| // Copyright 2014 The Chromium Authors. All rights reserved. |
| // Use of this source code is governed by a BSD-style license that can be |
| // found in the LICENSE file. |
| |
| #ifndef CAST_STREAMING_CLOCK_DRIFT_SMOOTHER_H_ |
| #define CAST_STREAMING_CLOCK_DRIFT_SMOOTHER_H_ |
| |
| #include <chrono> |
| |
| #include "platform/api/time.h" |
| |
| namespace openscreen { |
| namespace cast { |
| |
| // Tracks the jitter and drift between clocks, providing a smoothed offset. |
| // Internally, a Simple IIR filter is used to maintain a running average that |
| // moves at a rate based on the passage of time. |
| class ClockDriftSmoother { |
| public: |
| // |time_constant| is the amount of time an impulse signal takes to decay by |
| // ~62.6%. Interpretation: If the value passed to several Update() calls is |
| // held constant for T seconds, then the running average will have moved |
| // towards the value by ~62.6% from where it started. |
| explicit ClockDriftSmoother(Clock::duration time_constant); |
| ~ClockDriftSmoother(); |
| |
| // Returns the current offset. |
| Clock::duration Current() const; |
| |
| // Discard all history and reset to exactly |offset|, measured |now|. |
| void Reset(Clock::time_point now, Clock::duration offset); |
| |
| // Update the current offset, which was measured |now|. The weighting that |
| // |measured_offset| will have on the running average is influenced by how |
| // much time has passed since the last call to this method (or Reset()). |
| // |now| should be monotonically non-decreasing over successive calls of this |
| // method. |
| void Update(Clock::time_point now, Clock::duration measured_offset); |
| |
| // A time constant suitable for most use cases, where the clocks are expected |
| // to drift very little with respect to each other, and the jitter caused by |
| // clock imprecision is effectively canceled out. |
| static constexpr std::chrono::seconds kDefaultTimeConstant{30}; |
| |
| private: |
| const std::chrono::duration<double, Clock::duration::period> time_constant_; |
| |
| // The time at which |estimated_tick_offset_| was last updated. |
| Clock::time_point last_update_time_; |
| |
| // The current estimated offset, as number of Clock::duration ticks. |
| double estimated_tick_offset_; |
| }; |
| |
| } // namespace cast |
| } // namespace openscreen |
| |
| #endif // CAST_STREAMING_CLOCK_DRIFT_SMOOTHER_H_ |