rtt calculation handles time go backwards

webrtc/modules/rtp_rtcp/source/time_util.cc

+/*
+ *  Copyright (c) 2016 The WebRTC project authors. All Rights Reserved.
+ *
+ *  Use of this source code is governed by a BSD-style license
+ *  that can be found in the LICENSE file in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#include "webrtc/modules/rtp_rtcp/source/time_util.h"
+
+#include <algorithm>
+
+namespace webrtc {
+namespace {
+// TODO(danilchap): Make generic, optimize and move to base.
+inline int64_t DivideRoundToNearest(int64_t x, uint32_t y) {
+  // Caller ensure x is positive by converting unsigned value into it.
+  // So this Divide doesn't need to handle negative argument case.
+  return (x + y / 2) / y;
+}
+}  // namespace
+
+int64_t CompactNtpRttToMs(uint32_t compact_ntp_interval) {
+  // Interval to convert expected to be positive, e.g. rtt or delay.
+  // Because interval can be derived from non-monotonic ntp clock,
+  // it might become negative that is indistinguishable from very large values.
+  // Since very large rtt/delay are less likely than non-monotonic ntp clock,
+  // those values consider to be negative and convert to minimum value of 1ms.
+  if (compact_ntp_interval > 0x80000000)
+    return 1;
+  // Convert to 64bit value to avoid multiplication overflow.
+  int64_t value = static_cast<int64_t>(compact_ntp_interval);
+  // To convert to milliseconds need to divide by 2^16 to get seconds,
+  // then multiply by 1000 to get milliseconds. To avoid float operations,
+  // multiplication and division swapped.
+  int64_t ms = DivideRoundToNearest(value * 1000, 1 << 16);
+  // Rtt value 0 considered too good to be true and increases to 1.
+  return std::max<int64_t>(ms, 1);
+}
+}  // namespace webrtc