Code refactorization

Should even work if the subatoms of "----" are out of order

mp4.go

@@ -6,6 +6,7 @@ package tag
 
 import (
 	"encoding/binary"
+	"errors"
 	"fmt"
 	"io"
 	"os"
@@ -69,10 +70,62 @@ func ReadAtoms(r io.ReadSeeker) (Metadata, error) {
 	return m, err
 }
 
+func readCustomAtom(r io.ReadSeeker, size uint32) (string, uint32, error) {
+	var datasize uint32
+	var datapos int64
+	var name string
+	var mean string
+
+	for size > 8 {
+		var subsize uint32
+		err := binary.Read(r, binary.BigEndian, &subsize)
+		if err != nil {
+			return "----", size - 4, err
+		}
+		subname, err := readString(r, 4)
+		if err != nil {
+			return "----", size - 8, err
+		}
+		b, err := readBytes(r, int(subsize-8))
+		if err != nil {
+			return "----", size - subsize, err
+		}
+		// Remove the size of the mean atom from the size counter
+		size -= subsize
+
+		switch string(subname) {
+		case "mean":
+			mean = string(b[4:])
+		case "name":
+			name = string(b[4:])
+		case "data":
+			datapos, err = r.Seek(0, os.SEEK_CUR)
+			if err != nil {
+				return "----", size, err
+			}
+			datasize = subsize
+			datapos -= int64(subsize)
+		}
+	}
+	// there should remain only the header size
+	if size != 8 {
+		return "----", size, errors.New("---- atom out of bound")
+	}
+	if mean == "com.apple.iTunes" && datasize != 0 && name != "" {
+		// we jump just before the data subatom
+		_, err := r.Seek(datapos, os.SEEK_SET)
+		if err != nil {
+			return "----", size, err
+		}
+		return name, datasize + 8, nil
+	}
+	return "----", size, nil
+}
+
 func (m metadataMP4) readAtoms(r io.ReadSeeker) error {
 	for {
 		var size uint32
-		var subname string
+		ok := false
 		err := binary.Read(r, binary.BigEndian, &size)
 		if err != nil {
 			if err == io.EOF {
@@ -109,69 +162,20 @@ func (m metadataMP4) readAtoms(r io.ReadSeeker) error {
 			}
 			continue
 		case "----":
-			/* Generic atom.
-			   Should have 3 sub atoms : mean, name and data.
-			   We check that mean=="com.apple.iTunes" and we use the subname as
-			   the name, and move to the data atom if anything goes wrong, we jump
-			   at the end of the "----" atom.  */
-
-			// let's read the mean atom
-			var subsize uint32
-			err := binary.Read(r, binary.BigEndian, &subsize)
+			// Generic atom.
+			// Should have 3 sub atoms : mean, name and data.
+			// We check that mean=="com.apple.iTunes" and we use the subname as
+			// the name, and move to the data atom if anything goes wrong,
+			// we jump at the end of the "----" atom.
+			name, size, err = readCustomAtom(r, size)
 			if err != nil {
 				return err
 			}
-			sub, err := readString(r, 4)
-			if err != nil {
-				return err
+			if name != "----" {
+				ok = true
 			}
-
-			if sub != "mean" {
-				// Something's wrong. Remove 8 read bytes from the size counter
-				// since "----" is not a known atom name, the whole data will
-				// be skipped
-				size -= 8
-				break
-			}
-
-			mean, err := readBytes(r, int(subsize-8))
-			if err != nil {
-				return err
-			}
-			// Remove the size of the mean atom from the size counter
-			size -= subsize
-
-			if string(mean[4:]) != "com.apple.iTunes" {
-				// Something's wrong, skip this atom
-				break
-			}
-
-			// Let's read the name atom
-			err = binary.Read(r, binary.BigEndian, &subsize)
-			if err != nil {
-				return err
-			}
-			sub, err = readString(r, 4)
-			if err != nil {
-				return err
-			}
-
-			if sub != "name" {
-				// Something's wrong
-				size -= 8
-				break
-			}
-
-			b, err := readBytes(r, int(subsize-8))
-			if err != nil {
-				return err
-			}
-			/* Remove the size of the name atom from the size counter.
-			We should now be at the start of the data subatom and size should
-			be equal to the size of the data atom and its header */
-			size -= subsize
-
-			subname = string(b[4:])
+		default:
+			_, ok = atoms[name]
 		}
 
 		b, err := readBytes(r, int(size-8))
@@ -179,14 +183,8 @@ func (m metadataMP4) readAtoms(r io.ReadSeeker) error {
 			return err
 		}
 
-		// Allow all known atoms and the valid "----" atoms
-		_, ok := atoms[name]
-		switch {
-		case name == "----" && subname == "":
-			continue
-		case name == "----":
-			name = subname
-		case !ok:
+		// At this point, we allow all known atoms and the valid "----" atoms
+		if !ok {
 			continue
 		}