ffmpeg: Use replaygain tags if found

[Almoped]

Playing .m4a files with FFmpeg can use something like this to read ReplayGain tags.
FFmpeg also reads RG tags in other files like .ogg or .flac, should those decoders be unavailable.

[radioactiveman]

@jlindgren90: Can you explain please why some plugins read the RG tags in play() and others in read_tag()? What is better and would it make sense to implement both for ffaudio?

[jlindgren90]

Either way is fine. I would suggest doing it in read_tag for new code, if the information is available at that point.

What each plugin does is perhaps mostly for historical reasons at this point. I think for some formats (e.g. Vorbis) there is a concern that multiple segments may be included in a stream and have different tags, but I'm not sure how common that is in practice.

[radioactiveman]

The read_tag approach could be implemented as shown below.

@Almoped: Do you want to test this more carefully? I only used a single FLAC file as sample.

@jlindgren90: Looking good for you?

diff --git a/src/ffaudio/ffaudio-core.cc b/src/ffaudio/ffaudio-core.cc
index 253eed6c5..d21c480c4 100644
--- a/src/ffaudio/ffaudio-core.cc
+++ b/src/ffaudio/ffaudio-core.cc
@@ -416,6 +416,10 @@ static const struct {
     {Tuple::String, Tuple::Genre, {"genre", "WM/Genre", nullptr}},
     {Tuple::String, Tuple::Comment, {"comment", nullptr}},
     {Tuple::String, Tuple::Composer, {"composer", nullptr}},
+    {Tuple::String, Tuple::AlbumGain, {"replaygain_album_gain", nullptr}},
+    {Tuple::String, Tuple::AlbumPeak, {"replaygain_album_peak", nullptr}},
+    {Tuple::String, Tuple::TrackGain, {"replaygain_track_gain", nullptr}},
+    {Tuple::String, Tuple::TrackPeak, {"replaygain_track_peak", nullptr}},
     {Tuple::Int, Tuple::Year, {"year", "WM/Year", "date", nullptr}},
     {Tuple::Int, Tuple::Track, {"track", "WM/TrackNumber", nullptr}},
     {Tuple::Int, Tuple::Disc, {"disc", "WM/PartOfSet", nullptr}},
@@ -432,7 +436,11 @@ static void read_metadata_dict (Tuple & tuple, AVDictionary * dict)
 
         if (entry && entry->value)
         {
-            if (meta.ttype == Tuple::String)
+            if (meta.field == Tuple::AlbumGain || meta.field == Tuple::TrackGain)
+                tuple.set_gain (meta.field, Tuple::GainDivisor, entry->value);
+            else if (meta.field == Tuple::AlbumPeak || meta.field == Tuple::TrackPeak)
+                tuple.set_gain (meta.field, Tuple::PeakDivisor, entry->value);
+            else if (meta.ttype == Tuple::String)
                 tuple.set_str (meta.field, entry->value);
             else if (meta.ttype == Tuple::Int)
                 tuple.set_int (meta.field, atoi (entry->value));

[radioactiveman]

Pushed now as ae51bc8, closing this PR.