Validating card number with Luhn algorithm in dart/flutter

card_utils.dart

  static String validateCardNumWithLuhnAlgorithm(String input) {
    if (input.isEmpty) {
      return Strings.fieldReq;
    }

    input = getCleanedNumber(input);

    if (input.length < 8) { // No need to even proceed with the validation if it's less than 8 characters
      return Strings.numberIsInvalid;
    }

    int sum = 0;
    int length = input.length;
    for (var i = 0; i < length; i++) {
      // get digits in reverse order
      int digit = int.parse(input[length - i - 1]);

      // every 2nd number multiply with 2
      if (i % 2 == 1) {
        digit *= 2;
      }
      sum += digit > 9 ? (digit - 9) : digit;
    }

    if (sum % 10 == 0) {
      return null;
    }

    return Strings.numberIsInvalid;
  }