[Chromoting] Add AudioVolumeApplier to reduce the complexity and the dependency of kChannels

remoting/host/audio_volume_filter.cc

+// Copyright 2017 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.
+
+#include "remoting/host/audio_volume_filter.h"
+
+namespace remoting {
+
+AudioVolumeFilter::AudioVolumeFilter(int silence_threshold)
+    : silence_detector_(silence_threshold) {}
+AudioVolumeFilter::~AudioVolumeFilter() = default;
+
+bool AudioVolumeFilter::Apply(int16_t* data, size_t frames) {
+  if (frames == 0) {

[Sergey Ulanov, 2017/05/23 00:02:14]

I think this can be replaced with a DCHECK. IAudioCaptureClient::GetBuffer()
returns S_OK only when *pNumFramesToRead > 0.

See
https://msdn.microsoft.com/en-us/library/windows/desktop/dd370859(v=vs.85).aspx

[Hzj_jie, 2017/05/23 02:54:27]

But if it's used on other platforms, assuming it's always larger than 0 may not
that safe.

+    return false;
+  }
+
+  if (silence_detector_.IsSilence(data, frames)) {
+    return false;
+  }
+
+  float level = GetAudioLevel();
+  if (level == 0) {
+    return false;
+  }
+
+  if (level == 1) {
+    return true;
+  }
+
+  const int sample_count = frames * silence_detector_.channels();
+  const int32_t level_int = static_cast<int32_t>(level * 65536);
+  for (int i = 0; i < sample_count; i++) {
+    data[i] = (static_cast<int32_t>(data[i]) * level_int) >> 16;
+  }
+
+  return true;
+}
+
+void AudioVolumeFilter::Initialize(int sampling_rate, int channels) {
+  silence_detector_.Reset(sampling_rate, channels);
+}
+
+}  // namespace remoting