person_obstructed_at(const person_t* person, double x, double y, person_t** out_obstructing_person, int* out_tile_index)
{
	rect_t           area;
	rect_t           base, my_base;
	double           cur_x, cur_y;
	bool             is_obstructed = false;
	int              layer;
	const obsmap_t*  obsmap;
	int              tile_w, tile_h;
	const tileset_t* tileset;

	int i, i_x, i_y;

	map_normalize_xy(&x, &y, person->layer);
	person_get_xyz(person, &cur_x, &cur_y, &layer, true);
	my_base = rect_translate(person_base(person), x - cur_x, y - cur_y);
	if (out_obstructing_person != NULL)
		*out_obstructing_person = NULL;
	if (out_tile_index != NULL)
		*out_tile_index = -1;

	if (!person->ignore_all_persons) {
		for (i = 0; i < s_num_persons; ++i) {
			if (s_persons[i] == person)  // these persons aren't going to obstruct themselves!
				continue;
			if (s_persons[i]->layer != layer)
				continue;  // ignore persons not on the same layer
			if (person_following(s_persons[i], person))
				continue;  // ignore own followers
			base = person_base(s_persons[i]);
			if (do_rects_overlap(my_base, base) && !person_ignored_by(person, s_persons[i])) {
				is_obstructed = true;
				if (out_obstructing_person)
					*out_obstructing_person = s_persons[i];
				break;
			}
		}
	}

	obsmap = layer_obsmap(layer);
	if (obsmap_test_rect(obsmap, my_base))
		is_obstructed = true;

	if (!person->ignore_all_tiles) {
		tileset = map_tileset();
		tileset_get_size(tileset, &tile_w, &tile_h);
		area.x1 = my_base.x1 / tile_w;
		area.y1 = my_base.y1 / tile_h;
		area.x2 = area.x1 + (my_base.x2 - my_base.x1) / tile_w + 2;
		area.y2 = area.y1 + (my_base.y2 - my_base.y1) / tile_h + 2;
		for (i_x = area.x1; i_x < area.x2; ++i_x) for (i_y = area.y1; i_y < area.y2; ++i_y) {
			base = rect_translate(my_base, -(i_x * tile_w), -(i_y * tile_h));
			obsmap = tileset_obsmap(tileset, map_tile_at(i_x, i_y, layer));
			if (obsmap != NULL && obsmap_test_rect(obsmap, base)) {
				is_obstructed = true;
				if (out_tile_index)
					*out_tile_index = map_tile_at(i_x, i_y, layer);
				break;
			}
		}
	}

	return is_obstructed;
}

The product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.