rtt calculation handles time go backwards

webrtc/modules/rtp_rtcp/source/time_util.cc

Index: webrtc/modules/rtp_rtcp/source/time_util.cc
diff --git a/webrtc/modules/rtp_rtcp/source/time_util.cc b/webrtc/modules/rtp_rtcp/source/time_util.cc
new file mode 100644
index 0000000000000000000000000000000000000000..73b320c5db14183c141bee5f7a0454dce0c145a0
--- /dev/null
+++ b/webrtc/modules/rtp_rtcp/source/time_util.cc
@@ -0,0 +1,30 @@
+/*
+ *  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) {
+  return (x + y/2) / y;
+}
+}  // namespace
+
+int64_t CompactNtpRttToMs(uint32_t compact_ntp_interval) {
+  if (compact_ntp_interval > 0x80000000)
+    return 1;
+  int64_t value = static_cast<int64_t>(compact_ntp_interval);
+  value = DivideRoundToNearest(value * 1000, 1 << 16);

[stefan-webrtc, 2016/03/04 14:07:33]

I think you should comment on how and why this division can be negative. I
assume compact_ntp_interval can't be negative?

[danilchap, 2016/03/04 15:05:50]

negative case handled before division (otherwise division becomes unnecessary
complicated.) Added comments to clarify this (and other lines in this function,
since all of them might be unobvious)

+  return std::max<int64_t>(value, 1);
+}
+}  // namespace webrtc