Bulstat EIK validation implemented in C# in T-SQL

BulstatEikValidation.cs

using System.Text.RegularExpressions;

namespace Erpy.Domain;

// based on https://tsvetanv.wordpress.com/2011/04/01/eik/
public partial class BulstatEikValidation
{
    private static readonly int[] FIRST_SUM_9DIGIT_WEIGHTS = [1, 2, 3, 4, 5, 6, 7, 8];
    private static readonly int[] SECOND_SUM_9DIGIT_WEIGHTS = [3, 4, 5, 6, 7, 8, 9, 10];
    private static readonly int[] FIRST_SUM_13DIGIT_WEIGHTS = [2, 7, 3, 5];
    private static readonly int[] SECOND_SUM_13DIGIT_WEIGHTS = [4, 9, 5, 7];
    
    [GeneratedRegex(@"^\d+$", RegexOptions.Singleline)]
    private static partial Regex EikRegex();

    public static bool IsValid(string eik)
    {
        ArgumentNullException.ThrowIfNull(eik);

        if (eik.Length != 9 && eik.Length != 13)
        {
            return false;
        }

        if (!EikRegex().IsMatch(eik))
        {
            return false;
        }

        int[] digits = eik.Select(c => int.Parse(c.ToString())).ToArray();
        return
            digits.Length switch
            {
                9 => CalculateChecksum(digits, FIRST_SUM_9DIGIT_WEIGHTS, SECOND_SUM_9DIGIT_WEIGHTS) == digits[8],
                13 => CalculateChecksum(digits[0..9], FIRST_SUM_9DIGIT_WEIGHTS, SECOND_SUM_9DIGIT_WEIGHTS) == digits[8] &&
                        CalculateChecksum(digits[8..13], FIRST_SUM_13DIGIT_WEIGHTS, SECOND_SUM_13DIGIT_WEIGHTS) == digits[12],
                _ => throw new Exception("Should not happen.")
            };
    }

    private static int CalculateChecksum(int[] digits, int[] firstSumWeights, int[] secondSumWeights)
    {
        if (digits.Length - 1 != firstSumWeights.Length || digits.Length - 1 != secondSumWeights.Length)
        {
            throw new ArgumentException("The length of the digits minus 1 should be equal to the length of the weights.");
        }

        int sum = 0;
        for (int i = 0; i < digits.Length - 1; i++)
        {
            sum += digits[i] * firstSumWeights[i];
        }
        int remainder = sum % 11;
        if (remainder != 10)
        {
            return remainder;
        }

        int secondSum = 0;
        for (int i = 0; i < digits.Length - 1; i++)
        {
            secondSum += digits[i] * secondSumWeights[i];
        }
        
        int secondRem = secondSum % 11;
        if (secondRem != 10)
        {
            return secondRem;
        }

        return 0;
    }
}

BulstatEikValidation.sql

-- based on https://tsvetanv.wordpress.com/2011/04/01/eik/

CREATE TYPE IntArray
AS TABLE (
    -- indeces are 1 based
    [Index] INT,
    [Value] INT
)
GO 

CREATE OR ALTER FUNCTION fn_eik_is_valid(@eik NVARCHAR(MAX))
RETURNS BIT
AS
BEGIN
    IF @eik IS NULL
        RETURN 0;

    IF LEN(@eik) != 9 AND LEN(@eik) != 13
        RETURN 0;

    IF PATINDEX('%[^0-9]%', @eik) > 0
        RETURN 0;

    DECLARE @i INT = 1;
    DECLARE @len INT = LEN(@eik);
    DECLARE @digits IntArray;
    
    WHILE @i <= @len
    BEGIN
        INSERT INTO @digits ([Index], [Value])
        VALUES (@i, CAST(SUBSTRING(@eik, @i, 1) AS INT));
        SET @i = @i + 1;
    END;

    DECLARE @result BIT;

    IF @len = 9
    BEGIN
        IF dbo.fn_eik_calc_checksum(@digits, 1, 9) = (SELECT [Value] FROM @digits WHERE [Index] = 9)
            SET @result = 1;
        ELSE
            SET @result = 0;
    END
    ELSE IF @len = 13
    BEGIN
        IF dbo.fn_eik_calc_checksum(@digits, 1, 9) = (SELECT [Value] FROM @digits WHERE [Index] = 9)
            AND dbo.fn_eik_calc_checksum(@digits, 9, 13) = (SELECT [Value] FROM @digits WHERE [Index] = 13)
            SET @result = 1;
        ELSE
            SET @result = 0;
    END;

    RETURN @result;
END;
GO

CREATE OR ALTER FUNCTION dbo.fn_eik_calc_checksum(@digits IntArray READONLY, @start INT, @end INT)
RETURNS INT
AS
BEGIN
    DECLARE @sum INT = 0;
    DECLARE @i INT = @start;
    DECLARE @currentValue INT;

    WHILE @i < @end
    BEGIN
        SET @currentValue = (SELECT [Value] FROM @digits WHERE [Index] = @i)
        SET @sum = @sum + @currentValue * dbo.fn_eik_get_weight(@i);
        SET @i = @i + 1;
    END;

    DECLARE @remainder INT = @sum % 11;

    IF @remainder != 10
        RETURN @remainder;

    DECLARE @secondSum INT = 0;
    SET @i = @start;

    WHILE @i < @end
    BEGIN
        SET @currentValue = (SELECT [Value] FROM @digits WHERE [Index] = @i)
        SET @secondSum = @secondSum + @currentValue * dbo.fn_eik_get_second_weight(@i);
        SET @i = @i + 1;
    END;

    RETURN @secondSum % 11;
END;
GO

CREATE OR ALTER FUNCTION dbo.fn_eik_get_weight(@position INT)
RETURNS INT
AS
BEGIN
    RETURN CASE 
        WHEN @position = 1 THEN 1
        WHEN @position = 2 THEN 2
        WHEN @position = 3 THEN 3
        WHEN @position = 4 THEN 4
        WHEN @position = 5 THEN 5
        WHEN @position = 6 THEN 6
        WHEN @position = 7 THEN 7
        WHEN @position = 8 THEN 8
        WHEN @position = 9 THEN 2
        WHEN @position = 10 THEN 7
        WHEN @position = 11 THEN 3
        WHEN @position = 12 THEN 5
        ELSE 0
    END;
END;
GO

CREATE OR ALTER FUNCTION dbo.fn_eik_get_second_weight(@position INT)
RETURNS INT
AS
BEGIN
    RETURN CASE 
        WHEN @position = 1 THEN 3
        WHEN @position = 2 THEN 4
        WHEN @position = 3 THEN 5
        WHEN @position = 4 THEN 6
        WHEN @position = 5 THEN 7
        WHEN @position = 6 THEN 8
        WHEN @position = 7 THEN 9
        WHEN @position = 8 THEN 10
        WHEN @position = 9 THEN 4
        WHEN @position = 10 THEN 9
        WHEN @position = 11 THEN 5
        WHEN @position = 12 THEN 7
        ELSE 0
    END;
END;
GO