diff -r 000000000000 -r 6474c204b198 content/media/AudioMixer.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/content/media/AudioMixer.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,95 @@
+/* -*- 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"
+
+namespace mozilla {
+typedef void(*MixerFunc)(AudioDataValue* aMixedBuffer,
+                         AudioSampleFormat aFormat,
+                         uint32_t aChannels,
+                         uint32_t aFrames,
+                         uint32_t aSampleRate);
+
+/**
+ * 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.
+ *
+ * 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(MixerFunc aCallback)
+    : mCallback(aCallback),
+      mFrames(0),
+      mChannels(0),
+      mSampleRate(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() {
+    mCallback(mMixedAudio.Elements(),
+              AudioSampleTypeToFormat<AudioDataValue>::Format,
+              mChannels,
+              mFrames,
+              mSampleRate);
+    PodZero(mMixedAudio.Elements(), mMixedAudio.Length());
+    mSampleRate = mChannels = mFrames = 0;
+  }
+
+  /* Add a buffer to the mix. aSamples is interleaved. */
+  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];
+    }
+  }
+private:
+  void EnsureCapacityAndSilence() {
+    if (mFrames * mChannels > mMixedAudio.Length()) {
+      mMixedAudio.SetLength(mFrames* mChannels);
+    }
+    PodZero(mMixedAudio.Elements(), mMixedAudio.Length());
+  }
+
+  /* Function that is called when the mixing is done. */
+  MixerFunc mCallback;
+  /* 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;
+};
+}
+
+#endif // MOZILLA_AUDIOMIXER_H_