On 5/25/07, David Carlisle <davidc@xxxxxxxxx> wrote:

>   <xsl:variable name="calc" select="(10 - ((sum($odd) * 3) +
> sum($even)) mod 10) mod 10" as="xs:integer"/>
>     <xsl:if test="$calc ne $check-digit">UPC not legal</xsl:if>

foo mod 10 mod 10  is the same as foo mod 10, and 10 mod 10 is 0 so the
above is

<xsl:if test="($check-digit + ((sum($odd) * 3) + sum($even))) mod 10 =0">

Nice!

That give a boolean to check the UPC is correct - is it ok to re-write it as:

10 - ((sum($odd) * 3) + sum($even)) mod 10

to give you the check-digit?  If so it makes the outermost "mod 10" of
$calc redundant?  (if it is then step #4 on the wikipedia article isnt
needed)

Example:
Suppose your 11-digit number is 12345678950
When you apply the algorithm sums it gives you 100
Step 3 is now: 10 - 100 mod 10 = 10 - 0 = 10
Step 4 (now necessary) gives: 10 mod 10 = 0
ergo: check digit is 0. The UPC-A string becomes 123456789500

Cheers,
-- Abel Braaksma