Rework XDR encoding

protocol/header.go

@@ -0,0 +1,34 @@
+package protocol
+
+import "github.com/calmh/syncthing/xdr"
+
+type header struct {
+	version int
+	msgID   int
+	msgType int
+}
+
+func (h header) encodeXDR(xw *xdr.Writer) (int, error) {
+	u := encodeHeader(h)
+	return xw.WriteUint32(u)
+}
+
+func (h *header) decodeXDR(xr *xdr.Reader) error {
+	u := xr.ReadUint32()
+	*h = decodeHeader(u)
+	return xr.Error()
+}
+
+func encodeHeader(h header) uint32 {
+	return uint32(h.version&0xf)<<28 +
+		uint32(h.msgID&0xfff)<<16 +
+		uint32(h.msgType&0xff)<<8
+}
+
+func decodeHeader(u uint32) header {
+	return header{
+		version: int(u>>28) & 0xf,
+		msgID:   int(u>>16) & 0xfff,
+		msgType: int(u>>8) & 0xff,
+	}
+}