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diff --git a/dom/smil/nsSMILMilestone.h b/dom/smil/nsSMILMilestone.h
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+++ b/dom/smil/nsSMILMilestone.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_SMILMILESTONE_H_
+#define NS_SMILMILESTONE_H_
+
+/*
+ * A significant moment in an nsSMILTimedElement's lifetime where a sample is
+ * required.
+ *
+ * Animations register the next milestone in their lifetime with the time
+ * container that they belong to. When the animation controller goes to run
+ * a sample it first visits all the animations that have a registered milestone
+ * in order of their milestone times. This allows interdependencies between
+ * animations to be correctly resolved and events to fire in the proper order.
+ *
+ * A distinction is made between a milestone representing the end of an interval
+ * and any other milestone. This is because if animation A ends at time t, and
+ * animation B begins at the same time t (or has some other significant moment
+ * such as firing a repeat event), SMIL's endpoint-exclusive timing model
+ * implies that the interval end occurs first. In fact, interval ends can be
+ * thought of as ending an infinitesimally small time before t. Therefore,
+ * A should be sampled before B.
+ *
+ * Furthermore, this distinction between sampling the end of an interval and
+ * a regular sample is used within the timing model (specifically in
+ * nsSMILTimedElement) to ensure that all intervals ending at time t are sampled
+ * before any new intervals are entered so that we have a fully up-to-date set
+ * of instance times available before committing to a new interval. Once an
+ * interval is entered, the begin time is fixed.
+ */
+class nsSMILMilestone
+{
+public:
+  nsSMILMilestone(nsSMILTime aTime, bool aIsEnd)
+    : mTime(aTime), mIsEnd(aIsEnd)
+  { }
+
+  nsSMILMilestone()
+    : mTime(0), mIsEnd(false)
+  { }
+
+  bool operator==(const nsSMILMilestone& aOther) const
+  {
+    return mTime == aOther.mTime && mIsEnd == aOther.mIsEnd;
+  }
+
+  bool operator!=(const nsSMILMilestone& aOther) const
+  {
+    return !(*this == aOther);
+  }
+
+  bool operator<(const nsSMILMilestone& aOther) const
+  {
+    // Earlier times sort first, and for equal times end milestones sort first
+    return mTime < aOther.mTime ||
+          (mTime == aOther.mTime && mIsEnd && !aOther.mIsEnd);
+  }
+
+  bool operator<=(const nsSMILMilestone& aOther) const
+  {
+    return *this == aOther || *this < aOther;
+  }
+
+  bool operator>=(const nsSMILMilestone& aOther) const
+  {
+    return !(*this < aOther);
+  }
+
+  nsSMILTime   mTime;  // The milestone time. This may be in container time or
+                       // parent container time depending on where it is used.
+  bool mIsEnd; // true if this milestone corresponds to an interval
+                       // end, false otherwise.
+};
+
+#endif // NS_SMILMILESTONE_H_