very slim skelleton for downloading

internal/plugin/interface.go

@@ -189,3 +189,76 @@ func Search(search string, config SearchConfig) ([]data.MusicObject, error) {
 
 	return res, nil
 }
+
+type downloadState struct {
+	artist *data.Artist
+	album  *data.Album
+	song   *data.Song
+}
+
+var variousArtist = data.Artist{
+	Name: "VariousArtist",
+}.Compile().(data.Artist)
+
+var variousAlbum = data.Album{
+	Name: "VariousAlbum",
+}.Compile().(data.Album)
+
+func downloadSong(song data.Song, state downloadState) {}
+
+func Download(musicObject data.MusicObject, statesInput ...downloadState) {
+	state := downloadState{}
+	if len(statesInput) > 0 {
+		state = statesInput[0]
+	}
+
+	musicObject = musicObject.Compile()
+
+	if song, ok := musicObject.(data.Song); ok {
+		state.song = &song
+
+		if state.artist == nil {
+			if len(song.Artists) > 0 {
+				state.artist = &song.Artists[0]
+			} else {
+				state.artist = &variousArtist
+			}
+		}
+
+		if state.album == nil {
+			if song.Album.Name != "" {
+				state.album = &song.Album
+			} else {
+				state.album = &variousAlbum
+			}
+		}
+		downloadSong(song, state)
+		return
+	}
+
+	if album, ok := musicObject.(data.Album); ok {
+		state.album = &album
+
+		if state.artist == nil {
+			if len(album.Artists) > 0 {
+				state.artist = &album.Artists[0]
+			} else {
+				state.artist = &variousArtist
+			}
+		}
+
+		for _, song := range album.Songs {
+			Download(song, state)
+		}
+		return
+	}
+
+	if artist, ok := musicObject.(data.Artist); ok {
+		state.artist = &artist
+
+		for _, album := range artist.Albums {
+			Download(album, state)
+		}
+		return
+	}
+}