Lithuanian national id validator in C# .NET. Lietuviško asmens kodo validavimas.

NationalIdValidator.cs

//Fastest c# nationalid validator with birthdate extraction
public static class NationalIdValidator
{
	public static bool ValidateNationalId(string nationalId, out DateTime? birthdate)
	{
		birthdate = null;
		if (nationalId.Length != 11 || !long.TryParse(nationalId, out long nid))
			return false;
		return ValidateNationalId(nid, out birthdate);
	}
	public static bool ValidateNationalId(long nationalId, out DateTime? birthdate)
	{
		//https://lt.wikipedia.org/wiki/Asmens_kodas
		birthdate = null;
		if (nationalId < 10000001009 || nationalId > 99912319992)
			return false;
		long lastDigit = nationalId % 10;
		long sum = 0;
		long tmp = nationalId / 10;
		int multiplier = 1;
		for (int i = 9; i >= 0; i--)
		{
			multiplier = (i) % 9 + 1;
			sum += (tmp % 10) * multiplier;
			tmp /= 10;
		}
		var crc = sum % 11;
		if (crc == 10)
		{
			tmp = nationalId / 10;
			sum = 0;
			for (int i = 9; i >= 0; i--)
			{
				multiplier = (i + 2) % 9 + 1;
				sum += (tmp % 10) * multiplier;
				tmp /= 10;
			}
			crc = sum % 11;
		}
		crc = crc == 10 ? 0 : crc;
		var valid = crc == lastDigit;
		if (valid)
		{
			var firstDigit = nationalId / 10000000000;
			int year = (int)(nationalId / 100000000 % 100);
			int yearPrefix = 0;
			int month = (int)(nationalId / 1000000 % 100);
			int day = (int)(nationalId / 10000 % 100);
			if (firstDigit == 1 || firstDigit == 2)
				yearPrefix = 1800;
			else if (firstDigit == 3 || firstDigit == 4)
				yearPrefix = 1900;
			else if (firstDigit == 5 || firstDigit == 6)
				yearPrefix = 2000;

			if (month <= 0 || month > 12 || day <= 0 || day > DateTime.DaysInMonth(yearPrefix + year, month))
				return (year == 0 && month == 0 && day == 0) && valid; //allow special cases where national id contains no birthdate

			birthdate = new DateTime(yearPrefix + year, month, day);
		}
		return valid;
	}
}