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diff --git a/third_party/libwebrtc/audio/utility/channel_mixing_matrix.h b/third_party/libwebrtc/audio/utility/channel_mixing_matrix.h
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+/*
+ *  Copyright (c) 2019 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 AUDIO_UTILITY_CHANNEL_MIXING_MATRIX_H_
+#define AUDIO_UTILITY_CHANNEL_MIXING_MATRIX_H_
+
+#include <vector>
+
+#include "api/audio/channel_layout.h"
+
+namespace webrtc {
+
+class ChannelMixingMatrix {
+ public:
+  ChannelMixingMatrix(ChannelLayout input_layout,
+                      int input_channels,
+                      ChannelLayout output_layout,
+                      int output_channels);
+
+  ~ChannelMixingMatrix();
+
+  // Create the transformation matrix of input channels to output channels.
+  // Updates the empty matrix with the transformation, and returns true
+  // if the transformation is just a remapping of channels (no mixing).
+  // The size of `matrix` is `output_channels` x `input_channels`, i.e., the
+  // number of rows equals the number of output channels and the number of
+  // columns corresponds to the number of input channels.
+  // This file is derived from Chromium's media/base/channel_mixing_matrix.h.
+  bool CreateTransformationMatrix(std::vector<std::vector<float>>* matrix);
+
+ private:
+  const bool use_voip_channel_mapping_adjustments_;
+
+  // Result transformation of input channels to output channels
+  std::vector<std::vector<float>>* matrix_;
+
+  // Input and output channel layout provided during construction.
+  ChannelLayout input_layout_;
+  int input_channels_;
+  ChannelLayout output_layout_;
+  int output_channels_;
+
+  // Helper variable for tracking which inputs are currently unaccounted,
+  // should be empty after construction completes.
+  std::vector<Channels> unaccounted_inputs_;
+
+  // Helper methods for managing unaccounted input channels.
+  void AccountFor(Channels ch);
+  bool IsUnaccounted(Channels ch) const;
+
+  // Helper methods for checking if `ch` exists in either `input_layout_` or
+  // `output_layout_` respectively.
+  bool HasInputChannel(Channels ch) const;
+  bool HasOutputChannel(Channels ch) const;
+
+  // Helper methods for updating `matrix_` with the proper value for
+  // mixing `input_ch` into `output_ch`.  MixWithoutAccounting() does not
+  // remove the channel from `unaccounted_inputs_`.
+  void Mix(Channels input_ch, Channels output_ch, float scale);
+  void MixWithoutAccounting(Channels input_ch, Channels output_ch, float scale);
+
+  // Delete the copy constructor and assignment operator.
+  ChannelMixingMatrix(const ChannelMixingMatrix& other) = delete;
+  ChannelMixingMatrix& operator=(const ChannelMixingMatrix& other) = delete;
+};
+
+}  // namespace webrtc
+
+#endif  // AUDIO_UTILITY_CHANNEL_MIXING_MATRIX_H_