# NAME

CheckDigits::M16_001 - compute check digits for ISAN

# SYNOPSIS

``````  use Algorithm::CheckDigits;

\$isan = CheckDigits('isan');

if (\$isan->is_valid('123A567B8912E01A')) {
# do something
}

\$cn = \$isan->complete('123A567B8912E01');
# \$cn = '123A567B8912E01A'

\$cd = \$isan->checkdigit('123A567B8912E01A');
# \$cd = '4'

\$bn = \$isan->basenumber('123A567B8912E01A');
# \$bn = '123A567B8912E01'
``````

# DESCRIPTION

## ALGORITHM

1. `a(1) = 16`

`b(i) = a(i) % 17 +d(i)`, where `d(i)` is the decimal value of the hexdigit at position i.

`c(i) = b(i) % 16`

`a(i) = c(i-1) * 2`, for i greater than 1

2. Beginning left for each i = 1..16, `a`, `b`, `c` are computed.

3. The check digit is the value for `d(16)` where `c(16)` equals 1.

4. The check digit is appended as hexadecimal value to the number.

## 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, `<mamawe@cpan.org>`