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And should perhaps have had:

 'enRoute'                      2014 eller 2149          15

...but that card uses either another control algorithm or no control
digits at all. So C<enRoute> is never returned here.

If the control digits is valid, but the input does not match anything in the column C<starts on>, 1 is returned.

(This is also the same control digit mechanism used in Norwegian KID numbers on payment bills)

The first digit in a credit card number is supposed to tell what "industry" the card is meant for:

 MII Digit Value             Issuer Category
 --------------------------- ----------------------------------------------------
 0                           ISO/TC 68 and other industry assignments
 1                           Airlines
 2                           Airlines and other industry assignments
 3                           Travel and entertainment
 4                           Banking and financial
 5                           Banking and financial
 6                           Merchandizing and banking
 7                           Petroleum
 8                           Telecommunications and other industry assignments
 9                           National assignment

...although this has no meaning to C<Acme::Tools::ccn_ok()>.

The first six digits is I<Issuer Identifier>, that is the bank
(probably). The rest in the "account number", except the last digits,
which is the control digit. Max length on credit card numbers are 19
digits.

=cut

sub ccn_ok {
    my $ccn=shift(); #credit card number
    $ccn=~s/\D+//g;
    if(KID_ok($ccn)){
	return "MasterCard"                   if $ccn=~/^5[1-5]\d{14}$/;
	return "Visa"                         if $ccn=~/^4\d{12}(?:\d{3})?$/;
	return "American Express"             if $ccn=~/^3[47]\d{13}$/;
	return "Discover"                     if $ccn=~/^6011\d{12}$/;
	return "Diners Club / Carte Blanche"  if $ccn=~/^3(?:0[0-5]\d{11}|[68]\d{12})$/;
	return "JCB"                          if $ccn=~/^(?:3\d{15}|(?:2131|1800)\d{11})$/;
	return 1;
    }
    #return "enRoute"                        if $ccn=~/^(?:2014|2149)\d{11}$/; #ikke LUHN-krav?
    return 0;
}

=head2 KID_ok

Checks if a norwegian KID number has an ok control digit.

To check if a customer has typed the number correctly.

This uses the  LUHN algorithm (also known as mod-10) from 1960 which is also used
internationally in control digits for credit card numbers, and Canadian social security ID numbers as well.

The algorithm, as described in Phrack (47-8) (a long time hacker online publication):

 "For a card with an even number of digits, double every odd numbered
 digit and subtract 9 if the product is greater than 9. Add up all the
 even digits as well as the doubled-odd digits, and the result must be
 a multiple of 10 or it's not a valid card. If the card has an odd
 number of digits, perform the same addition doubling the even numbered
 digits instead."

B<Input:> A KID-nummer. Must consist of digits 0-9 only, otherwise a die (croak) happens.

B<Output:>

- Returns undef if the input argument is missing.

- Returns 0 if the control digit (the last digit) does not satify the LUHN/mod-10 algorithm.

- Returns 1 if ok

B<See also:> L</ccn_ok>

=cut

sub KID_ok {
  croak "Non-numeric argument" if $_[0]=~/\D/;
  my @k=split//,shift or return undef;
  my $s;$s+=pop(@k)+[qw/0 2 4 6 8 1 3 5 7 9/]->[pop@k] while @k;
  $s%10==0?1:0;
}



=head2 range

B<Input:>

One or more numeric arguments:

First: x (first returned element)

Second: y (up to y but not including y)

Third: step, default 1. The step between each returned element

If a fourth, fifth and so on arguments are given, they change the step for each returned element. As first derivative, second derivative.

B<Output:>

If one argument: returns the array C<(0 .. x-1)>

If two arguments: returns the array C<(x .. y-1)>

If three arguments: The default step is 1. Use a third argument to use a different step.

B<Examples:>

 print join ",", range(11);         # prints 0,1,2,3,4,5,6,7,8,9,10  (but not 11)
 print join ",", range(2,11);       # 2,3,4,5,6,7,8,9,10             (but not 11)
 print join ",", range(11,2,-1);    # 11,10,9,8,7,6,5,4,3
 print join ",", range(2,11,3);     # 2,5,8
 print join ",", range(11,2,-3);    # 11,8,5