#!/usr/bin/perl -wl
use strict;
use Getopt::Long;
use Pod::Usage;

my ($IN,$OUT,$COUNT);

GetOptions(
	'help|?'  => sub { pod2usage(-verbose => 2);exit } ,
	'in=s'    => \$IN,
	'out=s'   => \$OUT,
    'count=i' => \$COUNT
);
die pod2usage {-verbose=>2} unless $IN;

open IN, "< $IN" or die "Cannot open ($IN) for reading: $!";
chomp (my @items = <IN>);
close IN;

$COUNT ||= scalar @items;

my @permutes;
push @permutes => \@items for 1 .. @items;
my %results = map { join( '',@$_[0..($COUNT-1)]) => 1 } permute(@permutes);

if ( $OUT ) {
    open OUT, "> $OUT" or die "Cannot open ($OUT) for writing: $!";
    print OUT $_ foreach sort keys %results;
    close OUT;
}
else {
    print foreach sort keys %results;
}

sub permute {
    my $last = pop @_;
    return map [$_], @$last unless @_;
    return map { my $left = $_; map [@$left, $_], @$last } permute(@_);
}

__END__

=head1 NAME

permutations.pl -- A simple tool to generate permutations of items

=head1 SYNOPSIS

B<permutations.pl --help> for more information
 
    perl permutations.pl [options]

Options:

    --help    Display help
    --?       Same as --help
    --infile  File with list of items (one on each line)
    --outfile File to write permutations to
    --count   Number of items in permutation

=head1 DESCRIPTION

This program will generate a list of all possible permutations of data items
listed in an infile.  It will limit the number of items in the permutations to
the number of items specified in the C<--count> option.  For example, with the
following items in an input file:

 .jpg
 .exe
 .com

If you want to print all permutations with a limit of two items per
permutation, you would use the following command (assuming that the input and
output files are named C<infile.txt> and C<outfile.txt>, respectively).

 perl permutations.pl --count 2 --in infile.txt --out outfile.txt

You may use shortcuts, too:

 perl permutations.pl -c 2 -i infile.txt -o outfile.txt

The above command will result in the following output:

 .com.com
 .com.exe
 .com.jpg
 .exe.com
 .exe.exe
 .exe.jpg
 .jpg.com
 .jpg.exe
 .jpg.jpg

 
If C<--count> is omitted, the count will default to the number of items in the
infile.

If C<--out> is omitted, the results will be printed to C<STDOUT>.