class LuhnValidator

class LuhnValidator extends ConstraintValidator {
    public function validate(mixed $value, Constraint $constraint) : void {
        if (!$constraint instanceof Luhn) {
            throw new UnexpectedTypeException($constraint, Luhn::class);
        }
        if (null === $value || '' === $value) {
            return;
        }
        // Work with strings only, because long numbers are represented as floats
        // internally and don't work with strlen()
        if (!\is_string($value) && !$value instanceof \Stringable) {
            throw new UnexpectedValueException($value, 'string');
        }
        $value = (string) $value;
        if (!ctype_digit($value)) {
            $this->context
                ->buildViolation($constraint->message)
                ->setParameter('{{ value }}', $this->formatValue($value))
                ->setCode(Luhn::INVALID_CHARACTERS_ERROR)
                ->addViolation();
            return;
        }
        $checkSum = 0;
        $length = \strlen($value);
        for ($i = $length - 1; $i >= 0; --$i) {
            if ($i % 2 ^ $length % 2) {
                // Starting with the last digit and walking left, add every second
                // digit to the check sum
                // e.g. 7  9  9  2  7  3  9  8  7  1  3
                //      ^     ^     ^     ^     ^     ^
                //    = 7  +  9  +  7  +  9  +  7  +  3
                $checkSum += (int) $value[$i];
            }
            else {
                // Starting with the second last digit and walking left, double every
                // second digit and add it to the check sum
                // For doubles greater than 9, sum the individual digits
                // e.g. 7  9  9  2  7  3  9  8  7  1  3
                //         ^     ^     ^     ^     ^
                //    =    1+8 + 4  +  6  +  1+6 + 2
                $checkSum += (int) (2 * $value[$i] / 10) + 2 * $value[$i] % 10;
            }
        }
        if (0 === $checkSum || 0 !== $checkSum % 10) {
            $this->context
                ->buildViolation($constraint->message)
                ->setParameter('{{ value }}', $this->formatValue($value))
                ->setCode(Luhn::CHECKSUM_FAILED_ERROR)
                ->addViolation();
        }
    }

}