This only does 12 digit barcodes for now. if your interested in how this works go to
www.howstuffworks.com/upc.htm whichi is where I got most of my information. I just thought it would be nice to have a quick function to do the math for me and figured I should share ;)
a basic summary goes something like this:
To figure out what the chekdigit is suposed to be add together all of the odd digits and multiply by 3, then add all the even digits and add to the last result. Then find the number that when added to that makes it evenly divisible by to (10-(total mod 10))
To make sure a barcode is valid just do the same math but at the end say (total mod 10)==0
Its fairly basic but seems a little backwards on a string since they are zero based arrays. You just swap evens for odds