use strict;
use warnings;
use Digest::MD5 qw(md5 md5_hex);

# needs a file name and a value for the multi-part chunk size in MB.
print "etag for $ARGV[0] = " . calculate_etag(@ARGV) . "\n";
sub calculate_etag {
    my ($file, $chunk_size) = @_;
    print "Calculating etag of $file...\n";
    my $string;
    my $count = 0;
    open(my $FILE, '<', $file);
    binmode $FILE;
    while (read($FILE, my $data, 1048576 * $chunk_size) {
        my $chunk_md5 = md5($data);
        $string .= $chunk_md5;
        $count++;
    }
    close($FILE);
    return(md5_hex($string) . "-$count");
}