diff --git a/mp4tag.go b/mp4tag.go
new file mode 100644
index 0000000..a01cd4a
--- /dev/null
+++ b/mp4tag.go
@@ -0,0 +1,476 @@
+package mp4tag
+
+import (
+	"encoding/binary"
+	"errors"
+	"io"
+	"os"
+	"path/filepath"
+	"reflect"
+	"strings"
+
+	"github.com/abema/go-mp4"
+	"github.com/sunfish-shogi/bufseekio"
+)
+
+var atomsMap = map[string]mp4.BoxType{
+	"Album":       {'\251', 'a', 'l', 'b'},
+	"AlbumArtist": {'a', 'A', 'R', 'T'},
+	"Artist":      {'\251', 'A', 'R', 'T'},
+	"Comment":     {'\251', 'c', 'm', 't'},
+	"Composer":    {'\251', 'w', 'r', 't'},
+	"Copyright":   {'c', 'p', 'r', 't'},
+	"Cover":       {'c', 'o', 'v', 'r'},
+	"Disk":        {'d', 'i', 's', 'k'},
+	"Genre":       {'\251', 'g', 'e', 'n'},
+	"Label":       {'\251', 'l', 'a', 'b'},
+	"Title":       {'\251', 'n', 'a', 'm'},
+	"Track":       {'t', 'r', 'k', 'n'},
+	"Year":        {'\251', 'd', 'a', 'y'},
+}
+
+func copy(w *mp4.Writer, h *mp4.ReadHandle) error {
+	_, err := w.StartBox(&h.BoxInfo)
+	if err != nil {
+		return err
+	}
+	box, _, err := h.ReadPayload()
+	if err != nil {
+		return err
+	}
+	_, err = mp4.Marshal(w, box, h.BoxInfo.Context)
+	if err != nil {
+		return err
+	}
+	_, err = h.Expand()
+	if err != nil {
+		return err
+	}
+	_, err = w.EndBox()
+	return err
+}
+
+// See which atoms don't already exist and will need creating.
+func populateAtoms(f *os.File, _tags *Tags) (map[string]bool, error) {
+	ilst, err := mp4.ExtractBox(
+		f, nil, mp4.BoxPath{mp4.BoxTypeMoov(), mp4.BoxTypeUdta(), mp4.BoxTypeMeta(), mp4.BoxTypeIlst()})
+	if err != nil {
+		return nil, err
+	}
+	if len(ilst) == 0 {
+		return nil, errors.New("Ilst atom is missing. Implement me.")
+	}
+	atoms := map[string]bool{}
+	fields := reflect.VisibleFields(reflect.TypeOf(*_tags))
+	for _, field := range fields {
+		fieldName := field.Name
+		if fieldName == "Custom" || fieldName == "TrackTotal" || fieldName == "DiskTotal" {
+			continue
+		}
+		if fieldName == "TrackNumber" {
+			fieldName = "Track"
+		} else if fieldName == "DiskNumber" {
+			fieldName = "Disk"
+		}
+		boxType, ok := atomsMap[fieldName]
+		if !ok {
+			continue
+		}
+		boxes, err := mp4.ExtractBox(
+			f, nil, mp4.BoxPath{mp4.BoxTypeMoov(), mp4.BoxTypeUdta(), mp4.BoxTypeMeta(), mp4.BoxTypeIlst(), boxType})
+		if err != nil {
+			return nil, err
+		}
+		atoms[fieldName] = len(boxes) == 0
+	}
+	return atoms, nil
+}
+
+func marshalData(w *mp4.Writer, ctx mp4.Context, val interface{}) error {
+	_, err := w.StartBox(&mp4.BoxInfo{Type: mp4.BoxTypeData()})
+	if err != nil {
+		return err
+	}
+	var boxData mp4.Data
+	switch v := val.(type) {
+	case string:
+		boxData.DataType = mp4.DataTypeStringUTF8
+		boxData.Data = []byte(v)
+	case []byte:
+		boxData.DataType = mp4.DataTypeBinary
+		boxData.Data = v
+	}
+	_, err = mp4.Marshal(w, &boxData, ctx)
