#!/usr/perl/bin
#Simple program to check validity of 16 digit credit card numbers

use strict;
use warnings;

print processCard($_)
    for 'x', '41245678912345', '3124567891234560', '3324567891234560',
    '4124567891234560';


sub processCard {
    my ($cardNum) = @_;

    return "Invalid Input: $cardNum.\n" if $cardNum !~ /^[0-9]+$/;
    return "16 digit cards only for now ($cardNum).\n"
        if length $cardNum != 16;
    return "Invalid Card Number $cardNum\n" if !numberOK($cardNum);
    return cardInfo($cardNum);
}


sub numberOK {
    my @cardNum = split //, shift;

    for my $i (map {$_ * 2} 0 .. $#cardNum / 2) {
        $cardNum[$i] *= 2;
        $cardNum[$i] -= 9 if $cardNum[$i] >= 10;
    }

    my $total = 0;
    $total += $_ for (@cardNum);
    return $total % 10 == 0;
}


sub cardInfo {
    my ($cardNum) = @_;
    my %issuers = (4 => "VISA", 5 => "MasterCard", 6 => "Discover");
    my ($iDigit) = $cardNum =~ /^(\d)/;

    return "Unknown issuer code: $iDigit\n" if !exists $issuers{$iDigit};
    return <<INFO;
    
   Card Number: $cardNum
   Issuer:      $issuers{$iDigit}

INFO
}