mersenne.pl

#!/usr/bin/perl
use warnings;
use strict;
use bignum;
$|=1;

my $p = $ARGV[0];

die "Usage: $0 number\n" if !$p;

my $number = ( 2**$p ) - 1;
printf "Testing 2^%s - 1 == %s\n", $p, $number;

my $divisor = find_divisor($number);
printf "2^%s - 1 is %s.\n", $p, ($divisor ? "divisible by $divisor" : 'prime');

exit;

sub find_divisor {
    my $number = shift;
    my $limit = sqrt($number);
    my $divisor = 3;
    my $passes = 0;
    my $result;
    while ($divisor <= $limit) {
        $passes +=1;
        if ($number % $divisor == 0) {
            $result = $divisor;
            last;
        }
        $divisor +=2;
        print "\x0d".$passes if $passes % 1000 == 0;
    }
    print "\x0d".$passes." passes\n";
    return $result;
}