NAME

       CheckDigits::M11_001 - compute check digits for ISBN, ISSN, VAT RN (PT), HKID (HK), Wagon number (BR),
       NHS (GB), VAT (SL)

SYNOPSIS

         use Algorithm::CheckDigits;

         $isbn = CheckDigits('isbn');

         if ($isbn->is_valid('3-88229-192-3')) {
               # do something
         }

         $cn = $isbn->complete('3-88229-192-');
         # $cn = '3-88229-192-3'

         $cd = $isbn->checkdigit('3-88229-192-3');
         # $cd = '3'

         $bn = $isbn->basenumber('3-88229-192-3');
         # $bn = '3-88229-192-'

DESCRIPTION

   ALGORITHM
       1.  The sequence of digits is processed right to left.  Every digit is multiplied with their position in
           the sequence (i.e.  the digit left to the check digit has the weight 2 then 3 etc.).

           With a Hongkong ID (hkid) the leftmost char is replaced with its position in the alphabet and then
           multiplied with 8 (its weight).

       2.  The sum of all products is computed.

       3.  The sum of step 2 is taken modulo 11.

       4.  The checkdigit is the difference of the sum from step 3 to eleven under the following conditions:

           isbn,issn
                   If the difference is 10, the check digit is 'X'.

                   If the difference is 11, the check digit is 0.

           ustid_pt
                   If the difference is greater then 9, the check digit is '0'.

           hkid    If the difference is 10, the check digit is 'A'.

                   If the difference is 11, the check digit is 0.

           wagonnr_br
                   If the difference is 10, the check digit is 0.

                   If the difference is 11, the check digit is 1.

           nhs_gb  If the difference is 10, the number would not be taken.

                   If the difference is 11, the check digit is 0.

           vat_sl  This is a little bit unclear, don't trust on the method for this type.

   METHODS
       is_valid($number)
           Returns true only if $number consists solely of numbers and the last digit is a valid check digit
           according to the algorithm given above.

           Returns false otherwise,

       complete($number)
           The check digit for $number is computed and concatenated to the end of $number.

           Returns the complete number with check digit or '' if $number does not consist solely of digits and
           spaces.

       basenumber($number)
           Returns the basenumber of $number if $number has a valid check digit.

           Return '' otherwise.

       checkdigit($number)
           Returns the checkdigit of $number if $number has a valid check digit.

           Return '' otherwise.

   EXPORT
       None by default.

AUTHOR

       Mathias Weidner, <mathias@weidner.in-bad-schmiedeberg.de>

SEE ALSO

       perl, CheckDigits, www.pruefziffernberechnung.de.