Function CheckTrackCombination

src/rail_cmd.cpp:238–255

* Check that the new track bits may be built. * @param tile %Tile to build on. * @param to_build New track bits. * @return Succeeded or failed command. */

 * @return Succeeded or failed command.
 */
static CommandCost CheckTrackCombination(TileIndex tile, TrackBits to_build)
{
	if (!IsPlainRail(tile)) return CommandCost(STR_ERROR_IMPOSSIBLE_TRACK_COMBINATION);

	/* So, we have a tile with tracks on it (and possibly signals). Let's see
	 * what tracks first */
	TrackBits current = GetTrackBits(tile); // The current track layout.
	TrackBits future = current | to_build;  // The track layout we want to build.

	/* Are we really building something new? */
	if (current == future) {
		/* Nothing new is being built */
		return CommandCost(STR_ERROR_ALREADY_BUILT);
	}

	/* Normally, we may overlap and any combination is valid */
	return CommandCost();
}


/** Valid TrackBits on a specific (non-steep)-slope without foundation */