CheckDigits::M11_001(3) compute check digits for ISBN, ISSN, VAT RN

## 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-'

## 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.
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.

None by default.

## AUTHOR

Mathias Weidner, <[email protected]>