sub get_next_file {
  my $s = shift;

  if (!$s->{_selected_file}) {
    my @files = @_ ? @_ : $s->get_files;
    $s->{_file_queue} = \@files;
  }
  my $next_file = shift @{$s->{_file_queue}};
  $s->{_selected_file} = $next_file;
  return $next_file;
}