Support 4 channel mic in Windows Core Audio

webrtc/audio/utility/audio_frame_operations.h

Index: webrtc/audio/utility/audio_frame_operations.h
diff --git a/webrtc/audio/utility/audio_frame_operations.h b/webrtc/audio/utility/audio_frame_operations.h
index d16b163e7d1139f10ba00e87e73524648e01bebd..1d8b9a11dd2b74d2822156f83b78efae3854c956 100644
--- a/webrtc/audio/utility/audio_frame_operations.h
+++ b/webrtc/audio/utility/audio_frame_operations.h
@@ -54,6 +54,41 @@ class AudioFrameOperations {
   // |num_channels_| is stereo.
   static int StereoToMono(AudioFrame* frame);
 
+  // Downmixes 4 channels |src_audio| to stereo |dst_audio|. This is an in-place
+  // operation, meaning |src_audio| and |dst_audio| may point to the same
+  // buffer.
+  static void QuadToStereo(const int16_t* src_audio,
+                           size_t samples_per_channel,
+                           int16_t* dst_audio);

[aleloi, 2017/02/24 10:39:31]

Add blank line

[jens.nielsen, 2017/02/24 14:55:49]

Done.

+  // |frame.num_channels_| will be updated. This version checks that
+  // |num_channels_| is 4 channels.
+  static int QuadToStereo(AudioFrame* frame);
+
+  // Downmixes 4 channels |src_audio| to mono |dst_audio|. This is an in-place
+  // operation, meaning |src_audio| and |dst_audio| may point to the same
+  // buffer.
+  static void QuadToMono(const int16_t* src_audio,
+                         size_t samples_per_channel,
+                         int16_t* dst_audio);

[aleloi, 2017/02/24 10:39:31]

Add blank line

[jens.nielsen, 2017/02/24 14:55:48]

Done.

+  // |frame.num_channels_| will be updated. This version checks that
+  // |num_channels_| is stereo.

[aleloi, 2017/02/24 10:39:31]

Shouldn't |num_channels_| be 4 here and not stereo? Or am I missing something?

[jens.nielsen, 2017/02/24 14:55:48]

Done. (Silly copy-paste error, sorry about that)

+  static int QuadToMono(AudioFrame* frame);
+
+  // Downmixes |src_channels| |src_audio| to |dst_channels| |dst_audio|.
+  // This is an in-place operation, meaning |src_audio| and |dst_audio|
+  // may point to the same buffer.

[hlundin-webrtc, 2017/02/24 09:46:36]

Comment on which combinations of src_channels and dst_channels are supported.

[jens.nielsen, 2017/02/24 14:55:49]

Done.

+  static void AudioFrameOperations::DownmixChannels(const int16_t* src_audio,
+                                                    size_t src_channels,
+                                                    size_t samples_per_channel,
+                                                    int16_t* dst_audio,
+                                                    size_t dst_channels);

[aleloi, 2017/02/24 10:39:31]

Please move the dst_audio to the end.

Quote from style guide: "When defining a function, parameter order is: inputs,
then outputs."

https://google.github.io/styleguide/cppguide.html#Function_Parameter_Ordering

[jens.nielsen, 2017/02/24 14:55:49]

Done.

+
+  // |frame.num_channels_| will be updated. This version checks that
+  // |num_channels_| and |dst_channels| are valid and performs relevant
+  // downmix.

[hlundin-webrtc, 2017/02/24 09:46:36]

Comment on which combinations of src_channels and dst_channels are supported.

[jens.nielsen, 2017/02/24 14:55:49]

Done.

+  static int AudioFrameOperations::DownmixChannels(AudioFrame* frame,
+                                                   size_t dst_channels);

[aleloi, 2017/02/24 10:39:31]

Please swap function parameters here as well. |dst_channels| is input, |frame|
is both, so it should be last.

[jens.nielsen, 2017/02/24 14:55:49]

Done.

+
   // Swap the left and right channels of |frame|. Fails silently if |frame| is
   // not stereo.
   static void SwapStereoChannels(AudioFrame* frame);