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diff --git a/src/3rdparty/resonance-audio/resonance_audio/dsp/gain_mixer.h b/src/3rdparty/resonance-audio/resonance_audio/dsp/gain_mixer.h
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+/*
+Copyright 2018 Google Inc. All Rights Reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS-IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+#ifndef RESONANCE_AUDIO_DSP_GAIN_MIXER_H_
+#define RESONANCE_AUDIO_DSP_GAIN_MIXER_H_
+
+#include <memory>
+#include <unordered_map>
+#include <vector>
+
+#include "base/audio_buffer.h"
+#include "base/constants_and_types.h"
+#include "dsp/gain_processor.h"
+
+namespace vraudio {
+
+class GainMixer {
+ public:
+  GainMixer(size_t num_channels, size_t frames_per_buffer);
+
+  // Adds a separately scaled version of each channel of the input buffer to the
+  // output buffer's corresponding channels.
+  //
+  // @param input Input buffer to be added.
+  // @param gains Gains to be applied to the buffers channels. Must be equal in
+  //     length to the number of channels in |input|.
+  void AddInput(const AudioBuffer& input, const std::vector<float>& gains);
+
+  // Adds a single input channel to each of the output buffer's channels, with
+  // a separate gain applied to the input channel per output channel.
+  //
+  // @param input Input channel to be added.
+  // @param source_id Identifier corresponding to the input.
+  // @param gains Gains to be applied to the buffers channels. Must be equal in
+  //     length to the number of channels in the output buffer.
+  void AddInputChannel(const AudioBuffer::Channel& input, SourceId source_id,
+                       const std::vector<float>& gains);
+
+  // Returns a pointer to the accumulator.
+  //
+  // @return Pointer to the processed (mixed) output buffer, or nullptr if no
+  //     input has been added to the accumulator.
+  const AudioBuffer* GetOutput() const;
+
+  // Resets the state of the accumulator.
+  void Reset();
+
+ private:
+  // Comprises one |GainProcessor| per channel of a source and a boolean to
+  // denote whether that source is active.
+  struct GainProcessors {
+    explicit GainProcessors(size_t num_channels);
+
+    // Bool to signify if a given source is still passing data to a processor.
+    bool processors_active;
+
+    // Scale and accumulation processors, one per channel for each source.
+    std::vector<GainProcessor> processors;
+  };
+
+  // Returns the |GainProcessor|s associated with a |source_id| (or creates
+  // one if needed) and sets the corresponding |processors_active| flag to true.
+  //
+  // @param source_id Identifier for a given input.
+  // @return The corresponding |ScalingAccumulators|.
+  std::vector<GainProcessor>* GetOrCreateProcessors(SourceId source_id);
+
+  // Number of channels.
+  const size_t num_channels_;
+
+  // Output buffer (accumulator).
+  AudioBuffer output_;
+
+  // Denotes whether the accumulator has processed any inputs or not.
+  bool is_empty_;
+
+  // Scale and accumulation processors, one per channel for each source.
+  std::unordered_map<SourceId, GainProcessors> source_gain_processors_;
+};
+
+}  // namespace vraudio
+
+#endif  // RESONANCE_AUDIO_DSP_GAIN_MIXER_H_