command: add track-list/N/audio-channels property

Signed-off-by: wm4 <wm4@nowhere>

(With some heavy modifications from the original patch.)

DOCS/interface-changes.rst

@@ -20,6 +20,7 @@ Interface changes
 ::
 
  --- mpv 0.10.0 will be released ---
+    - add ``track-list/N/audio-channels`` property
     - change --screenshot-tag-colorspace default value
     - add --stretch-image-subs-to-screen
     - add "playlist/N/title" property

DOCS/man/input.rst

@@ -1574,6 +1574,10 @@ Property list
     ``track-list/N/lang``
         Track language as identified by the file. Not always available.
 
+    ``track-list/N/audio-channels``
+        For audio tracks, the number of audio channels in the audio stream.
+        Not always accurate (depends on container hints). Not always available.
+
     ``track-list/N/albumart``
         ``yes`` if this is a video track that consists of a single picture,
         ``no`` or unavailable otherwise. This is used for video tracks that are
@@ -1622,6 +1626,7 @@ Property list
                 "src-id"            MPV_FORMAT_INT64
                 "title"             MPV_FORMAT_STRING
                 "lang"              MPV_FORMAT_STRING
+                "audio-channels"    MPV_FORMAT_INT64
                 "albumart"          MPV_FORMAT_FLAG
                 "default"           MPV_FORMAT_FLAG
                 "forced"            MPV_FORMAT_FLAG

player/command.c

@@ -1861,6 +1861,12 @@ static int property_switch_track_ff(void *ctx, struct m_property *prop,
     return mp_property_generic_option(mpctx, prop, action, arg);
 }
 
+static int track_channels(struct track *track)
+{
+    return track->stream && track->stream->audio
+        ? track->stream->audio->channels.num : 0;
+}
+
 static int get_track_entry(int item, int action, void *arg, void *ctx)
 {
     struct MPContext *mpctx = ctx;
@@ -1878,6 +1884,8 @@ static int get_track_entry(int item, int action, void *arg, void *ctx)
                         .unavailable = !track->title},
         {"lang",        SUB_PROP_STR(track->lang),
                         .unavailable = !track->lang},
+        {"audio-channels", SUB_PROP_INT(track_channels(track)),
+                        .unavailable = track_channels(track) <= 0},
         {"albumart",    SUB_PROP_FLAG(track->attached_picture)},
         {"default",     SUB_PROP_FLAG(track->default_track)},
         {"forced",      SUB_PROP_FLAG(track->forced_track)},