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diff --git a/third_party/libwebrtc/modules/audio_processing/aec3/alignment_mixer.h b/third_party/libwebrtc/modules/audio_processing/aec3/alignment_mixer.h
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+++ b/third_party/libwebrtc/modules/audio_processing/aec3/alignment_mixer.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 MODULES_AUDIO_PROCESSING_AEC3_ALIGNMENT_MIXER_H_
+#define MODULES_AUDIO_PROCESSING_AEC3_ALIGNMENT_MIXER_H_
+
+#include <vector>
+
+#include "api/array_view.h"
+#include "api/audio/echo_canceller3_config.h"
+#include "modules/audio_processing/aec3/aec3_common.h"
+#include "modules/audio_processing/aec3/block.h"
+
+namespace webrtc {
+
+// Performs channel conversion to mono for the purpose of providing a decent
+// mono input for the delay estimation. This is achieved by analyzing all
+// incoming channels and produce one single channel output.
+class AlignmentMixer {
+ public:
+  AlignmentMixer(size_t num_channels,
+                 const EchoCanceller3Config::Delay::AlignmentMixing& config);
+
+  AlignmentMixer(size_t num_channels,
+                 bool downmix,
+                 bool adaptive_selection,
+                 float excitation_limit,
+                 bool prefer_first_two_channels);
+
+  void ProduceOutput(const Block& x, rtc::ArrayView<float, kBlockSize> y);
+
+  enum class MixingVariant { kDownmix, kAdaptive, kFixed };
+
+ private:
+  const size_t num_channels_;
+  const float one_by_num_channels_;
+  const float excitation_energy_threshold_;
+  const bool prefer_first_two_channels_;
+  const MixingVariant selection_variant_;
+  std::array<size_t, 2> strong_block_counters_;
+  std::vector<float> cumulative_energies_;
+  int selected_channel_ = 0;
+  size_t block_counter_ = 0;
+
+  void Downmix(const Block& x, rtc::ArrayView<float, kBlockSize> y) const;
+  int SelectChannel(const Block& x);
+};
+}  // namespace webrtc
+
+#endif  // MODULES_AUDIO_PROCESSING_AEC3_ALIGNMENT_MIXER_H_