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 "modules/rtp_rtcp/source/time_util.h"

#include <algorithm>

#include "rtc_base/checks.h"
#include "rtc_base/timeutils.h"

namespace webrtc {
namespace {
// TODO(danilchap): Make generic, optimize and move to base.
inline int64_t DivideRoundToNearest(int64_t x, uint32_t y) {
  // Callers ensure x is positive and x + y / 2 doesn't overflow.
  return (x + y / 2) / y;
}

int64_t NtpOffsetMsCalledOnce() {
  constexpr int64_t kNtpJan1970Sec = 2208988800;
  int64_t clock_time = rtc::TimeMillis();
  int64_t utc_time = rtc::TimeUTCMillis();
  return utc_time - clock_time + kNtpJan1970Sec * rtc::kNumMillisecsPerSec;
}

}  // namespace

int64_t NtpOffsetMs() {
  // Calculate the offset once.
  static int64_t ntp_offset_ms = NtpOffsetMsCalledOnce();
  return ntp_offset_ms;
}

NtpTime TimeMicrosToNtp(int64_t time_us) {
  // Since this doesn't return a wallclock time, but only NTP representation
  // of rtc::TimeMillis() clock, the exact offset doesn't matter.
  // To simplify conversions between NTP and RTP time, this offset is
  // limited to milliseconds in resolution.
  int64_t time_ntp_us = time_us + NtpOffsetMs() * 1000;
  RTC_DCHECK_GE(time_ntp_us, 0);  // Time before year 1900 is unsupported.

  // TODO(danilchap): Convert both seconds and fraction together using int128
  // when that type is easily available.
  // Currently conversion is done separetly for seconds and fraction of a second
  // to avoid overflow.

  // Convert seconds to uint32 through uint64 for well-defined cast.
  // Wrap around (will happen in 2036) is expected for ntp time.
  uint32_t ntp_seconds =
      static_cast<uint64_t>(time_ntp_us / rtc::kNumMicrosecsPerSec);

  // Scale fractions of the second to ntp resolution.
  constexpr int64_t kNtpInSecond = 1LL << 32;
  int64_t us_fractions = time_ntp_us % rtc::kNumMicrosecsPerSec;
  uint32_t ntp_fractions =
      us_fractions * kNtpInSecond / rtc::kNumMicrosecsPerSec;
  return NtpTime(ntp_seconds, ntp_fractions);
}

uint32_t SaturatedUsToCompactNtp(int64_t us) {
  constexpr uint32_t kMaxCompactNtp = 0xFFFFFFFF;
  constexpr int kCompactNtpInSecond = 0x10000;
  if (us <= 0)
    return 0;
  if (us >= kMaxCompactNtp * rtc::kNumMicrosecsPerSec / kCompactNtpInSecond)
    return kMaxCompactNtp;
  // To convert to compact ntp need to divide by 1e6 to get seconds,
  // then multiply by 0x10000 to get the final result.
  // To avoid float operations, multiplication and division swapped.
  return DivideRoundToNearest(us * kCompactNtpInSecond,
                              rtc::kNumMicrosecsPerSec);
}

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