// Codec encodes/decodes various types of messages used within go-micro.
// ReadHeader and ReadBody are called in pairs to read requests/responses
// from the connection. Close is called when finished with the
// connection. ReadBody may be called with a nil argument to force the
// body to be read and discarded.
type Codec interface {
	ReadHeader(*Message, MessageType) error
	ReadBody(interface{}) error
	Write(*Message, interface{}) error
	Close() error
	String() string
}

// Message represents detailed information about
// the communication, likely followed by the body.
// In the case of an error, body may be nil.
type Message struct {
	Id     uint64
	Type   MessageType
	Target string
	Method string
	Error  string
	Header map[string]string
}

// WriteNetString writes data to a big-endian netstring on a Writer.
// Size is always a 32-bit unsigned int.
func WriteNetString(w io.Writer, data []byte) (written int, err error) {
	size := make([]byte, 4)
	binary.BigEndian.PutUint32(size, uint32(len(data)))
	if written, err = w.Write(size); err != nil {
		return
	}
	return w.Write(data)
}

// ReadNetString reads data from a big-endian netstring.
func ReadNetString(r io.Reader) (data []byte, err error) {
	sizeBuf := make([]byte, 4)
	_, err = r.Read(sizeBuf)
	if err != nil {
		return nil, err
	}
	size := binary.BigEndian.Uint32(sizeBuf)
	if size == 0 {
		return nil, nil
	}
	data = make([]byte, size)
	_, err = r.Read(data)
	if err != nil {
		return nil, err
	}
	return
}