feat(artwork): improve album cover art selection by considering disc numbers

Signed-off-by: Deluan <deluan@navidrome.org>

model/mediafile.go

@@ -183,6 +183,8 @@ func (mfs MediaFiles) ToAlbum() Album {
 	tags := make(TagList, 0, len(mfs[0].Tags)*len(mfs))
 
 	a.Missing = true
+	embedArtPath := ""
+	embedArtDisc := 0
 	for _, m := range mfs {
 		// We assume these attributes are all the same for all songs in an album
 		a.ID = m.AlbumID
@@ -218,7 +220,7 @@ func (mfs MediaFiles) ToAlbum() Album {
 		a.Participants.Merge(m.Participants)
 
 		// Find the MediaFile with cover art and the lowest disc number to use for album cover
-		a.EmbedArtPath = firstArtPath(a.EmbedArtPath, m)
+		embedArtPath, embedArtDisc = firstArtPath(embedArtPath, embedArtDisc, m)
 
 		if m.ExplicitStatus == "c" && a.ExplicitStatus != "e" {
 			a.ExplicitStatus = "c"
@@ -231,6 +233,7 @@ func (mfs MediaFiles) ToAlbum() Album {
 		a.Missing = a.Missing && m.Missing
 	}
 
+	a.EmbedArtPath = embedArtPath
 	a.SetTags(tags)
 	a.FolderIDs = slice.Unique(slice.Map(mfs, func(m MediaFile) string { return m.FolderID }))
 	a.Date, _ = allOrNothing(dates)
@@ -307,15 +310,24 @@ func fixAlbumArtist(a *Album) {
 
 // firstArtPath determines which media file path should be used for album artwork
 // based on disc number (preferring lower disc numbers) and path (for consistency)
-func firstArtPath(currentPath string, m MediaFile) string {
+func firstArtPath(currentPath string, currentDisc int, m MediaFile) (string, int) {
 	if !m.HasCoverArt {
-		return currentPath
+		return currentPath, currentDisc
 	}
 
-	if m.Path < currentPath || currentPath == "" {
-		return m.Path
+	// If current has no disc number (currentDisc == 0) or new file has lower disc number
+	if currentDisc == 0 || (m.DiscNumber < currentDisc && m.DiscNumber > 0) {
+		return m.Path, m.DiscNumber
 	}
-	return currentPath
+
+	// If disc numbers are equal, use path for ordering
+	if m.DiscNumber == currentDisc {
+		if m.Path < currentPath || currentPath == "" {
+			return m.Path, m.DiscNumber
+		}
+	}
+
+	return currentPath, currentDisc
 }
 
 type MediaFileCursor iter.Seq2[MediaFile, error]

model/mediafile_test.go

@@ -373,6 +373,32 @@ var _ = Describe("MediaFiles", func() {
 						Expect(album.EmbedArtPath).To(Equal("Artist/Album/Disc2/01.mp3"))
 					})
 				})
+
+				When("we have media files with path names that don't correlate with disc numbers", func() {
+					BeforeEach(func() {
+						mfs = MediaFiles{
+							{
+								Path:        "Artist/Album/file-z.mp3", // Path would be sorted last alphabetically
+								HasCoverArt: true,
+								DiscNumber:  1, // But it has lowest disc number
+							},
+							{
+								Path:        "Artist/Album/file-a.mp3", // Path would be sorted first alphabetically
+								HasCoverArt: true,
+								DiscNumber:  2, // But it has higher disc number
+							},
+							{
+								Path:        "Artist/Album/file-m.mp3",
+								HasCoverArt: true,
+								DiscNumber:  3,
+							},
+						}
+					})
+					It("selects the cover art from the lowest disc number regardless of path", func() {
+						album := mfs.ToAlbum()
+						Expect(album.EmbedArtPath).To(Equal("Artist/Album/file-z.mp3"))
+					})
+				})
 			})
 		})
 	})