/**
 * Get a list of all bridges and tunnels that can be built from the
 * current tile. Tunnels will only be built if no terraforming
 * is needed on both ends.
 */
function Road::_GetTunnelsBridges(last_node, cur_node, bridge_dir)
{
	local tiles = [];

	for (local i = 2; i < this._max_bridge_length; i++) {
		local bridge_list = AIBridgeList_Length(i + 1);
		local target = cur_node + i * (cur_node - last_node);
		if (!bridge_list.IsEmpty() && AIBridge.BuildBridge(AIVehicle.VT_ROAD, bridge_list.Begin(), cur_node, target)) {
			tiles.push([target, bridge_dir]);
		}
	}

	local slope = AITile.GetSlope(cur_node);
	if (slope != AITile.SLOPE_SW && slope != AITile.SLOPE_NW && slope != AITile.SLOPE_SE && slope != AITile.SLOPE_NE) return tiles;
	local other_tunnel_end = AITunnel.GetOtherTunnelEnd(cur_node);
	if (!AIMap.IsValidTile(other_tunnel_end)) return tiles;

	local tunnel_length = AIMap.DistanceManhattan(cur_node, other_tunnel_end);
	local prev_tile = cur_node + (cur_node - other_tunnel_end) / tunnel_length;
	if (AITunnel.GetOtherTunnelEnd(other_tunnel_end) == cur_node && tunnel_length >= 2 &&
			prev_tile == last_node && tunnel_length < _max_tunnel_length && AITunnel.BuildTunnel(AIVehicle.VT_ROAD, cur_node)) {
		tiles.push([other_tunnel_end, bridge_dir]);
	}
	return tiles;
}