fix: resolve bugs found in pre-release review

internal/tag/id3v2.go

@@ -3,6 +3,7 @@ package tag
 import (
 	"fmt"
 	"strconv"
+	"strings"
 
 	"github.com/bogem/id3v2/v2"
 )
@@ -14,20 +15,19 @@ type id3v2Tagger struct {
 func (t *id3v2Tagger) write(m Metadata) error {
 	defer t.tag.Close()
 
-	duration, err := strconv.Atoi(m.Duration)
-	if err != nil {
-		return err
-	}
-	length := fmt.Sprintf("%d", duration*1000)
-
 	if m.Album != nil {
+		year := m.Album.ReleaseDate
+		if parts := strings.Split(year, "-"); len(parts) == 3 {
+			year = parts[0]
+		}
+
 		t.addTag("TRCK", m.TrackNumber)
 		t.addTag("TPE2", m.Album.Artist)
 		t.addTag("TALB", m.Album.Title)
 		t.addTag("TPUB", m.Album.Label)
 		t.addTag("TDOR", m.Album.OriginalReleaseDate)
-		t.addTag("TYER", m.Album.ReleaseDate)
-		t.addTag("COMM", m.Album.ProducerLine)
+		t.addTag("TYER", year)
+		t.addComment(m.Album.ProducerLine)
 		t.addTag("TCOP", m.Album.Copyright)
 	}
 
@@ -36,19 +36,23 @@ func (t *id3v2Tagger) write(m Metadata) error {
 	t.addTag("TCOM", m.Composers)
 	t.addTag("TEXT", m.Lyricists)
 	t.addTag("TCON", m.Genre)
-	t.addTag("TLEN", length)
+	if duration, err := strconv.Atoi(m.Duration); err == nil {
+		t.addTag("TLEN", fmt.Sprintf("%d", duration*1000))
+	}
 	t.addTag("TBPM", m.BPM)
 	t.addTag("TKEY", m.Key)
 	t.addTXXX("GAIN", m.Gain)
 	t.addTXXX("ISRC", m.ISRC)
 
-	t.tag.AddAttachedPicture(id3v2.PictureFrame{
-		Encoding:    t.tag.DefaultEncoding(),
-		MimeType:    "image/jpeg",
-		PictureType: id3v2.PTFrontCover,
-		Description: "Cover",
-		Picture:     m.Cover,
-	})
+	if len(m.Cover) > 0 {
+		t.tag.AddAttachedPicture(id3v2.PictureFrame{
+			Encoding:    t.tag.DefaultEncoding(),
+			MimeType:    "image/jpeg",
+			PictureType: id3v2.PTFrontCover,
+			Description: "Cover",
+			Picture:     m.Cover,
+		})
+	}
 
 	return t.tag.Save()
 }
@@ -59,6 +63,16 @@ func (t *id3v2Tagger) addTag(name, value string) {
 	}
 }
 
+func (t *id3v2Tagger) addComment(value string) {
+	if value != "" {
+		t.tag.AddCommentFrame(id3v2.CommentFrame{
+			Encoding: t.tag.DefaultEncoding(),
+			Language: "eng",
+			Text:     value,
+		})
+	}
+}
+
 func (t *id3v2Tagger) addTXXX(description, value string) {
 	if value != "" {
 		t.tag.AddUserDefinedTextFrame(id3v2.UserDefinedTextFrame{