avutil: Add av_get_default_channel_layout()

Also, use the new function in the AC-3 encoder.

doc/APIchanges

@@ -12,6 +12,9 @@ libavutil:   2011-04-18
 
 API changes, most recent first:
 
+2012-xx-xx - xxxxxxx - lavu 51.26.0 - audioconvert.h
+  Add av_get_default_channel_layout()
+
 2012-03-06 - 4d851f8 - lavu 51.25.0 - cpu.h
   Add av_set_cpu_flags_mask().
 

libavcodec/ac3enc.c

@@ -2074,7 +2074,7 @@ static av_cold int set_channel_info(AC3EncodeContext *s, int channels,
         return AVERROR(EINVAL);
     ch_layout = *channel_layout;
     if (!ch_layout)
-        ch_layout = avcodec_guess_channel_layout(channels, CODEC_ID_AC3, NULL);
+        ch_layout = av_get_default_channel_layout(channels);
 
     s->lfe_on       = !!(ch_layout & AV_CH_LOW_FREQUENCY);
     s->channels     = channels;

libavutil/audioconvert.c

@@ -153,3 +153,18 @@ int av_get_channel_layout_nb_channels(uint64_t channel_layout)
         x &= x-1; // unset lowest set bit
     return count;
 }
+
+uint64_t av_get_default_channel_layout(int nb_channels)
+{
+    switch(nb_channels) {
+    case 1: return AV_CH_LAYOUT_MONO;
+    case 2: return AV_CH_LAYOUT_STEREO;
+    case 3: return AV_CH_LAYOUT_SURROUND;
+    case 4: return AV_CH_LAYOUT_QUAD;
+    case 5: return AV_CH_LAYOUT_5POINT0;
+    case 6: return AV_CH_LAYOUT_5POINT1;
+    case 7: return AV_CH_LAYOUT_6POINT1;
+    case 8: return AV_CH_LAYOUT_7POINT1;
+    default: return 0;
+    }
+}

libavutil/audioconvert.h

@@ -124,6 +124,11 @@ void av_get_channel_layout_string(char *buf, int buf_size, int nb_channels, uint
  */
 int av_get_channel_layout_nb_channels(uint64_t channel_layout);
 
+/**
+ * Return default channel layout for a given number of channels.
+ */
+uint64_t av_get_default_channel_layout(int nb_channels);
+
 /**
  * @}
  */

libavutil/avutil.h

@@ -152,7 +152,7 @@
  */
 
 #define LIBAVUTIL_VERSION_MAJOR 51
-#define LIBAVUTIL_VERSION_MINOR 25
+#define LIBAVUTIL_VERSION_MINOR 26
 #define LIBAVUTIL_VERSION_MICRO  0
 
 #define LIBAVUTIL_VERSION_INT   AV_VERSION_INT(LIBAVUTIL_VERSION_MAJOR, \