[oak perl] Re: Customizing a Barcode Checkdigit Subroutine

[Belden Lyman]

I just realized that there's a small difference between
LUHN-10 and what Larry wants.

At the end of all the math, Larry wants the checkdigit to
be the tens-complement for the sum; however, LUHN-10 returns
``(!($sum % 10))'' rather than ``10 - sum % 10''.

So the algorithms, though very similar, are not identical.

Belden

David B. Peterson wrote:
> 
> There is an interesting property of numbers that you can use for this 
> calculation.
> 
> If you add the individual digits of a number and subtract 9 from it as 
> many times as necessary, you will eventually get the sum of the two 
> numbers.
> 
> So, if you have a number such as 17  1+7 = 8, 17-9 = 8 too.  Since you 
> are only dealing with single digits multiplied by 2, the highest number 
> you will use is 18.  So you will only need to ever subtract 9 once.
> 
> I believe your library card system uses what we call a LUHN-10 check. 
> This is the same check used to determine a 'valid' credit card number 
> (for some definition of valid. :)  This works for visa, mastercard, 
> american express, discover, and many others.
> 
> Here is a little routine I wrote a hell of a long time ago for this 
> purpose.
> 
> sub LUHN10 {
>   my ($a, @cc, $sum);
>   my ($cc = @_[0]);
>   $cc =~ tr/0-9//cd;
>   @cc = split('', $cc);
>   while (@cc) {
>     $sum += pop(@cc);
>     $a = pop(@cc) * 2;
>     $a -= 9 if $a > 9;
>     $sum += $a;
>   }
>   return (!($sum % 10));
> }
> 
> used as such:
> 
> LUHN10('visa: 4444444444445555');
> LUHN10('4444 4444 4444 5555');
> 
> It doesn't care about any non digit chars.
> 
> <rant>
> And as such, I'm always disgusted by commerce sites that require me to 
> leave the spaces out of my credit card number.  It shouldn't matter one 
> iota to the computer, but it sure makes it easier for me to see if I've 
> made a typo when I can put spaces in there.
> </rant>
> 
> Have fun.
> 
> -Dave