Adding SmoothingFilter to audio network adaptor.

webrtc/modules/audio_coding/audio_network_adaptor/smoothing_filter.h

Index: webrtc/modules/audio_coding/audio_network_adaptor/smoothing_filter.h
diff --git a/webrtc/modules/audio_coding/audio_network_adaptor/smoothing_filter.h b/webrtc/modules/audio_coding/audio_network_adaptor/smoothing_filter.h
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+/*
+ *  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.
+ */
+
+#ifndef WEBRTC_MODULES_AUDIO_CODING_AUDIO_NETWORK_ADAPTOR_SMOOTHING_FILTER_H_
+#define WEBRTC_MODULES_AUDIO_CODING_AUDIO_NETWORK_ADAPTOR_SMOOTHING_FILTER_H_
+
+#include "webrtc/base/constructormagic.h"
+#include "webrtc/base/exp_filter.h"
+#include "webrtc/base/optional.h"
+#include "webrtc/system_wrappers/include/clock.h"
+
+namespace webrtc {
+
+class SmoothingFilter {
+ public:
+  virtual ~SmoothingFilter() = default;
+  virtual void AddSample(float sample) = 0;
+  virtual rtc::Optional<float> GetAverage() const = 0;
+};
+
+// SmoothingFilterImpl applies an exponential filter
+//   alpha = exp(-sample_interval / time_constant);
+//   y[t] = alpha * y[t-1] + (1 - alpha) * sample;
+class SmoothingFilterImpl final : public SmoothingFilter {
+ public:
+  // |time_constant_ms| is the time constant for the exponential filter.
+  SmoothingFilterImpl(int time_constant_ms, const Clock* clock);
+
+  void AddSample(float sample) override;
+  rtc::Optional<float> GetAverage() const override;
+
+ private:
+  const int time_constant_ms_;
+  const Clock* const clock_;
+
+  bool first_sample_received_;
+  bool initialized_;
+  int64_t first_sample_time_ms_;
+  int64_t last_sample_time_ms_;
+  rtc::ExpFilter filter_;
+
+  RTC_DISALLOW_COPY_AND_ASSIGN(SmoothingFilterImpl);
+};
+
+}  // namespace webrtc
+
+#endif  // WEBRTC_MODULES_AUDIO_CODING_AUDIO_NETWORK_ADAPTOR_SMOOTHING_FILTER_H_