#! perl -w

use strict;

my $element;

my @union = ();

my @intersection = ();

my @difference = ();

my %count = ();

my $file1 = shift or die "Usage: $0 file1 file2 | optional -I[ntersection]\n";

my $file2 = shift or die "Usage: $0 file1 file2 | optional -I[ntersection]\n";

my $mode  = shift;

open FILE1, "<$file1" or die "ERROR: Can't read $file1";

open FILE2, "<$file2" or die "ERROR: Can't read $file2";

my @file1_content = <FILE1>;

my @file2_content = <FILE2>;

close FILE1;

close FILE2;

print "* Comparing $file1 and $file2\n";

foreach $element (@file1_content, @file2_content) { $count{$element}++ }

foreach $element (keys %count) {

    push @union, $element;

    push @{ $count{$element} > 1 ? \@intersection : \@difference }, $element;

}

if (!defined $mode) {

	if (@difference > 0) {

		foreach (@difference){

			print $_;

		}

	} else {

		print "* Files are identical\n";

	}

} elsif ($mode eq "-I") {

	if (@intersection > 0) {

		foreach (@intersection){

			print $_;

		}

	}

} else {

	print "Usage: $0 file1 file2 | optional -I[ntersection]\n";

}