feat: enforce call order

Author: nerg4l

decode.go

@@ -76,7 +76,7 @@ func (d *Decoder) DecodeHeader() (*EBML, error) {
 			d.r.MaxIDLength = DefaultMaxIDLength
 			d.r.MaxSizeLength = DefaultMaxSizeLength
 			var h EBML
-			err := d.Decode(&h)
+			err := d.Decode(el, &h)
 			if err != nil {
 				return nil, err
 			}
@@ -109,41 +109,29 @@ func (d *Decoder) DecodeBody(v interface{}) error {
 			}
 			continue
 		case d.def.Root.ID:
-			return d.Decode(v)
+			return d.Decode(el, v)
 		}
 	}
 }
 
 func (d *Decoder) SkipByte() error {
 	_, err := io.CopyN(io.Discard, d.r, 1)
-	d.delay = false
 	d.el = nil
 	return err
 }
 
-func (d *Decoder) Skip() error {
-	if d.el == nil {
-		return fmt.Errorf("ebml: missing decoded element (forgotten call Next?)")
-	}
-	_, err := io.CopyN(io.Discard, d.r, d.el.DataSize)
-	d.delay = false
-	d.el = nil
+func (d *Decoder) Skip(el Element) error {
+	_, err := io.CopyN(io.Discard, d.r, el.DataSize)
 	return err
 }
 
-func (d *Decoder) Decode(v interface{}) error {
+func (d *Decoder) Decode(el Element, v interface{}) error {
 	val := reflect.ValueOf(v)
 	if val.Kind() != reflect.Ptr || val.IsNil() {
 		return &InvalidDecodeError{reflect.TypeOf(v)}
 	}
-	if d.el == nil {
-		return fmt.Errorf("ebml: missing decoded element (forgotten call Next?)")
-	}
 	d.skippedErrs = nil
-	err := d.decodeSingle(*d.el, val.Elem())
-	if !d.delay {
-		d.el = nil
-	}
+	err := d.decodeSingle(el, val.Elem())
 	if d.skippedErrs != nil {
 		err = errors.Join(err, d.skippedErrs)
 	}

ebml.go

@@ -167,9 +167,8 @@ type Decoder struct {
 	r   *ebmltext.Decoder
 	def *Def
 
-	el    *Element
-	delay bool
-	n     int
+	el *Element
+	n  int
 	// skippedErrs signals to return errors at the end of Decode.
 	skippedErrs error
 
@@ -211,7 +210,6 @@ func (d *Decoder) Next() (el Element, n int, err error) {
 		return Element{}, n, err
 	}
 	n += d.r.Release()
-	d.el = &el
 	d.n = n
 	sch, ok := d.def.Get(el.ID)
 	if !ok {
@@ -239,20 +237,21 @@ func (d *Decoder) NextOf(parent Element, offset int64) (el Element, n int, err e
 	if end := d.EndOfKnownDataSize(parent, offset); end {
 		return Element{}, 0, io.EOF
 	}
-	if !d.delay {
+	if d.el != nil {
+		el = *d.el
+		d.el = nil
+	} else {
 		el, n, err = d.Next()
 		if err != nil {
 			return Element{}, n, err
 		}
-	} else {
-		d.delay = false
 	}
-	el = *d.el
 	if parent.DataSize != -1 && offset+el.DataSize > parent.DataSize {
 		err = ErrElementOverflow
 	}
 	if end := d.EndOfUnknownDataSize(parent, el); end {
-		d.delay = true
+		tmp := el // This is unexpected. I cannot use the pointer to the return parameter variable.
+		d.el = &tmp
 		return Element{}, 0, io.EOF
 	}
 	return el, n, err
@@ -274,7 +273,6 @@ type DecodeSeeker struct {
 func (s DecodeSeeker) Seek(offset int64, whence int) (ret int64, err error) {
 	d, ss := s.d, s.ss
 	if offset != 0 && whence != io.SeekCurrent {
-		d.delay = false
 		d.el = nil
 	}
 	return ss.Seek(offset, whence)