#!env perl
#
# factor.pl <Number>
#
# Print the list of prime factors for Number
#
# 20130206 Can't find a copy on my hard drive, so making it again
use strict;
use warnings;

my $num = shift or die "Expected number to factor!";
print "$num: ", join(", ", factors($num)), "\n";

sub factors {
    my $val=shift;
    return unless $val;

    my @primes = (1);
    if ($val < 0) {
        unshift @primes, -1;
        $val = -$val;
    }

    while ($val % 2 == 0) {
        push @primes, 2;
        $val = $val/2;
    }
    my $fac=3;
    while ($fac*$fac <= $val) {
        while ($val % $fac == 0) {
            push @primes, $fac;
            $val = $val / $fac;
        }
        $fac += 2;
    }
    push @primes, $val if $val > 1;
    return @primes;
}

##</code><code>##

sub do_tests {
    use Test::More;
    use Data::Dump 'pp';
    my @test_cases = (
        [   0, [] ],
        [   1, [1] ],
        [   2, [1, 2] ],
        [   3, [1, 3] ],
        [   4, [1, 2, 2] ],
        [ -72, [-1, 1, 2, 2, 2, 3, 3] ],
    );

    for my $case (@test_cases) {
        my @f = factors($case->[0]);
        # Uncomment for debugging
        #print "$case->[0], exp: (", join(",",@{$case->[1]}), "), got(", join(",",@f),")\n";
        is_deeply([@f], $case->[1]);
    }
    done_testing();
}

##</code><code>##

if ($num eq "-test") {
    do_tests();
}
else {
    print "$num: ", join(", ", factors($num)), "\n";
}

##</code><code>##

$ perl pm_1224882.pl 1234
1234: 1, 2, 617

Roboticus@Waubli ~
$ perl pm_1224882.pl -test
ok 1
ok 2
ok 3
ok 4
ok 5
ok 6
1..6