Adding SmoothingFilter to audio network adaptor.

webrtc/modules/audio_coding/audio_network_adaptor/smoothing_filter.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 <cmath>
+
+#include "webrtc/modules/audio_coding/audio_network_adaptor/smoothing_filter.h"
+
+namespace webrtc {
+
+SmoothingFilterImpl::SmoothingFilterImpl(int time_constant_ms,
+                                         const Clock* clock)
+    : time_constant_ms_(time_constant_ms),
+      clock_(clock),
+      first_sample_received_(false),
+      initialized_(false),
+      first_sample_time_ms_(0),
+      last_sample_time_ms_(0),
+      filter_(0.0) {}
+
+void SmoothingFilterImpl::AddSample(float sample) {
+  if (!first_sample_received_) {
+    last_sample_time_ms_ = first_sample_time_ms_ = clock_->TimeInMilliseconds();
+    first_sample_received_ = true;
+    RTC_DCHECK_EQ(rtc::ExpFilter::kValueUndefined, filter_.filtered());
+
+    // Since this is first sample, any value for argument 1 should work.
+    filter_.Apply(0.0f, sample);
+    return;
+  }
+
+  int64_t now_ms = clock_->TimeInMilliseconds();
+  if (!initialized_) {
+    float duration = now_ms - first_sample_time_ms_;
+    if (duration < static_cast<int64_t>(time_constant_ms_)) {
+      filter_.UpdateBase(exp(1.0f / duration));
+    } else {
+      initialized_ = true;
+      filter_.UpdateBase(exp(1.0f / time_constant_ms_));
+    }
+  }
+
+  // The filter will do the following:
+  //   float alpha = pow(base, last_update_time_ms_ - now_ms);
+  //   filtered_ = alpha * filtered_ + (1 - alpha) * sample;
+  filter_.Apply(static_cast<float>(last_sample_time_ms_ - now_ms), sample);
+  last_sample_time_ms_ = now_ms;
+}
+
+rtc::Optional<float> SmoothingFilterImpl::GetAverage() const {
+  float value = filter_.filtered();
+  return value == rtc::ExpFilter::kValueUndefined ? rtc::Optional<float>()
+                                                  : rtc::Optional<float>(value);
+}
+
+}  // namespace webrtc