/**
 * Build a single piece of rail
 * @param tile tile  to build on
 * @param flags operation to perform
 * @param p1 railtype of being built piece (normal, mono, maglev)
 * @param p2 rail track to build
 * @param text unused
 * @return the cost of this operation or an error
 */
CommandCost CmdBuildSingleRail(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32 p2, const char *text)
{
	RailType railtype = Extract<RailType, 0, 4>(p1);
	Track track = Extract<Track, 0, 3>(p2);
	CommandCost cost(EXPENSES_CONSTRUCTION);

	if (!ValParamRailtype(railtype) || !ValParamTrackOrientation(track)) return CMD_ERROR;

	Slope tileh = GetTileSlope(tile);
	TrackBits trackbit = TrackToTrackBits(track);

	switch (GetTileType(tile)) {
		case MP_RAILWAY: {
			CommandCost ret = CheckTileOwnership(tile);
			if (ret.Failed()) return ret;

			if (!IsPlainRail(tile)) return CMD_ERROR;

			if (!IsCompatibleRail(GetRailType(tile), railtype)) return_cmd_error(STR_ERROR_IMPOSSIBLE_TRACK_COMBINATION);

			ret = CheckTrackCombination(tile, trackbit, flags);
			if (ret.Succeeded()) ret = EnsureNoTrainOnTrack(tile, track);
			if (ret.Failed()) return ret;

			ret = CheckRailSlope(tileh, trackbit, GetTrackBits(tile), tile);
			if (ret.Failed()) return ret;
			cost.AddCost(ret);

			/* If the rail types don't match, try to convert only if engines of
			 * the new rail type are not powered on the present rail type and engines of
			 * the present rail type are powered on the new rail type. */
			if (GetRailType(tile) != railtype && !HasPowerOnRail(railtype, GetRailType(tile))) {
				if (HasPowerOnRail(GetRailType(tile), railtype)) {
					ret = DoCommand(tile, tile, railtype, flags, CMD_CONVERT_RAIL);
					if (ret.Failed()) return ret;
					cost.AddCost(ret);
				} else {
					return CMD_ERROR;
				}
			}

			if (flags & DC_EXEC) {
				SetRailGroundType(tile, RAIL_GROUND_BARREN);
				TrackBits bits = GetTrackBits(tile);
				SetTrackBits(tile, bits | trackbit);
				/* Subtract old infrastructure count. */
				uint pieces = CountBits(bits);
				if (TracksOverlap(bits)) pieces *= pieces;
				Company::Get(GetTileOwner(tile))->infrastructure.rail[GetRailType(tile)] -= pieces;
				/* Add new infrastructure count. */
				pieces = CountBits(bits | trackbit);
				if (TracksOverlap(bits | trackbit)) pieces *= pieces;
				Company::Get(GetTileOwner(tile))->infrastructure.rail[GetRailType(tile)] += pieces;
				DirtyCompanyInfrastructureWindows(GetTileOwner(tile));
			}
			break;
		}

		case MP_ROAD: {
			/* Level crossings may only be built on these slopes */
			if (!HasBit(VALID_LEVEL_CROSSING_SLOPES, tileh)) return_cmd_error(STR_ERROR_LAND_SLOPED_IN_WRONG_DIRECTION);

			CommandCost ret = EnsureNoVehicleOnGround(tile);
			if (ret.Failed()) return ret;

			if (IsNormalRoad(tile)) {
				if (HasRoadWorks(tile)) return_cmd_error(STR_ERROR_ROAD_WORKS_IN_PROGRESS);

				if (GetDisallowedRoadDirections(tile) != DRD_NONE) return_cmd_error(STR_ERROR_CROSSING_ON_ONEWAY_ROAD);

				if (RailNoLevelCrossings(railtype)) return_cmd_error(STR_ERROR_CROSSING_DISALLOWED);

