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..fc2c13f5b285c4f7be78663dd254e4ee5b3557bb 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 +98,86 @@ void Interleave(const T* const* deinterleaved, int samples_per_channel,
   }
 }
 
+template <typename T, typename Intermediate>
+void DownmixStereoToMono(const T* left,
+                         const T* right,
+                         int num_frames,
+                         T* out) {
+  for (int i = 0; i < num_frames; ++i) {
+    out[i] = (static_cast<Intermediate>(left[i]) + right[i]) / 2;
+  }
+}
+
+template <typename T, typename Intermediate>
+void DownmixToMono(const T* const* input_channels,
+                   int num_frames,
+                   int num_channels,
+                   T* out) {
+  if (num_channels == 2) {
+    DownmixStereoToMono<T, Intermediate>(input_channels[0], input_channels[1],
+                                         num_frames, out);
+  } else {
+    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,
+                                  int num_channels,
+                                  T* deinterleaved) {
+  assert(num_channels > 0);
+  assert(num_multichannel_frames > 0);
+
+  const T* const end = interleaved + num_multichannel_frames * num_channels;
+
+  if (num_channels == 1) {
+    std::memmove(deinterleaved, interleaved,
+                 num_multichannel_frames * sizeof(*deinterleaved));
+  } else if (num_channels == 2) {
+    // Explicitly unroll for the common stereo case.
+    while (interleaved < end) {
+      *deinterleaved++ =
+          (static_cast<Intermediate>(*interleaved) + *(interleaved + 1)) / 2;
+      interleaved += 2;
+    }
+  } else {
+    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) {
+  DownmixInterleavedToMonoImpl<T, T>(interleaved, num_multichannel_frames,
+                                     num_channels, 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_