Add a simple, safe gain PostProcessor.

chromecast/media/cma/backend/alsa/post_processors/saturated_gain.cc

Index: chromecast/media/cma/backend/alsa/post_processors/saturated_gain.cc
diff --git a/chromecast/media/cma/backend/alsa/post_processors/saturated_gain.cc b/chromecast/media/cma/backend/alsa/post_processors/saturated_gain.cc
new file mode 100644
index 0000000000000000000000000000000000000000..a2d15e8502bd2215b3ec43aede2951858d97c785
--- /dev/null
+++ b/chromecast/media/cma/backend/alsa/post_processors/saturated_gain.cc
@@ -0,0 +1,73 @@
+// 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 "chromecast/media/cma/backend/alsa/post_processors/saturated_gain.h"
+
+#include <algorithm>
+#include <cmath>
+
+#include "base/logging.h"
+#include "base/values.h"
+#include "chromecast/base/serializers.h"
+#include "chromecast/media/cma/backend/alsa/slew_volume.h"
+
+namespace chromecast {
+namespace media {
+
+namespace {
+const int kNoSampleRate = -1;
+
+// Configuration strings:
+const char kGainKey[] = "gain_db";
+
+float DbFsToScale(float db) {
+  return std::pow(10, db / 20);
+}
+
+}  // namespace
+
+SaturatedGain::SaturatedGain(const std::string& config, int channels)
+    : channels_(channels), sample_rate_(kNoSampleRate), last_volume_(-1) {
+  auto config_dict = base::DictionaryValue::From(DeserializeFromJson(config));
+  CHECK(config_dict) << "SaturatedGain config is not valid json: " << config;
+  double gain_db;
+  CHECK(config_dict->GetDouble(kGainKey, &gain_db)) << config;
+  gain_ = DbFsToScale(gain_db);
+  LOG(INFO) << "Created a SaturatedGain: gain = " << gain_db;
+}
+
+SaturatedGain::~SaturatedGain() = default;
+
+bool SaturatedGain::SetSampleRate(int sample_rate) {
+  sample_rate_ = sample_rate;
+  slew_volume_.SetSampleRate(sample_rate);
+  return true;
+}
+
+int SaturatedGain::ProcessFrames(float* data, int frames, float volume) {
+  if (volume != last_volume_) {
+    last_volume_ = volume;
+    // Don't apply more gain than attenuation.
+    float effective_gain = std::min(
+        1.0f / DbFsToScale(volume_map_.VolumeToDbFS(last_volume_)), gain_);
+    slew_volume_.SetVolume(effective_gain);
+  }
+
+  slew_volume_.ProcessFMUL(false, data, frames, channels_, data);
+
+  return 0;  // No delay in this pipeline.
+}
+
+int SaturatedGain::GetRingingTimeInFrames() {
+  return 0;
+}
+
+}  // namespace media
+}  // namespace chromecast
+
+chromecast::media::AudioPostProcessor* AudioPostProcessorShlib_Create(
+    const std::string& config,
+    int channels) {
+  return new chromecast::media::SaturatedGain(config, channels);
+}