				RoadTypes roadtypes = GetRoadTypes(tile);
				RoadBits road = GetRoadBits(tile, ROADTYPE_ROAD);
				RoadBits tram = GetRoadBits(tile, ROADTYPE_TRAM);
				if ((track == TRACK_X && ((road | tram) & ROAD_X) == 0) ||
						(track == TRACK_Y && ((road | tram) & ROAD_Y) == 0)) {
					Owner road_owner = GetRoadOwner(tile, ROADTYPE_ROAD);
					Owner tram_owner = GetRoadOwner(tile, ROADTYPE_TRAM);
					/* Disallow breaking end-of-line of someone else
					 * so trams can still reverse on this tile. */
					if (Company::IsValidID(tram_owner) && HasExactlyOneBit(tram)) {
						CommandCost ret = CheckOwnership(tram_owner);
						if (ret.Failed()) return ret;
					}
					/* Crossings must always have a road... */
					uint num_new_road_pieces = 2 - CountBits(road);
					if (road == ROAD_NONE) road_owner = _current_company;
					roadtypes |= ROADTYPES_ROAD;
					/* ...but tram is not required. */
					uint num_new_tram_pieces = (tram != ROAD_NONE) ? 2 - CountBits(tram) : 0;

					cost.AddCost((num_new_road_pieces + num_new_tram_pieces) * _price[PR_BUILD_ROAD]);

					if (flags & DC_EXEC) {
						MakeRoadCrossing(tile, road_owner, tram_owner, _current_company, (track == TRACK_X ? AXIS_Y : AXIS_X), railtype, roadtypes, GetTownIndex(tile));
						UpdateLevelCrossing(tile, false);
						Company::Get(_current_company)->infrastructure.rail[railtype] += LEVELCROSSING_TRACKBIT_FACTOR;
						DirtyCompanyInfrastructureWindows(_current_company);
						if (num_new_road_pieces > 0 && Company::IsValidID(road_owner)) {
							Company::Get(road_owner)->infrastructure.road[ROADTYPE_ROAD] += num_new_road_pieces;
							DirtyCompanyInfrastructureWindows(road_owner);
						}
						if (num_new_tram_pieces > 0 && Company::IsValidID(tram_owner)) {
							Company::Get(tram_owner)->infrastructure.road[ROADTYPE_TRAM] += num_new_tram_pieces;
							DirtyCompanyInfrastructureWindows(tram_owner);
						}
					}
					break;
				}
			}

			if (IsLevelCrossing(tile) && GetCrossingRailBits(tile) == trackbit) {
				return_cmd_error(STR_ERROR_ALREADY_BUILT);
			}
			/* FALL THROUGH */
		}

		default: {
			/* Will there be flat water on the lower halftile? */
			bool water_ground = IsTileType(tile, MP_WATER) && IsSlopeWithOneCornerRaised(tileh);

			CommandCost ret = CheckRailSlope(tileh, trackbit, TRACK_BIT_NONE, tile);
			if (ret.Failed()) return ret;
			cost.AddCost(ret);

			if (!water_ground) {
				ret = DoCommand(tile, 0, 0, flags, CMD_LANDSCAPE_CLEAR);
				if (ret.Failed()) return ret;
				cost.AddCost(ret);
			} else {
				cost.AddCost(_price[PR_CLEAR_ROUGH]);
			}

			if (flags & DC_EXEC) {
				MakeRailNormal(tile, _current_company, trackbit, railtype);
				if (water_ground) SetRailGroundType(tile, RAIL_GROUND_WATER);
				Company::Get(_current_company)->infrastructure.rail[railtype]++;
				DirtyCompanyInfrastructureWindows(_current_company);
			}
			break;
		}
	}

	if (flags & DC_EXEC) {
		MarkTileDirtyByTile(tile);
		AddTrackToSignalBuffer(tile, track, _current_company);
		YapfNotifyTrackLayoutChange(tile, track);
	}

	cost.AddCost(RailBuildCost(railtype));
	return cost;
}