keys/bcrypt/bcrypt.go
func Cost(hashedPassword []byte) (int, error) {
	p, err := newFromHash(hashedPassword)
	if err != nil {
		return 0, err
	}
	return p.cost, nil
}
func Cost(hashedPassword []byte) (int, error) {
	p, err := newFromHash(hashedPassword)
	if err != nil {
		return 0, err
	}
	return p.cost, nil
}
// Cost returns the hashing cost used to create the given hashed
// password. When, in the future, the hashing cost of a password system needs
// to be increased in order to adjust for greater computational power, this
// function allows one to establish which passwords need to be updated.