sub rank_objects {
  my @objects = @_;
  my $n = 0;
  my @next_objects = grep {not $_->depends_on} @objects;
  my @current_objects;

  while (@next_objects and $n < @objects) {
    $n++;
    @current_objects = @next_objects;
    @next_objects = ();

    for my $object (@current_objects) {
      $object->set_rank($n);
      push @next_objects, $object->depended_on_by;
    }
  }

  my @circular = @next_objects, grep {not $_->rank} @objects;
  if (@circular) {
    # We have circular dependencies, report that somehow.
    # In production code I'd actually try to report one of
    # the cycles here.
    die "Circular dependencies found.";
  }
}