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diff --git a/dom/media/AudioMixer.h b/dom/media/AudioMixer.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef MOZILLA_AUDIOMIXER_H_
+#define MOZILLA_AUDIOMIXER_H_
+
+#include "AudioSampleFormat.h"
+#include "nsTArray.h"
+#include "mozilla/PodOperations.h"
+#include "mozilla/LinkedList.h"
+#include "AudioStream.h"
+
+namespace mozilla {
+
+struct MixerCallbackReceiver {
+  virtual void MixerCallback(AudioDataValue* aMixedBuffer,
+                             AudioSampleFormat aFormat,
+                             uint32_t aChannels,
+                             uint32_t aFrames,
+                             uint32_t aSampleRate) = 0;
+};
+/**
+ * This class mixes multiple streams of audio together to output a single audio
+ * stream.
+ *
+ * AudioMixer::Mix is to be called repeatedly with buffers that have the same
+ * length, sample rate, sample format and channel count. This class works with
+ * interleaved and plannar buffers, but the buffer mixed must be of the same
+ * type during a mixing cycle.
+ *
+ * When all the tracks have been mixed, calling FinishMixing will call back with
+ * a buffer containing the mixed audio data.
+ *
+ * This class is not thread safe.
+ */
+class AudioMixer
+{
+public:
+  AudioMixer()
+    : mFrames(0),
+      mChannels(0),
+      mSampleRate(0)
+  { }
+
+  ~AudioMixer()
+  {
+    MixerCallback* cb;
+    while ((cb = mCallbacks.popFirst())) {
+      delete cb;
+    }
+  }
+
+  void StartMixing()
+  {
+    mSampleRate = mChannels = mFrames = 0;
+  }
+
+  /* Get the data from the mixer. This is supposed to be called when all the
+   * tracks have been mixed in. The caller should not hold onto the data. */
+  void FinishMixing() {
+    MOZ_ASSERT(mChannels && mFrames && mSampleRate, "Mix not called for this cycle?");
+    for (MixerCallback* cb = mCallbacks.getFirst();
+         cb != nullptr; cb = cb->getNext()) {
+      cb->mReceiver->MixerCallback(mMixedAudio.Elements(),
+                                   AudioSampleTypeToFormat<AudioDataValue>::Format,
+                                   mChannels,
+                                   mFrames,
+                                   mSampleRate);
+    }
+    PodZero(mMixedAudio.Elements(), mMixedAudio.Length());
+    mSampleRate = mChannels = mFrames = 0;
+  }
+
+  /* Add a buffer to the mix. */
+  void Mix(AudioDataValue* aSamples,
+           uint32_t aChannels,
+           uint32_t aFrames,
+           uint32_t aSampleRate) {
+    if (!mFrames && !mChannels) {
+      mFrames = aFrames;
+      mChannels = aChannels;
+      mSampleRate = aSampleRate;
+      EnsureCapacityAndSilence();
+    }
+
+    MOZ_ASSERT(aFrames == mFrames);
+    MOZ_ASSERT(aChannels == mChannels);
+    MOZ_ASSERT(aSampleRate == mSampleRate);
+
+    for (uint32_t i = 0; i < aFrames * aChannels; i++) {
+      mMixedAudio[i] += aSamples[i];
+    }
+  }
+
+  void AddCallback(MixerCallbackReceiver* aReceiver) {
+    mCallbacks.insertBack(new MixerCallback(aReceiver));
+  }
+
+  bool FindCallback(MixerCallbackReceiver* aReceiver) {
+    for (MixerCallback* cb = mCallbacks.getFirst();
+         cb != nullptr; cb = cb->getNext()) {
+      if (cb->mReceiver == aReceiver) {
+        return true;
+      }
+    }
+    return false;
+  }
+
+  bool RemoveCallback(MixerCallbackReceiver* aReceiver) {
+    for (MixerCallback* cb = mCallbacks.getFirst();
+         cb != nullptr; cb = cb->getNext()) {
+      if (cb->mReceiver == aReceiver) {
+        cb->remove();
+        delete cb;
+        return true;
+      }
+    }
+    return false;
+  }
+private:
+  void EnsureCapacityAndSilence() {
+    if (mFrames * mChannels > mMixedAudio.Length()) {
+      mMixedAudio.SetLength(mFrames* mChannels);
+    }
+    PodZero(mMixedAudio.Elements(), mMixedAudio.Length());
+  }
+
+  class MixerCallback : public LinkedListElement<MixerCallback>
+  {
+  public:
+    explicit MixerCallback(MixerCallbackReceiver* aReceiver)
+      : mReceiver(aReceiver)
+    { }
+    MixerCallbackReceiver* mReceiver;
+  };
+
+  /* Function that is called when the mixing is done. */
+  LinkedList<MixerCallback> mCallbacks;
+  /* Number of frames for this mixing block. */
+  uint32_t mFrames;
+  /* Number of channels for this mixing block. */
+  uint32_t mChannels;
+  /* Sample rate the of the mixed data. */
+  uint32_t mSampleRate;
+  /* Buffer containing the mixed audio data. */
+  nsTArray<AudioDataValue> mMixedAudio;
+};
+
+} // namespace mozilla
+
+#endif // MOZILLA_AUDIOMIXER_H_