public string Generate()
{
var nipNumberBuilder = new StringBuilder();
string taxOfficePrefix = TaxOffice.Codes[_random.Next(TaxOffice.Codes.Length)];
nipNumberBuilder.Append(taxOfficePrefix);
nipNumberBuilder.Append(GenerateRandomNumbers(6));
int checksum = NipCheckSumCalculator.Calculate(nipNumberBuilder.ToString());
while (checksum == 10)
{
// change last number, check sum must be different from 10
nipNumberBuilder.Remove(nipNumberBuilder.Length - 1, 1);
nipNumberBuilder.Append(_random.Next(10).ToString());
checksum = NipCheckSumCalculator.Calculate(nipNumberBuilder.ToString());
}
nipNumberBuilder.Append(checksum);
return(nipNumberBuilder.ToString());
}
public static bool IsValid(string nip)
{
var regex = new Regex("^\\d{10}$");
if (!regex.IsMatch(nip))
{
return(false);
}
int checkSum = NipCheckSumCalculator.Calculate(nip);
if (nip.Last() - '0' != checkSum)
{
return(false);
}
return(TaxOffice.Codes.Contains(nip.Substring(0, 3)));
}