimedElement and its
+ *                      SMILInstanceTimes 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   | SMILTimeValue | true       | false        | true
+ * -----------+---------------+------------+--------------+------------
+ * Indefinite | --            | false      | true         | true
+ * -----------+---------------+------------+--------------+------------
+ * Unresolved | --            | false      | false        | false
+ *
+ */
+
+class SMILTimeValue {
+ public:
+  // Creates an unresolved time value
+  SMILTimeValue()
+      : mMilliseconds(kUnresolvedMillis), mState(STATE_UNRESOLVED) {}
+
+  // Creates a resolved time value
+  explicit SMILTimeValue(SMILTime aMillis)
+      : mMilliseconds(aMillis), mState(STATE_DEFINITE) {}
+
+  // Named constructor to create an indefinite time value
+  static SMILTimeValue Indefinite() {
+    SMILTimeValue 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; }
+  SMILTime GetMillis() const {
+    MOZ_ASSERT(mState == STATE_DEFINITE,
+               "GetMillis() called for unresolved or indefinite time");
+
+    return mState == STATE_DEFINITE ? mMilliseconds : kUnresolvedMillis;
+  }
+
+  void SetMillis(SMILTime aMillis) {
+    mState = STATE_DEFINITE;
+    mMilliseconds = aMillis;
+  }
+
+  int8_t CompareTo(const SMILTimeValue& aOther) const;
+
+  bool operator==(const SMILTimeValue& aOther) const {
+    return CompareTo(aOther) == 0;
+  }
+
+  bool operator!=(const SMILTimeValue& aOther) const {
+    return CompareTo(aOther) != 0;
+  }
+
+  bool operator<(const SMILTimeValue& aOther) const {
+    return CompareTo(aOther) < 0;
+  }
+
+  bool operator>(const SMILTimeValue& aOther) const {
+    return CompareTo(aOther) > 0;
+  }
+
+  bool operator<=(const SMILTimeValue& aOther) const {
+    return CompareTo(aOther) <= 0;
+  }
+
+  bool operator>=(const SMILTimeValue& aOther) const {
+    return CompareTo(aOther) >= 0;
+  }
+
+ private:
+  static const SMILTime kUnresolvedMillis;
+
+  SMILTime mMilliseconds;
+  enum { STATE_DEFINITE, STATE_INDEFINITE, STATE_UNRESOLVED } mState;
+};
+
+}  // namespace mozilla
+
+#endif  // DOM_SMIL_SMILTIMEVALUE_H_