internal: add RawXMLValue to defer XML encoding/decoding

internal/xml.go

@@ -0,0 +1,109 @@
+package internal
+
+import (
+	"encoding/xml"
+	"io"
+)
+
+// RawXMLValue is a raw XML value. It implements xml.Unmarshaler and
+// xml.Marshaler and can be used to delay XML decoding or precompute an XML
+// encoding.
+type RawXMLValue struct {
+	tok      xml.Token // guaranteed not to be xml.EndElement
+	children []RawXMLValue
+}
+
+// UnmarshalXML implements xml.Unmarshaler.
+func (val *RawXMLValue) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error {
+	val.tok = start
+	val.children = nil
+
+	for {
+		tok, err := d.Token()
+		if err != nil {
+			return err
+		}
+		switch tok := tok.(type) {
+		case xml.StartElement:
+			child := RawXMLValue{}
+			if err := child.UnmarshalXML(d, tok); err != nil {
+				return err
+			}
+			val.children = append(val.children, child)
+		case xml.EndElement:
+			return nil
+		default:
+			val.children = append(val.children, RawXMLValue{tok: xml.CopyToken(tok)})
+		}
+	}
+}
+
+// MarshalXML implements xml.Marshaler.
+func (val *RawXMLValue) MarshalXML(e *xml.Encoder, start xml.StartElement) error {
+	switch tok := val.tok.(type) {
+	case xml.StartElement:
+		if err := e.EncodeToken(tok); err != nil {
+			return err
+		}
+		for _, child := range val.children {
+			// TODO: find a sensible value for the start argument?
+			if err := child.MarshalXML(e, xml.StartElement{}); err != nil {
+				return err
+			}
+		}
+		return e.EncodeToken(tok.End())
+	case xml.EndElement:
+		panic("unexpected end element")
+	default:
+		return e.EncodeToken(tok)
+	}
+}
+
+var _ xml.Marshaler = (*RawXMLValue)(nil)
+var _ xml.Unmarshaler = (*RawXMLValue)(nil)
+
+// TokenReader returns a stream of tokens for the XML value.
+func (val *RawXMLValue) TokenReader() xml.TokenReader {
+	return &rawXMLValueReader{val: val}
+}
+
+type rawXMLValueReader struct {
+	val         *RawXMLValue
+	start, end  bool
+	child       int
+	childReader xml.TokenReader
+}
+
+func (tr *rawXMLValueReader) Token() (xml.Token, error) {
+	if tr.end {
+		return nil, io.EOF
+	}
+
+	start, ok := tr.val.tok.(xml.StartElement)
+	if !ok {
+		tr.end = true
+		return tr.val.tok, nil
+	}
+
+	if !tr.start {
+		tr.start = true
+		return start, nil
+	}
+
+	for tr.child < len(tr.val.children) {
+		if tr.childReader == nil {
+			tr.childReader = tr.val.children[tr.child].TokenReader()
+		}
+
+		tok, err := tr.childReader.Token()
+		if err == io.EOF {
+			tr.childReader = nil
+			tr.child++
+		} else {
+			return tok, err
+		}
+	}
+
+	tr.end = true
+	return start.End(), nil
+}