add similar songs fallback when server returns none

backend/mediaprovider/helpers/similar.go

@@ -0,0 +1,22 @@
+package helpers
+
+import (
+	"github.com/dweymouth/supersonic/backend/mediaprovider"
+	"github.com/dweymouth/supersonic/sharedutil"
+)
+
+func GetSimilarSongsFallback(mp mediaprovider.MediaProvider, track *mediaprovider.Track, count int) []*mediaprovider.Track {
+	var tracks []*mediaprovider.Track
+	if len(track.ArtistIDs) > 0 {
+		tracks, _ = mp.GetSimilarTracks(track.ArtistIDs[0], count)
+		if len(tracks) > 0 {
+			return tracks
+		}
+	}
+	tracks, _ = mp.GetRandomTracks(track.Genre, count)
+
+	// make sure to exclude the song itself from the similar list
+	return sharedutil.FilterSlice(tracks, func(t *mediaprovider.Track) bool {
+		return t.ID != track.ID
+	})
+}