BSN checker

bsn_check.dart

bool isNumeric(String? s) {
  if(s == null) {
    return false;
  }
  return double.tryParse(s) != null;
}

bool isValidBsn(String bsn) {
  /*
    Validator based on a variation of the Dutch check-digit validation used for checking IBAN.
    This variation is used for BSN.
    For more information visit https://nl.wikipedia.org/wiki/Elfproef#Burgerservicenummer
    :param bsn: customer service number (BSN), must be 9 digits
    :return: true if valid, false if invalid
    */
  int bsnLen = bsn.length;
  if (bsnLen != 9 || !isNumeric(bsn)) {
    return false;
  }

  int total = 0;
  for (int index=0; index<8; index++) {
    String t = bsn[index];
    total += int.parse(t) * (9 - index);
  }

  int lastNumber = int.parse(bsn[8]);

  // Validate if the remainder of the total divided by 11 is equal to the last number in the BSN
  if (total % 11 != lastNumber) {
    return false;
  }

  return true;
}