+	if err != nil {
+		return err
+	}
+	_, err = w.EndBox()
+	return err
+}
+
+func writeMeta(w *mp4.Writer, tag mp4.BoxType, ctx mp4.Context, val interface{}) error {
+	_, err := w.StartBox(&mp4.BoxInfo{Type: tag})
+	if err != nil {
+		return err
+	}
+	err = marshalData(w, ctx, val)
+	if err != nil {
+		return err
+	}
+	_, err = w.EndBox()
+	return err
+}
+
+func writeCustomMeta(w *mp4.Writer, ctx mp4.Context, field string, val interface{}) error {
+	_, err := w.StartBox(&mp4.BoxInfo{Type: mp4.BoxType{'-', '-', '-', '-'}, Context: ctx})
+	if err != nil {
+		return err
+	}
+	_, err = w.StartBox(&mp4.BoxInfo{Type: mp4.BoxType{'m', 'e', 'a', 'n'}, Context: ctx})
+	if err != nil {
+		return err
+	}
+	_, err = w.Write([]byte{'\x00', '\x00', '\x00', '\x00'})
+	if err != nil {
+		return err
+	}
+	_, err = io.WriteString(w, "com.apple.iTunes")
+	if err != nil {
+		return err
+	}
+	_, err = w.EndBox()
+	if err != nil {
+		return err
+	}
+	_, err = w.StartBox(&mp4.BoxInfo{Type: mp4.BoxType{'n', 'a', 'm', 'e'}, Context: ctx})
+	if err != nil {
+		return err
+	}
+	_, err = w.Write([]byte{'\x00', '\x00', '\x00', '\x00'})
+	if err != nil {
+		return err
+	}
+	_, err = io.WriteString(w, field)
+	if err != nil {
+		return err
+	}
+	_, err = w.EndBox()
+	if err != nil {
+		return err
+	}
+	err = marshalData(w, ctx, val)
+	if err != nil {
+		return err
+	}
+	_, err = w.EndBox()
+	return err
+}
+
+// func writeCover(h *mp4.ReadHandle, w *mp4.Writer, ctx mp4.Context, coverData []byte) error {
+// 	_, err := w.StartBox(&h.BoxInfo)
+// 	if err != nil {
+// 		return err
+// 	}
+// 	box, _, err := h.ReadPayload()
+// 	if err != nil {
+// 		return err
+// 	}
+// 	_, err = mp4.Marshal(w, box, h.BoxInfo.Context)
+// 	if err != nil {
+// 		return err
+// 	}
+// 	err = writeMeta(w, h.BoxInfo.Type, ctx, coverData)
+// 	if err != nil {
+// 		return err
+// 	}
+// 	_, err = w.EndBox()
+// 	if err != nil {
+// 		return err
+// 	}
+// 	return nil
+// }
+
+// Make new atoms and write to.
+func createAndWrite(h *mp4.ReadHandle, w *mp4.Writer, ctx mp4.Context, _tags *Tags, atoms map[string]bool) error {
+	_, err := w.StartBox(&h.BoxInfo)
+	if err != nil {
+		return err
+	}
+	box, _, err := h.ReadPayload()
+	if err != nil {
+		return err
+	}
+	_, err = mp4.Marshal(w, box, h.BoxInfo.Context)
+	if err != nil {
+		return err
+	}
+	if _tags.Cover != nil {
+		err := writeMeta(w, atomsMap["Cover"], ctx, _tags.Cover)
+		if err != nil {
+			return err
+		}
+	}
+	if _tags.TrackNumber > 0 {
+		trkn := make([]byte, 8)
+		binary.BigEndian.PutUint32(trkn, uint32(_tags.TrackNumber))
+		if _tags.TrackTotal > 0 {
+			binary.BigEndian.PutUint16(trkn[4:], uint16(_tags.TrackTotal))
+		}
+		err = writeMeta(w, atomsMap["Track"], ctx, trkn)
+		if err != nil {
+			return err
+		}
+	}
+	if _tags.DiskNumber > 0 {
