Check Digit Formula

proc checkDigit10 {number} {

   set tot 0
   set sub 0
   set ex 0
   set wy 0
   set checkDigit “”

   if {![lempty $number] && [string is digit $number] } {

     # trim the input value

       set number [string trim $number]

     # Split the number into individual digits

       set numList [split $number {}]

     # Work the digits of the number from right to left

       for {set ex [expr [llength $numList] -1]} {$ex >= 0} {incr ex -1} {

         # Multiply every other number by 2 beginning with right-most digit

           if [expr $wy % 2 == 0] {
               set sub [expr [lindex $numList $ex] * 2]
           } else {
               set sub [lindex $numList $ex]
           }

         # If more than 1 digit then add them to the total individually

           set subList [split $sub {}]
           foreach itm $subList {
               incr tot $itm
           }

           incr wy

       }

     # Subtract the $tot value from the next integer multiple of 10.

       set checkDigit [expr (10-($tot % 10)) % 10]

   }

   return $checkDigit

}