Complete rewrite.

Everything is done in the library now, Abema's go-mp4 lib no longer needed.

mp4tag.go

@@ -0,0 +1,69 @@
+package mp4tag
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"os"
+)
+
+func (mp4 *MP4) Close() error { 
+	return mp4.f.Close()
+}
+
+func (mp4 *MP4) Read() (*MP4Tags, error) {
+	tags, _, err := mp4.actualRead()
+	return tags, err
+}
+
+func (mp4 *MP4) Write(tags *MP4Tags, delStrings []string) error {
+	if tags == nil && len(delStrings) == 0 {
+		return nil
+	}
+	err := mp4.actualWrite(tags, delStrings)
+	return err
+}
+
+func (mp4 *MP4) checkHeader() error {
+	buf := make([]byte, 8)
+	_, err := mp4.f.Seek(4, io.SeekStart)
+	if err != nil {
+		return err
+	}
+	_, err = io.ReadFull(mp4.f, buf)
+	if err != nil {
+		return err
+	}
+
+	if !bytes.Equal(buf[:4], []byte{0x66, 0x74, 0x79, 0x70}) {
+		return &ErrInvalidMagic{}
+	}
+	for _, ftyp := range ftyps {
+		if bytes.Equal(buf[4:], ftyp) {
+			return nil
+		}
+	}
+	return &ErrUnsupportedFtyp{
+		Msg: "unsupported ftyp: " + fmt.Sprintf("%x", buf[4:]),
+	}
+}
+
+func Open(trackPath string) (*MP4, error) {
+	f, err := os.Open(trackPath)
+	if err != nil {
+		return nil, err
+	}
+	stat, err := f.Stat()
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+
+	mp4 := &MP4{f: f, size : stat.Size(), path: trackPath}
+	err = mp4.checkHeader()
+	if err != nil {
+		f.Close()
+		return nil, err
+	}
+	return mp4, nil
+}