In the Object Designer window, create a new codeunit. Assign a number and name (for example, 50000, Luhn Verification).

Click View, then C/AL Globals; in the Globals window open the Functions tab.

In the first line of the table enter the function name, SplitNumber.

The verification function receives a BigInteger number as an input parameter, but the algorithm works with separate digits. Therefore, before starting the validation we need to convert the number into an array of digits.

Position into the Split function, the number you just declared, and click View, then C/AL Locals. First tab in the Locals window is Parameters. Enter the first parameter of the function:

Still in the C/AL Locals window, click View, Properties to open the variable properties and set Dimensions = 11.

Close the variable properties window and add the second function parameter AccountNumber with type BigInteger.

The Parameters window with the list of properties for the variable Digits is shown in the following screenshot:

Next, navigate to the Variables tab. Insert a variable i of Integer type.

Close the local declarations window to return to the code editor and type the function code:

        FOR i := 11 DOWNTO 1 DO BEGIN 
          Digits[i] := AccountNumber MOD 10; 
          ccountNumber := AccountNumber DIV 10; 
        END; 

Open the C/AL Globals window again and insert the second function VerifyCheckSum. This is the main function that implements the verification algorithm.

In the C/AL Locals window, insert a single parameter of this function AccountNumber of type BigInteger.

Navigate to the Return Value tab and fill in the Return Type field. In this case, the type should be Boolean.

In the C/AL Locals window, declare three local variables as follows:

Select the Digits variable, open its properties, and set Dimensions to 11.

Type the following function code:

        SplitNumber(Digits,AccountNumber); 
        FOR i := 1 TO 10 DO BEGIN 
          IF i MOD 2 = 1 THEN 
            CheckSum += (Digits[i] * 2) DIV 10 +  
              (Digits[i] * 2) MOD 10; 
        END; 
        CheckSum := 10 - (CheckSum * 9) MOD 10; 
        EXIT(CheckSum = Digits[11]); 

In the OnRun trigger, place the code that will call the verification function:

        IF VerifyCheckSum(79927398712) THEN 
          MESSAGE(CorrectCheckSumMsg) 
        ELSE 
          MESSAGE(WrongCheckSumMsg); 

To present the execution result to the user, OnRun uses two text constants that we have not declared yet. To do it, open the C/AL Globals window in the View menu. In the Text Constants tab, enter the values as in the following table: