#!/usr/bin/perl -w
use strict;
use Digest::MD5;

if (! scalar(@ARGV)) {
    print <<USAGE;
Usage:
    $0 file1 file2 ... fileN

USAGE
    exit;
}

my (%md5sums);
foreach my $filename (@ARGV) {
    open(FILE, $filename) or die("Can't open '$filename': $!");
    binmode(FILE);
        my $digest = Digest::MD5->new->addfile(*FILE)->hexdigest;
        $md5sums{'by-name'}{$filename} = $digest;
        push(@{$md5sums{'by-digest'}{$digest}}, $filename);
    close(FILE);
}

print "By checksum:\n";
foreach my $digest (sort(keys(%{$md5sums{'by-digest'}}))) {
    print $digest, ": \n";
    foreach my $filename (sort(@{$md5sums{'by-digest'}{$digest}})) {
        print "\t", $filename, "\n";
    }
}
print "\n";
print "By filename:\n";
foreach my $filename (sort(keys(%{$md5sums{'by-name'}}))) {
    print $md5sums{'by-name'}{$filename}, ': ', $filename, "\n";
}