docs: static boolean isValidLuhnNumber(@Nullable String cardNumber) {
          if (cardNumber == null) {
              return false;
          }
  
          boolean isOdd = true;
          int sum = 0;
  
          for (int index = cardNumber.length() - 1; index >= 0; index--) {
              char c = cardNumber.charAt(index);
              if (!Character.isDigit(c)) {
                  return false;
              }
  
              int digitInteger = Character.getNumericValue(c);
              isOdd = !isOdd;
  
              if (isOdd) {
                  digitInteger *= 2;
              }
  
              if (digitInteger > 9) {
                  digitInteger -= 9;
              }
  
              sum += digitInteger;
          }
  
          return sum % 10 == 0;
      }