This tutorial shows how to call a simple DLL function, which performs a quick check on a credit card number. Click here to download the flowchart/zipfile, which accompanies this tutorial topic.

i.  What is a LUHN check?
Most international credit card numbers (including Visa and Mastercard) conform to a LUHN checksum algorithm. It is possible to calculate the last digit of a card number, by examining all the preceeding digits. This will not indicate if the card is stolen, but it can be used to check if the user has made a mistake when entering a card number.

1.  The DLL function
The zipfile which accompanies this topic contains the C source code for a DLL. This DLL contains a single function, the prototype is shown below:

/*
This function performs LUHN checksum validation for the specified card number.
It returns non-zero if the checksum is valid, zero if invalid.
*/
__declspec(dllexport)
int CheckLuhn(char* pstrCardNumber);

2.  Begin a new flowchart
Run the SQLphone Designer program, to begin a new flowchart. Remember to select Options from the Prompt menu, and setup the prompt directory. You can leave all the standard prompt options unticked. Note: This tutorial topic does not need a database. Click here to recall the "New flowchart checklist" topic.

3.  Add memories
Press the <New Memory> button on the toolbar, to create the following two memories:

Memories
Name	Description
Card	Stores the credit card number entered by a user
OK	Stores the DLL return code: zero = failure, non-zero = success

4.  Add prompts
Press the <New Prompt> button on the toolbar, to create the following three voice prompts:

Prompts (wave files)
Name	Description
EnterCard	"Please enter your credit card number"
CardGood	"Card number is good"
CardBad	"Card number is invalid"

When you create new prompts, you can press the <Sound recorder> button to record using your own voice, or you can press the <Generate...> button to record using a synthesized voice.

5.  Add a box to get a credit card number
Add a new Digits Box to the flowchart. Name the box Get Card, then click the Digits tab. Setup as shown in figure 1.

• Play the EnterCard prompt, asking the user to enter a card number
• Store the card number in the Card memory
• Set the maximum digits to be the largest number of digits expected for a card number

Figure 1

6.  Add a box to check the card number
Add a new DLL Box to the flowchart. Name the box Check Card, then click the DLL tab. Setup as shown in figure 2.

• The function return code is stored in the OK memory
• The function parameter is the value of the Card memory

	Figure 2
	Figure 3

7.  Add a box to report valid card
Add a new Play Box to the flowchart. Name the box Card OK, then click the Play tab. Setup as shown in figure 4.

• The CardGood prompt will get played

Figure 4

8.  Add a box to report invalid card
Add a new Play Box to the flowchart. Name the box Card Fail, then click the Play tab. Setup as shown in figure 5.

• The CardBad prompt will get played

Figure 5

9.  Allow the user to make a mistake
If the user enters an invalid card number, allow them to have another go. This is achieved by setting a retry counter. Double click the Get Card box, then click the Conditions tab. Setup the retry counter, see figure 6.

Figure 6

10.  Join the boxes
Join up the flowchart box conditions, and setup the Get Card box as the flowchart starting point. See figure 7.

Figure 7

11.  Test the flowchart
Press F5 to test the flowchart.