+		disk := make([]byte, 8)
+		binary.BigEndian.PutUint32(disk, uint32(_tags.DiskNumber))
+		if _tags.DiskTotal != 0 {
+			binary.BigEndian.PutUint16(disk[4:], uint16(_tags.DiskTotal))
+		}
+		err = writeMeta(w, atomsMap["Disk"], ctx, disk)
+		if err != nil {
+			return err
+		}
+	}
+	for tagName, needCreate := range atoms {
+		if tagName == "Cover" || tagName == "Track" || tagName == "Disk" {
+			continue
+		}
+		val := reflect.ValueOf(*_tags).FieldByName(tagName).String()
+		if !needCreate || val == "" {
+			continue
+		}
+		boxType := atomsMap[tagName]
+		err = writeMeta(w, boxType, ctx, val)
+		if err != nil {
+			return err
+		}
+	}
+	for field, value := range _tags.Custom {
+		if value == "" {
+			continue
+		}
+		err = writeCustomMeta(w, ctx, field, strings.ToUpper(value))
+		if err != nil {
+			return err
+		}
+	}
+	_, err = h.Expand()
+	if err != nil {
+		return err
+	}
+	_, err = w.EndBox()
+	return err
+}
+
+func writeExisting(h *mp4.ReadHandle, w *mp4.Writer, _tags *Tags, currentKey string, ctx mp4.Context) (bool, error) {
+	if currentKey == "Cover" && _tags.Cover == nil {
+		return true, nil
+	}
+	if currentKey == "Cover" && _tags.Cover != nil {
+		// err := writeCover(h, w, ctx, _tags.Cover)
+		// if err != nil {
+		// 	return false, nil
+		// }
+		//err := writeMeta(w, atomsMap["Cover"], ctx, _tags.Cover)
+		_, err := w.StartBox(&h.BoxInfo)
+		if err != nil {
+			return false, err
+		}
+		box, _, err := h.ReadPayload()
+		if err != nil {
+			return false, err
+		}
+		data := box.(*mp4.Data)
+		data.DataType = mp4.DataTypeBinary
+		data.Data = []byte(_tags.Cover)
+		_, err = mp4.Marshal(w, data, h.BoxInfo.Context)
+		if err != nil {
+			return false, err
+		}
+		_, err = w.EndBox()
+		if err != nil {
+			return false, err
+		}
+		// err := writeMeta(w, h.BoxInfo.Type, ctx, _tags.Cover)
+		// if err != nil {
+		// 	return false, err
+		// }
+	} else if currentKey == "Disk" {
+		if _tags.DiskNumber < 1 {
+			return true, nil
+		}
+		// disk := make([]byte, 8)
+		// binary.BigEndian.PutUint32(disk, uint32(_tags.DiskNumber))
+		// if _tags.DiskTotal > 0 {
+		// 	binary.BigEndian.PutUint16(disk[4:], uint16(_tags.DiskTotal))
+		// }
+		// err := writeMeta(w, h.BoxInfo.Type, ctx, disk)
+		// if err != nil {
+		// 	return false, err
+		// }
+	} else if currentKey == "Track" {
+		if _tags.TrackNumber < 1 {
+			return true, nil
+		}
+		// trkn := make([]byte, 8)
+		// binary.BigEndian.PutUint32(trkn, uint32(_tags.TrackNumber))
+		// if _tags.TrackTotal > 0 {
+		// 	binary.BigEndian.PutUint16(trkn[4:], uint16(_tags.TrackTotal))
+		// }
+		// // err := writeMeta(w, h.BoxInfo.Type, ctx, trkn)
+	} else {
+		toWrite := reflect.ValueOf(*_tags).FieldByName(currentKey).String()
+		if toWrite == "" {
+			return true, nil
+		}
+		// Not working here.
+		// err := writeMeta(w, h.BoxInfo.Type, ctx, toWrite)
+		// if err != nil {
+		// 	return false, err
+		// }
+		_, err := w.StartBox(&h.BoxInfo)
+		if err != nil {
+			return false, err
+		}
+		box, _, err := h.ReadPayload()
+		if err != nil {
+			return false, err
+		}
+		data := box.(*mp4.Data)
+		data.DataType = mp4.DataTypeStringUTF8
+		data.Data = []byte(toWrite)
+		_, err = mp4.Marshal(w, data, h.BoxInfo.Context)
+		if err != nil {
+			return false, err
+		}
+		_, err = w.EndBox()
+		return false, err
+
+	}
+	return false, nil
+}
+
+func containsAtom(boxType mp4.BoxType, boxes []mp4.BoxType) mp4.BoxType {
+	for _, _boxType := range boxes {
+		if boxType == _boxType {
+			return boxType
+		}
+	}
+	return mp4.BoxType{}
+}
+
+func containsTag(delete []string, currentTag string) bool {
+	for _, tag := range delete {
+		if strings.EqualFold(tag, currentTag) {
+			return true
+		}
+	}
+	return false
+}
+
+func getTag(boxType mp4.BoxType) string {
+	for k, v := range atomsMap {
+		if v == boxType {
+			return k
+		}
+	}
+	return ""
+}
+
+func getAtomsList() []mp4.BoxType {
+	var atomsList []mp4.BoxType
+	for _, atom := range atomsMap {
+		atomsList = append(atomsList, atom)
+	}
+	return atomsList
+}
+
+func copyTrack(srcPath, destPath string) error {
+	srcFile, err := os.OpenFile(srcPath, os.O_RDONLY, 0755)
+	if err != nil {
+		return err
+	}
+	defer srcFile.Close()
+	outFile, err := os.OpenFile(destPath, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0755)
+	if err != nil {
+		return err
+	}
+	defer outFile.Close()
+	_, err = io.Copy(outFile, srcFile)
+	return err
+}
+
+func Write(trackPath string, _tags *Tags) error {
+	var currentKey string
+	ctx := mp4.Context{UnderIlstMeta: true}
+	tempPath, err := os.MkdirTemp(os.TempDir(), "go-mp4tag")
+	if err != nil {
+		return errors.New(
+			"Failed to make temp directory.\n" + err.Error())
+	}
+	tempPath = filepath.Join(tempPath, "tmp.m4a")
+	defer os.RemoveAll(tempPath)
+	atomsList := getAtomsList()
+	outFile, err := os.OpenFile(trackPath, os.O_RDONLY, 0755)
+	if err != nil {
+		return err
+	}
+	tempFile, err := os.OpenFile(tempPath, os.O_CREATE|os.O_TRUNC|os.O_WRONLY, 0755)
+	if err != nil {
+		outFile.Close()
+		return err
+	}
+	r := bufseekio.NewReadSeeker(outFile, 128*1024, 4)
+	atoms, err := populateAtoms(outFile, _tags)
+	if err != nil {
+		outFile.Close()
+		tempFile.Close()
+		return err
+	}
+	w := mp4.NewWriter(tempFile)
+	_, err = mp4.ReadBoxStructure(r, func(h *mp4.ReadHandle) (interface{}, error) {
+		switch h.BoxInfo.Type {
+		case mp4.BoxTypeMoov(), mp4.BoxTypeUdta(), mp4.BoxTypeMeta():
+			err := copy(w, h)
+			return nil, err
+		case mp4.BoxTypeIlst():
+			err := createAndWrite(h, w, ctx, _tags, atoms)
+			return nil, err
+		case containsAtom(h.BoxInfo.Type, atomsList):
+			if h.BoxInfo.Type == atomsMap["Cover"] && _tags.Cover != nil {
+				return nil, nil
+			}
+			currentKey = getTag(h.BoxInfo.Type)
+			if containsTag(_tags.Delete, currentKey) {
+				return nil, nil
+			}
+			err = copy(w, h)
+			return nil, err
+		case mp4.BoxTypeData():
+			if currentKey == "" {
+				return nil, w.CopyBox(r, &h.BoxInfo)
+			}
+			needCreate := atoms[currentKey]
+			if !needCreate {
+				valEmpty, err := writeExisting(h, w, _tags, currentKey, ctx)
+				currentKey = ""
+				if err != nil {
+					return nil, err
+				} else if valEmpty {
+					return nil, w.CopyBox(r, &h.BoxInfo)
+				}
+			}
+			return nil, nil
+		default:
+			return nil, w.CopyBox(r, &h.BoxInfo)
+		}
+	})
+	outFile.Close()
+	tempFile.Close()
+	if err != nil {
+		return err
+	}
+	err = copyTrack(tempPath, trackPath)
+	return err
+}
diff --git a/structs.go b/structs.go
new file mode 100644
index 0000000..faf7263
--- /dev/null
+++ b/structs.go
@@ -0,0 +1,20 @@
+package mp4tag
+
+type Tags struct {
+	Album       string
+	AlbumArtist string
+	Artist      string
+	Comment     string
+	Composer    string
+	Cover       []byte
+	Custom      map[string]string
+	Delete      []string
+	DiskNumber  int
+	DiskTotal   int
+	Genre       string
+	Label       string
+	Title       string
+	TrackNumber int
+	TrackTotal  int
+	Year        string
+}