sub _file_containing {
    my($self, $id) = @_;

    opendir(my $dir, $self->{update_dir}) or return;
    my @files =
        map  { "$self->{update_dir}/$_" }
        sort { $a <=> $b }
        grep { /^\d+$/ && $_ <= $id } 
        readdir $dir;                 
    return $files[-1];
}