Function IsValidTileForWaypoint

src/waypoint_cmd.cpp:152–184

* Check whether the given tile is suitable for a waypoint. * @param tile the tile to check for suitability * @param axis the axis of the waypoint * @param waypoint Waypoint the waypoint to check for is already joined to. If we find another waypoint it can join to it will throw an error. */

 * @param waypoint Waypoint the waypoint to check for is already joined to. If we find another waypoint it can join to it will throw an error.
 */
static CommandCost IsValidTileForWaypoint(TileIndex tile, Axis axis, StationID *waypoint)
{
	/* if waypoint is set, then we have special handling to allow building on top of already existing waypoints.
	 * so waypoint points to StationID::Invalid() if we can build on any waypoint.
	 * Or it points to a waypoint if we're only allowed to build on exactly that waypoint. */
	if (waypoint != nullptr && IsTileType(tile, MP_STATION)) {
		if (!IsRailWaypoint(tile)) {
			return ClearTile_Station(tile, DoCommandFlag::Auto); // get error message
		} else {
			StationID wp = GetStationIndex(tile);
			if (*waypoint == StationID::Invalid()) {
				*waypoint = wp;
			} else if (*waypoint != wp) {
				return CommandCost(STR_ERROR_WAYPOINT_ADJOINS_MORE_THAN_ONE_EXISTING);
			}
		}
	}

	if (GetAxisForNewRailWaypoint(tile) != axis) return CommandCost(STR_ERROR_NO_SUITABLE_RAILROAD_TRACK);

	Owner owner = GetTileOwner(tile);
	CommandCost ret = CheckOwnership(owner);
	if (ret.Succeeded()) ret = EnsureNoVehicleOnGround(tile);
	if (ret.Failed()) return ret;

	Slope tileh = GetTileSlope(tile);
	if (tileh != SLOPE_FLAT &&
			(!_settings_game.construction.build_on_slopes || IsSteepSlope(tileh) || !(tileh & (0x3 << axis)) || !(tileh & ~(0x3 << axis)))) {
		return CommandCost(STR_ERROR_FLAT_LAND_REQUIRED);
	}

	return CommandCost();
}

extern CommandCost FindJoiningWaypoint(StationID existing_station, StationID station_to_join, bool adjacent, TileArea ta, Waypoint **wp, bool is_road);
extern CommandCost CanExpandRailStation(const BaseStation *st, TileArea &new_ta);