Add Identity function for identifying metadata/filetypes.

id.go

@@ -0,0 +1,70 @@
+package tag
+
+import (
+	"fmt"
+	"io"
+	"os"
+)
+
+// Identify reads from the given ReadSeaker and
+func Identify(r io.ReadSeeker) (format Format, fileType FileType, err error) {
+	b, err := readBytes(r, 11)
+	if err != nil {
+		return
+	}
+
+	_, err = r.Seek(-11, os.SEEK_CUR)
+	if err != nil {
+		err = fmt.Errorf("could not seek back to original position: %v", err)
+		return
+	}
+
+	switch {
+	case string(b[0:4]) == "fLaC":
+		return VORBIS, FLAC, nil
+
+	case string(b[0:4]) == "OggS":
+		return VORBIS, OGG, nil
+
+	case string(b[4:11]) == "ftypM4A":
+		return AAC, MP4, nil
+
+	case string(b[0:3]) == "ID3":
+		b := b[3:]
+		switch uint(b[0]) {
+		case 2:
+			format = ID3v2_2
+		case 3:
+			format = ID3v2_3
+		case 4:
+			format = ID3v2_4
+		case 0, 1:
+			fallthrough
+		default:
+			err = fmt.Errorf("ID3 version: %v, expected: 2, 3 or 4", uint(b[0]))
+			return
+		}
+		return format, MP3, nil
+	}
+
+	n, err := r.Seek(-128, os.SEEK_END)
+	if err != nil {
+		return
+	}
+
+	tag, err := readString(r, 3)
+	if err != nil {
+		return
+	}
+
+	_, err = r.Seek(-n, os.SEEK_CUR)
+	if err != nil {
+		return
+	}
+
+	if tag != "TAG" {
+		err = ErrNoTagsFound
+		return
+	}
+	return ID3v1, MP3, nil
+}