text: /*
   *  Copyright (c) 2018 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.
   */
  #ifndef CALL_RECEIVE_TIME_CALCULATOR_H_
  #define CALL_RECEIVE_TIME_CALCULATOR_H_
  
  #include <stdint.h>
  #include <memory>
  
  #include "absl/types/optional.h"
  
  namespace webrtc {
  
  // The receive time calculator serves the purpose of combining packet time
  // stamps with a safely incremental clock. This assumes that the packet time
  // stamps are based on lower layer timestamps that have more accurate time
  // increments since they are based on the exact receive time. They might
  // however, have large jumps due to clock resets in the system. To compensate
  // this they are combined with a safe clock source that is guaranteed to be
  // consistent, but it will not be able to measure the exact time when a packet
  // is received.
  class ReceiveTimeCalculator {
   public:
    static std::unique_ptr<ReceiveTimeCalculator> CreateFromFieldTrial();
    // The min delta is used to correct for jumps backwards in time, to allow some
    // packet reordering a small negative value is appropriate to use. The max
    // delta difference is used as margin when detecting when packet time
    // increases more than the safe clock. This should be larger than the largest
    // expected sysmtem induced delay in the safe clock timestamp.
    ReceiveTimeCalculator(int64_t min_delta_ms, int64_t max_delta_diff_ms);
    int64_t ReconcileReceiveTimes(int64_t packet_time_us_, int64_t safe_time_us_);
  
   private:
    const int64_t min_delta_us_;
    const int64_t max_delta_diff_us_;
    absl::optional<int64_t> receive_time_offset_us_;
    int64_t last_packet_time_us_ = 0;
    int64_t last_safe_time_us_ = 0;
  };
  }  // namespace webrtc
  #endif  // CALL_RECEIVE_TIME_CALCULATOR_H_
  