1 files changed, 99 insertions, 0 deletions
diff --git a/third_party/libwebrtc/audio/utility/channel_mixer.cc b/third_party/libwebrtc/audio/utility/channel_mixer.cc
new file mode 100644
index 0000000000..0f1e663873
--- /dev/null
+++ b/third_party/libwebrtc/audio/utility/channel_mixer.cc
@@ -0,0 +1,99 @@
+/*
+ *  Copyright (c) 2012 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 "audio/utility/channel_mixer.h"
+
+#include "audio/utility/channel_mixing_matrix.h"
+#include "rtc_base/checks.h"
+#include "rtc_base/logging.h"
+#include "rtc_base/numerics/safe_conversions.h"
+
+namespace webrtc {
+
+ChannelMixer::ChannelMixer(ChannelLayout input_layout,
+                           ChannelLayout output_layout)
+    : input_layout_(input_layout),
+      output_layout_(output_layout),
+      input_channels_(ChannelLayoutToChannelCount(input_layout)),
+      output_channels_(ChannelLayoutToChannelCount(output_layout)) {
+  // Create the transformation matrix.
+  ChannelMixingMatrix matrix_builder(input_layout_, input_channels_,
+                                     output_layout_, output_channels_);
+  remapping_ = matrix_builder.CreateTransformationMatrix(&matrix_);
+}
+
+ChannelMixer::~ChannelMixer() = default;
+
+void ChannelMixer::Transform(AudioFrame* frame) {
+  RTC_DCHECK(frame);
+  RTC_DCHECK_EQ(matrix_[0].size(), static_cast<size_t>(input_channels_));
+  RTC_DCHECK_EQ(matrix_.size(), static_cast<size_t>(output_channels_));
+
+  // Leave the audio frame intact if the channel layouts for in and out are
+  // identical.
+  if (input_layout_ == output_layout_) {
+    return;
+  }
+
+  if (IsUpMixing()) {
+    RTC_CHECK_LE(frame->samples_per_channel() * output_channels_,
+                 frame->max_16bit_samples());
+  }
+
+  // Only change the number of output channels if the audio frame is muted.
+  if (frame->muted()) {
+    frame->num_channels_ = output_channels_;
+    frame->channel_layout_ = output_layout_;
+    return;
+  }
+
+  const int16_t* in_audio = frame->data();
+
+  // Only allocate fresh memory at first access or if the required size has
+  // increased.
+  // TODO(henrika): we might be able to do downmixing in-place and thereby avoid
+  // extra memory allocation and a memcpy.
+  const size_t num_elements = frame->samples_per_channel() * output_channels_;
+  if (audio_vector_ == nullptr || num_elements > audio_vector_size_) {
+    audio_vector_.reset(new int16_t[num_elements]);
+    audio_vector_size_ = num_elements;
+  }
+  int16_t* out_audio = audio_vector_.get();
+
+  // Modify the number of channels by creating a weighted sum of input samples
+  // where the weights (scale factors) for each output sample are given by the
+  // transformation matrix.
+  for (size_t i = 0; i < frame->samples_per_channel(); i++) {
+    for (size_t output_ch = 0; output_ch < output_channels_; ++output_ch) {
+      float acc_value = 0.0f;
+      for (size_t input_ch = 0; input_ch < input_channels_; ++input_ch) {
+        const float scale = matrix_[output_ch][input_ch];
+        // Scale should always be positive.
+        RTC_DCHECK_GE(scale, 0);
+        // Each output sample is a weighted sum of input samples.
+        acc_value += scale * in_audio[i * input_channels_ + input_ch];
+      }
+      const size_t index = output_channels_ * i + output_ch;
+      RTC_CHECK_LE(index, audio_vector_size_);
+      out_audio[index] = rtc::saturated_cast<int16_t>(acc_value);
+    }
+  }
+
+  // Update channel information.
+  frame->num_channels_ = output_channels_;
+  frame->channel_layout_ = output_layout_;
+
+  // Copy the output result to the audio frame in `frame`.
+  memcpy(
+      frame->mutable_data(), out_audio,
+      sizeof(int16_t) * frame->samples_per_channel() * frame->num_channels());
+}
+
+}  // namespace webrtc