docs: @VisibleForTesting
      String stripSeparators(final String cardNumber) {
          final int length = cardNumber.length();
          final char[] result = new char[length];
          int count = 0;
          char cur;
          for (int i = 0; i < length; i++) {
              cur = cardNumber.charAt(i);
              if (!(cur == ' ' || cur == '-')) {
                  result[count++] = cur;
              }
          }
          if (count == length) {
              return cardNumber;
          }
          return new String(result, 0, count);
      }