https://github.com/Sorrow446/go-mp4tag/issues/11

- Issue #11 implemented.
- Fixed assignment to nil map error for the custom field.
- UpperCustom func, pass false to disable uppercasing custom tags' names for read & write.

mp4tag.go

@@ -7,6 +7,10 @@ import (
 	"os"
 )
 
+func (mp4 *MP4) UpperCustom(b bool) { 
+	mp4.upperCustom = b
+}
+
 func (mp4 *MP4) Close() error { 
 	return mp4.f.Close()
 }
@@ -59,7 +63,12 @@ func Open(trackPath string) (*MP4, error) {
 		return nil, err
 	}
 
-	mp4 := &MP4{f: f, size : stat.Size(), path: trackPath}
+	mp4 := &MP4{
+		f: f,
+		size : stat.Size(),
+		path: trackPath,
+		upperCustom: true,
+	}
 	err = mp4.checkHeader()
 	if err != nil {
 		f.Close()

objects.go

@@ -62,6 +62,7 @@ type MP4 struct {
 	f *os.File
 	path string
 	size int64
+	upperCustom bool
 }
 
 type MP4Box struct {
@@ -297,6 +298,7 @@ type MP4Tags struct {
 	ItunesArtistID int32
 	Lyrics string
 	Narrator string
+	OtherCustom map[string][]string
 	Pictures []*MP4Picture
 	Publisher string
 	Title string

read.go

@@ -229,7 +229,18 @@ func (mp4 MP4) readTrknDisk(boxes MP4Boxes, boxName string) (int16, int16, error
 	return num, total, nil
 }
 
-func (mp4 MP4) readCustom(boxes MP4Boxes) (map[string]string, error) {
+func addToOthers(others map[string][]string, key, val string) map[string][]string {
+	existingOthers, ok := others[key]
+	if ok {
+		existingOthers = append(existingOthers, val)
+		others[key] = existingOthers
+	} else {
+		others[key] = []string{val}
+	}
+	return others
+}
+
+func (mp4 MP4) readCustom(boxes MP4Boxes) (map[string]string, map[string][]string, error) {
 	var (
 		names []string
 		values []string
@@ -237,38 +248,67 @@ func (mp4 MP4) readCustom(boxes MP4Boxes) (map[string]string, error) {
 	path := "moov.udta.meta.ilst.----"
 	nameBoxes := boxes.getBoxesByPath(path+".name")
 	if nameBoxes == nil {
-		return nil, nil
+		return nil, nil, nil
 	}
 	for _, box := range nameBoxes {
 		_, err := mp4.f.Seek(box.StartOffset+12, io.SeekStart)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
 		}
 		name, err := mp4.readString(box.BoxSize-12)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
+		}
+		if mp4.upperCustom {
+			name = strings.ToUpper(name)
 		}
 		names = append(names, name)
 	}
 
+	others := map[string][]string{}
+
 	dataBoxes := boxes.getBoxesByPath(path+".data")
+
+	var (
+		prev int64
+		idx int
+	)
+
 	for _, box := range dataBoxes {
 		_, err := mp4.f.Seek(box.StartOffset+16, io.SeekStart)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
 		}
 		value, err := mp4.readString(box.BoxSize-16)
 		if err != nil {
-			return nil, err
+			return nil, nil, err
+		}
+		if box.StartOffset == prev {
+			others = addToOthers(others, names[idx-1], value)
+			prev = box.EndOffset
+			continue
 		}
 		values = append(values, value)
+		prev = box.EndOffset
+		idx++
 	}
 
 	custom := map[string]string{}
 	for idx, name := range names {
-		custom[name] = values[idx]
+		_, ok := custom[name]
+		if ok {
+			existingOthers, ok := others[name]
+			if ok {
+				existingOthers = append(existingOthers, values[idx])
+				others[name] = existingOthers			
+			} else {
+				others[name] = []string{values[idx]}
+			}
+		} else {
+			custom[name] = values[idx]
+		}
 	}
-	return custom, nil
+	return custom, others, nil
 }
 
 func (mp4 MP4) readITAlbumID(boxes MP4Boxes) (int32, error) {
@@ -373,7 +413,7 @@ func (mp4 MP4) readTags(boxes MP4Boxes) (*MP4Tags, error) {
 	if err != nil  {
 		return nil, err
 	}
-	custom, err := mp4.readCustom(boxes)
+	custom, otherCustom, err := mp4.readCustom(boxes)
 	if err != nil  {
 		return nil, err
 	}	
@@ -455,6 +495,7 @@ func (mp4 MP4) readTags(boxes MP4Boxes) (*MP4Tags, error) {
 		ItunesArtistID: artistID,
 		Lyrics: lyrics,
 		Narrator: narrator,
+		OtherCustom: otherCustom,
 		Pictures: pics,
 		Publisher: publisher,
 		Title: title,

