chmap_sel: add multichannel fallback heuristic

Instead of just failing during channel map selection, try to select a close
layout that makes most sense and upmix/downmix to that instead of failing AO
initialization. The heuristic is rather simple, and uses the following steps:

1) If mono is required always prefer stereo to a multichannel upmix.
2) Search for an upmix that is an exact superset of the required channel map.
3) Search for a downmix that is the exact subset of the required channel map.
4) Search for either an upmix or downmix that is the closest (minimum difference
   of channels) to the required channel map.

1 files changed, 5 insertions, 0 deletions

diff --git a/audio/chmap.h b/audio/chmap.h
index d0b9806ff5..9ec1c12d7b 100644
--- a/audio/chmap.h
+++ b/audio/chmap.h
@@ -120,6 +120,11 @@ void mp_chmap_reorder_to_lavc(struct mp_chmap *map);
 void mp_chmap_get_reorder(int dst[MP_NUM_CHANNELS], const struct mp_chmap *from,
                           const struct mp_chmap *to);
 
+void mp_chmap_diff(const struct mp_chmap *a, const struct mp_chmap *b,
+                   struct mp_chmap *diff);
+
+bool mp_chmap_contains(const struct mp_chmap *a, const struct mp_chmap *b);
+
 char *mp_chmap_to_str_buf(char *buf, size_t buf_size, const struct mp_chmap *src);
 #define mp_chmap_to_str(m) mp_chmap_to_str_buf((char[64]){0}, 64, (m))