Function ExtendTrainReservation

src/train_cmd.cpp:2513–2614

* Extend a train path as far as possible. Stops on encountering a safe tile, * another reservation or a track choice. * @return INVALID_TILE indicates that the reservation failed. */

 * @return INVALID_TILE indicates that the reservation failed.
 */
static PBSTileInfo ExtendTrainReservation(const Train *v, TrackBits *new_tracks, DiagDirection *enterdir)
{
	PBSTileInfo origin = FollowTrainReservation(v);

	CFollowTrackRail ft(v);

	std::vector<std::pair<TileIndex, Trackdir>> signals_set_to_red;

	TileIndex tile = origin.tile;
	Trackdir  cur_td = origin.trackdir;
	while (ft.Follow(tile, cur_td)) {
		if (KillFirstBit(ft.new_td_bits) == TRACKDIR_BIT_NONE) {
			/* Possible signal tile. */
			if (HasOnewaySignalBlockingTrackdir(ft.new_tile, FindFirstTrackdir(ft.new_td_bits))) break;
		}

		if (Rail90DegTurnDisallowed(GetTileRailType(ft.old_tile), GetTileRailType(ft.new_tile))) {
			ft.new_td_bits &= ~TrackdirCrossesTrackdirs(ft.old_td);
			if (ft.new_td_bits == TRACKDIR_BIT_NONE) break;
		}

		/* Station, depot or waypoint are a possible target. */
		bool target_seen = ft.is_station || (IsTileType(ft.new_tile, MP_RAILWAY) && !IsPlainRail(ft.new_tile));
		if (target_seen || KillFirstBit(ft.new_td_bits) != TRACKDIR_BIT_NONE) {
			/* Choice found or possible target encountered.
			 * On finding a possible target, we need to stop and let the pathfinder handle the
			 * remaining path. This is because we don't know if this target is in one of our
			 * orders, so we might cause pathfinding to fail later on if we find a choice.
			 * This failure would cause a bogous call to TryReserveSafePath which might reserve
			 * a wrong path not leading to our next destination. */
			if (HasReservedTracks(ft.new_tile, TrackdirBitsToTrackBits(TrackdirReachesTrackdirs(ft.old_td)))) break;

			/* If we did skip some tiles, backtrack to the first skipped tile so the pathfinder
			 * actually starts its search at the first unreserved tile. */
			if (ft.tiles_skipped != 0) ft.new_tile -= TileOffsByDiagDir(ft.exitdir) * ft.tiles_skipped;

			/* Choice found, path valid but not okay. Save info about the choice tile as well. */
			if (new_tracks != nullptr) *new_tracks = TrackdirBitsToTrackBits(ft.new_td_bits);
			if (enterdir != nullptr) *enterdir = ft.exitdir;
			return PBSTileInfo(ft.new_tile, ft.old_td, false);
		}

		tile = ft.new_tile;
		cur_td = FindFirstTrackdir(ft.new_td_bits);

		Trackdir rev_td = ReverseTrackdir(cur_td);
		if (IsSafeWaitingPosition(v, tile, cur_td, true, _settings_game.pf.forbid_90_deg)) {
			bool wp_free = IsWaitingPositionFree(v, tile, cur_td, _settings_game.pf.forbid_90_deg);
			if (!(wp_free && TryReserveRailTrack(tile, TrackdirToTrack(cur_td)))) break;
			/* Green path signal opposing the path? Turn to red. */
			if (HasPbsSignalOnTrackdir(tile, rev_td) && GetSignalStateByTrackdir(tile, rev_td) == SIGNAL_STATE_GREEN) {
				signals_set_to_red.emplace_back(tile, rev_td);
				SetSignalStateByTrackdir(tile, rev_td, SIGNAL_STATE_RED);
				MarkTileDirtyByTile(tile);
			}
			/* Safe position is all good, path valid and okay. */
			return PBSTileInfo(tile, cur_td, true);
		}