// of bytes decoded before the error.
func Decode(dst, src []byte) (int, error) {
	i, j := 0, 1
	for ; j < len(src); j += 2 {
		p := src[j-1]
		q := src[j]

		a := reverseHexTable[p]
		b := reverseHexTable[q]
		if a > 0x0f {
			return i, InvalidByteError(p)
		}
		if b > 0x0f {
			return i, InvalidByteError(q)
		}
		dst[i] = (a << 4) | b
		i++
	}
	if len(src)%2 == 1 {