rate128      = 168
	rate256      = 136
)

func bytepad(input []byte, w int) []byte {
	// leftEncode always returns max 9 bytes
	buf := make([]byte, 0, 9+len(input)+w)
	buf = append(buf, leftEncode(uint64(w))...)
	buf = append(buf, input...)
	padlen := w - (len(buf) % w)
	return append(buf, make([]byte, padlen)...)
}

func leftEncode(value uint64) []byte {
	var b [9]byte
	binary.BigEndian.PutUint64(b[1:], value)