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

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-'

- 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.

- 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.

None by default.

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

perl, CheckDigits,

*www.pruefziffernberechnung.de*.