Luhn check digit

Question

I cant seem to figure out what is wrong with my check digit code!

At times, it produces 2 length check digit values

Example

1277531815000110 <-- check digit is double value??????
1277532495000110 <-- check digit is double value???????
1277534649000110 <-- check digit is double value???????
127753185300011 <-- good!
127753208500019 <-- good!

All generated numbers are valid, it can be checked at http://www.ee.unb.ca/cgi-bin/tervo/luhn.pl?N=127753224800013

CODE: http://dl.dropbox.com/u/678582/Email/GenerateAN.txt

Your code link goes to a 404. Is the code too long to post here? — Bill the Lizard, Sep 14 '10 at 19:52

score 2 · Accepted Answer · answered Jun 26 '10 at 06:42

2

This line's the problem:

CheckSumNumber = (((sum / 10) + 1) * 10) - sum;

That will generate 10 when sum is already a multiple of 10. Basically you're just trying to round up. Here's an easy way of doing it:

CheckSumNumber = (((sum + 9) / 10) * 10) - sum;

answered Jun 26 '10 at 06:42

Jon Skeet

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Luhn check digit

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