write.go

@@ -20,6 +20,16 @@ func overwriteTags(mergedTags, tags *MP4Tags, delStrings []string) *MP4Tags{
 	} else if containsStr(delStrings, "allcustom") {
 		mergedTags.Custom = map[string]string{}
 	}
+	if containsStr(delStrings, "allothercustom") {
+		mergedTags.OtherCustom = map[string][]string{}
+	}
+
+	if mergedTags.Custom == nil {
+		mergedTags.Custom = map[string]string{}
+	}
+	if mergedTags.OtherCustom == nil {
+		mergedTags.OtherCustom = map[string][]string{}
+	}
 
 	if containsStr(delStrings,  "album") {
 		mergedTags.Album = ""
@@ -265,11 +275,27 @@ func overwriteTags(mergedTags, tags *MP4Tags, delStrings []string) *MP4Tags{
 	}
 
 	for k, v := range tags.Custom {
+		if containsStr(delStrings, "custom:"+k) {
+			continue
+		}
 		if v != "" {
 			mergedTags.Custom[k] = v
 		}
 	}
 
+	for k, v := range tags.OtherCustom {
+		if containsStr(delStrings, "custom:"+k) || len(v) < 1 {
+			continue
+		}
+		_, ok := mergedTags.OtherCustom[k]
+		if ok {
+			mergedTags.OtherCustom[k] = append(mergedTags.OtherCustom[k], v...)
+		} else {
+			mergedTags.OtherCustom[k] = v
+		}
+
+	}
+
 	var filteredPics []*MP4Picture
 
 	for idx, p := range mergedTags.Pictures {
@@ -595,13 +621,25 @@ func writeItunesArtistID(f *os.File, artistID int32) error {
 	return err
 }
 
-func writeCustom(f *os.File, name, value string) error {
-	nameUpperBytes := []byte(strings.ToUpper(name))
+func writeCustom(f *os.File, name, value string, upper bool, others map[string][]string) error {	
 	valueBytes := []byte(value)
-	nameSize := len(nameUpperBytes)
 	valueSize := len(valueBytes)
 
-	sizeBytes := putI32BE(int32(nameSize+valueSize)+64)
+	if upper {
+		name = strings.ToUpper(name)
+	}
+	nameUpperBytes := []byte(name)
+	nameSize := len(nameUpperBytes)
+
+	totalSize := nameSize+valueSize+64
+	otherCust, _ := others[name]
+
+	for _, other := range otherCust {
+		totalSize += len([]byte(other)) + 16
+	}
+
+	// 48
+	sizeBytes := putI32BE(int32(totalSize))
 	_, err := f.Write(sizeBytes)
 	if err != nil {
 		return err
@@ -658,7 +696,35 @@ func writeCustom(f *os.File, name, value string) error {
 		return err
 	}
 	_, err = f.Write(valueBytes)
-	return err
+	if err != nil {
+		return err
+	}
+
+
+	for _, v := range otherCust {
+		valueBytes = []byte(v)
+		valueSize = len(valueBytes)
+		sizeBytes = putI32BE(int32(valueSize)+16)
+		_, err = f.Write(sizeBytes)
+		if err != nil {
+			return err
+		}
+		_, err = f.WriteString("data")
+		if err != nil {
+			return err
+		}
+		_, err = f.Write(
+			[]byte{0x0, 0x0, 0x0, 0x01, 0x0, 0x0, 0x0, 0x0})
+		if err != nil {
+			return err
+		}
+		_, err = f.Write(valueBytes)
+		if err != nil {
+			return err
+		}
+	}
+
+	return nil
 }
 
 func getPicFormat(imageType ImageType, magic []byte) uint8 {
@@ -993,7 +1059,7 @@ func (mp4 MP4) writeTags(boxes MP4Boxes, tags *MP4Tags, tempPath string) error {
 	}
 	
 	for k, v := range tags.Custom {
-		err = writeCustom(f, k, v)
+		err = writeCustom(f, k, v, mp4.upperCustom, tags.OtherCustom)
 		if err != nil {
 			return err
 		}