function Road::_GetDirectionEfficient(from, to, is_bridge,
		map_size_x = Road._map_size_x)
{
	if (!is_bridge && AITile.GetSlope(to) == AITile.SLOPE_FLAT) return 0xFF;
	local diff = from - to;
	switch (diff) {
		case 1:
			return 1;
		case -1:
			return 2;
		case map_size_x:
			return 4;
		case -map_size_x:
			return 8;
		default:
			assert(false);
	}
}