Allow more than 2 input channels in AudioProcessing.

webrtc/common_audio/include/audio_util.h

Index: webrtc/common_audio/include/audio_util.h
diff --git a/webrtc/common_audio/include/audio_util.h b/webrtc/common_audio/include/audio_util.h
index 8262649145546180f23b6e82094d324b7f604e7f..909afed5156047fa701007fc513ab076f31b5944 100644
--- a/webrtc/common_audio/include/audio_util.h
+++ b/webrtc/common_audio/include/audio_util.h
@@ -12,6 +12,7 @@
 #define WEBRTC_COMMON_AUDIO_INCLUDE_AUDIO_UTIL_H_
 
 #include <limits>
+#include <cstring>
 
 #include "webrtc/base/scoped_ptr.h"
 #include "webrtc/typedefs.h"
@@ -97,6 +98,56 @@ void Interleave(const T* const* deinterleaved, int samples_per_channel,
   }
 }
 
+template <typename T, typename Intermediate>
+void DownmixToMono(const T* const* input_channels,
+                   int num_frames,
+                   int num_channels,
+                   T* out) {
+  for (int i = 0; i < num_frames; ++i) {
+    Intermediate value = input_channels[0][i];
+    for (int j = 1; j < num_channels; ++j) {
+      value += input_channels[j][i];
+    }
+    out[i] = value / num_channels;
+  }
+}
+
+// Downmixes an interleaved multichannel signal to a single channel by averaging
+// all channels.
+template <typename T, typename Intermediate>
+void DownmixInterleavedToMonoImpl(const T* interleaved,
+                                  int num_multichannel_frames,

[Andrew MacDonald, 2015/07/22 22:47:21]

Why "multichannel"? To differentiate from the "frame == block of audio"
interpretation? That could still be misconstrued.

I think we should go use num_frames or stick to the old samples_per_channel.

[mgraczyk, 2015/07/23 00:16:54]

Done.

+                                  int num_channels,
+                                  T* deinterleaved) {
+  assert(num_channels > 0);

[Andrew MacDonald, 2015/07/22 22:47:21]

For new code we should use DCHECK.

[mgraczyk, 2015/07/23 00:16:54]

Done.

+  assert(num_multichannel_frames > 0);
+
+  const T* const end = interleaved + num_multichannel_frames * num_channels;
+
+  while (interleaved < end) {
+    const T* const frame_end = interleaved + num_channels;
+
+    Intermediate value = *interleaved++;
+    while (interleaved < frame_end) {
+      value += *interleaved++;
+    }
+
+    *deinterleaved++ = value / num_channels;
+  }
+}
+
+template <typename T>
+void DownmixInterleavedToMono(const T* interleaved,
+                              int num_multichannel_frames,
+                              int num_channels,
+                              T* deinterleaved);
+
+template <>
+void DownmixInterleavedToMono<int16_t>(const int16_t* interleaved,
+                                       int num_multichannel_frames,
+                                       int num_channels,
+                                       int16_t* deinterleaved);
+
 }  // namespace webrtc
 
 #endif  // WEBRTC_COMMON_AUDIO_INCLUDE_AUDIO_UTIL_H_