library: fall back to the plural artist names for display

Keep the server's combined legacy artist/albumArtist string when it is
present, prefer the OpenSubsonic displayArtist when it is not, and otherwise
join the plural artist refs with Navidrome's separator. The normalized value
is stored in the display columns, so song rows, album cards, now playing,
menu subtitles, downloads, search and artist ordering all get it.

app/src/main/kotlin/com/subtracks/data/source/subsonic/SubsonicXml.kt

@@ -44,6 +44,7 @@ object SubsonicXml {
             artistId = attrs.attr("artistId").ifEmpty { null },
             name = attrs.attr("name"),
             albumArtist = attrs.attr("artist").ifEmpty { null },
+            displayArtist = attrs.attr("displayArtist").ifEmpty { null },
             created = attrs.dateAttr("created") ?: 0L,
             coverArt = attrs.attr("coverArt").ifEmpty { null },
             genre = attrs.attr("genre").ifEmpty { null },
@@ -69,6 +70,7 @@ object SubsonicXml {
             title = attrs.attr("title"),
             album = attrs.attr("album").ifEmpty { null },
             artist = attrs.attr("artist").ifEmpty { null },
+            displayArtist = attrs.attr("displayArtist").ifEmpty { null },
             duration = attrs.longAttr("duration"),
             track = attrs.longAttr("track"),
             disc = attrs.longAttr("discNumber"),
@@ -168,6 +170,7 @@ internal class AlbumDraft(
     var artistId: String? = null,
     var name: String = "",
     var albumArtist: String? = null,
+    var displayArtist: String? = null,
     var created: Long = 0L,
     var coverArt: String? = null,
     var genre: String? = null,
@@ -203,13 +206,14 @@ internal class AlbumDraft(
         artistContainer = false
     }
 
-    fun toAlbum(): Album =
-        Album(
+    fun toAlbum(): Album {
+        val refs = artistFallback(artists, artistId, albumArtist)
+        return Album(
             sourceId = sourceId,
             id = id,
             artistId = artistId,
             name = name,
-            albumArtist = albumArtist,
+            albumArtist = displayName(albumArtist, displayArtist, refs),
             created = created,
             coverArt = coverArt,
             genre = genre,
@@ -217,8 +221,9 @@ internal class AlbumDraft(
             starred = starred,
             songCount = songCount,
             discTitles = discTitles.toMap(),
-            artists = artistFallback(artists, artistId, albumArtist),
+            artists = refs,
         )
+    }
 }
 
 internal class SongDraft(
@@ -229,6 +234,7 @@ internal class SongDraft(
     private val title: String = "",
     private val album: String? = null,
     private val artist: String? = null,
+    private val displayArtist: String? = null,
     private val duration: Long? = null,
     private val track: Long? = null,
     private val disc: Long? = null,
@@ -279,15 +285,16 @@ internal class SongDraft(
         nestedArtists?.let { list -> attrs.artistRef()?.let { list += it } }
     }
 
-    fun toSong(): Song =
-        Song(
+    fun toSong(): Song {
+        val refs = artistFallback(artists, artistId, artist)
+        return Song(
             sourceId = sourceId,
             id = id,
             albumId = albumId,
             artistId = artistId,
             title = title,
             album = album,
-            artist = artist,
+            artist = displayName(artist, displayArtist, refs),
             duration = duration,
             track = track,
             disc = disc,
@@ -301,8 +308,9 @@ internal class SongDraft(
             trackPeak = trackPeak,
             albumPeak = albumPeak,
             genres = genres.toList(),
-            artists = artistFallback(artists, artistId, artist),
+            artists = refs,
         )
+    }
 }
 
 private fun Attributes.artistRef(): ArtistRef? {
@@ -323,6 +331,22 @@ private fun artistFallback(
     return listOf(ArtistRef(artistId, name.orEmpty()))
 }
 
+private const val ARTIST_JOINER = " • "
+
+private fun displayName(
+    legacy: String?,
+    openSubsonic: String?,
+    refs: List<ArtistRef>,
+): String? =
+    legacy?.takeIf { it.isNotBlank() }
+        ?: openSubsonic?.takeIf { it.isNotBlank() }
+        ?: refs
+            .map { it.name }
+            .filter { it.isNotBlank() }
+            .distinct()
+            .joinToString(ARTIST_JOINER)
+            .takeIf { it.isNotEmpty() }
+
 internal fun Attributes.attr(name: String): String = getValue(name) ?: ""
 
 internal fun Attributes.longAttr(name: String): Long? = attr(name).takeIf { it.isNotEmpty() }?.toLongOrNull()