Fix: Don't try to rename OWNER_DEITY signs in-game (#9716)

[openttd-github.git] / src / train_cmd.cpp

/*
 * This file is part of OpenTTD.
 * OpenTTD is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, version 2.
 * OpenTTD is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 * See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with OpenTTD. If not, see <http://www.gnu.org/licenses/>.
 */

/** @file train_cmd.cpp Handling of trains. */

#include "stdafx.h"
#include "error.h"
#include "articulated_vehicles.h"
#include "command_func.h"
#include "pathfinder/npf/npf_func.h"
#include "pathfinder/yapf/yapf.hpp"
#include "news_func.h"
#include "company_func.h"
#include "newgrf_sound.h"
#include "newgrf_text.h"
#include "strings_func.h"
#include "viewport_func.h"
#include "vehicle_func.h"
#include "sound_func.h"
#include "ai/ai.hpp"
#include "game/game.hpp"
#include "newgrf_station.h"
#include "effectvehicle_func.h"
#include "network/network.h"
#include "spritecache.h"
#include "core/random_func.hpp"
#include "company_base.h"
#include "newgrf.h"
#include "order_backup.h"
#include "zoom_func.h"
#include "newgrf_debug.h"
#include "framerate_type.h"

#include "table/strings.h"
#include "table/train_cmd.h"

#include "safeguards.h"

static Track ChooseTrainTrack(Train *v, TileIndex tile, DiagDirection enterdir, TrackBits tracks, bool force_res, bool *got_reservation, bool mark_stuck);
static bool TrainCheckIfLineEnds(Train *v, bool reverse = true);
bool TrainController(Train *v, Vehicle *nomove, bool reverse = true); // Also used in vehicle_sl.cpp.
static TileIndex TrainApproachingCrossingTile(const Train *v);
static void CheckIfTrainNeedsService(Train *v);
static void CheckNextTrainTile(Train *v);

static const byte _vehicle_initial_x_fract[4] = {10, 8, 4,  8};
static const byte _vehicle_initial_y_fract[4] = { 8, 4, 8, 10};

template <>
bool IsValidImageIndex<VEH_TRAIN>(uint8 image_index)
{
        return image_index < lengthof(_engine_sprite_base);
}


/**
 * Return the cargo weight multiplier to use for a rail vehicle
 * @param cargo Cargo type to get multiplier for
 * @return Cargo weight multiplier
 */
byte FreightWagonMult(CargoID cargo)
{
        if (!CargoSpec::Get(cargo)->is_freight) return 1;
        return _settings_game.vehicle.freight_trains;
}

/** Checks if lengths of all rail vehicles are valid. If not, shows an error message. */
void CheckTrainsLengths()
{
        bool first = true;

        for (const Train *v : Train::Iterate()) {
                if (v->First() == v && !(v->vehstatus & VS_CRASHED)) {
                        for (const Train *u = v, *w = v->Next(); w != nullptr; u = w, w = w->Next()) {
                                if (u->track != TRACK_BIT_DEPOT) {
                                        if ((w->track != TRACK_BIT_DEPOT &&
                                                        std::max(abs(u->x_pos - w->x_pos), abs(u->y_pos - w->y_pos)) != u->CalcNextVehicleOffset()) ||
                                                        (w->track == TRACK_BIT_DEPOT && TicksToLeaveDepot(u) <= 0)) {
                                                SetDParam(0, v->index);
                                                SetDParam(1, v->owner);
                                                ShowErrorMessage(STR_BROKEN_VEHICLE_LENGTH, INVALID_STRING_ID, WL_CRITICAL);

                                                if (!_networking && first) {
                                                        first = false;
                                                        DoCommandP(0, PM_PAUSED_ERROR, 1, CMD_PAUSE);
                                                }
                                                /* Break so we warn only once for each train. */
                                                break;
                                        }
                                }
                        }
                }
        }
}

/**
 * Recalculates the cached stuff of a train. Should be called each time a vehicle is added
 * to/removed from the chain, and when the game is loaded.
 * Note: this needs to be called too for 'wagon chains' (in the depot, without an engine)
 * @param allowed_changes Stuff that is allowed to change.
 */
void Train::ConsistChanged(ConsistChangeFlags allowed_changes)
{
        uint16 max_speed = UINT16_MAX;

        assert(this->IsFrontEngine() || this->IsFreeWagon());

        const RailVehicleInfo *rvi_v = RailVehInfo(this->engine_type);
        EngineID first_engine = this->IsFrontEngine() ? this->engine_type : INVALID_ENGINE;
        this->gcache.cached_total_length = 0;
        this->compatible_railtypes = RAILTYPES_NONE;

        bool train_can_tilt = true;
        int min_curve_speed_mod = INT_MAX;

        for (Train *u = this; u != nullptr; u = u->Next()) {
                const RailVehicleInfo *rvi_u = RailVehInfo(u->engine_type);

                /* Check the this->first cache. */
                assert(u->First() == this);

                /* update the 'first engine' */
                u->gcache.first_engine = this == u ? INVALID_ENGINE : first_engine;
                u->railtype = rvi_u->railtype;

                if (u->IsEngine()) first_engine = u->engine_type;

                /* Set user defined data to its default value */
                u->tcache.user_def_data = rvi_u->user_def_data;
                this->InvalidateNewGRFCache();
                u->InvalidateNewGRFCache();
        }

        for (Train *u = this; u != nullptr; u = u->Next()) {
                /* Update user defined data (must be done before other properties) */
                u->tcache.user_def_data = GetVehicleProperty(u, PROP_TRAIN_USER_DATA, u->tcache.user_def_data);
                this->InvalidateNewGRFCache();
                u->InvalidateNewGRFCache();
        }

        for (Train *u = this; u != nullptr; u = u->Next()) {
                const Engine *e_u = u->GetEngine();
                const RailVehicleInfo *rvi_u = &e_u->u.rail;

                if (!HasBit(e_u->info.misc_flags, EF_RAIL_TILTS)) train_can_tilt = false;
                min_curve_speed_mod = std::min(min_curve_speed_mod, u->GetCurveSpeedModifier());

                /* Cache wagon override sprite group. nullptr is returned if there is none */
                u->tcache.cached_override = GetWagonOverrideSpriteSet(u->engine_type, u->cargo_type, u->gcache.first_engine);

                /* Reset colour map */
                u->colourmap = PAL_NONE;

                /* Update powered-wagon-status and visual effect */
                u->UpdateVisualEffect(true);

                if (rvi_v->pow_wag_power != 0 && rvi_u->railveh_type == RAILVEH_WAGON &&
                                UsesWagonOverride(u) && !HasBit(u->vcache.cached_vis_effect, VE_DISABLE_WAGON_POWER)) {
                        /* wagon is powered */
                        SetBit(u->flags, VRF_POWEREDWAGON); // cache 'powered' status
                } else {
                        ClrBit(u->flags, VRF_POWEREDWAGON);
                }

                if (!u->IsArticulatedPart()) {
                        /* Do not count powered wagons for the compatible railtypes, as wagons always
                           have railtype normal */
                        if (rvi_u->power > 0) {
                                this->compatible_railtypes |= GetRailTypeInfo(u->railtype)->powered_railtypes;
                        }

                        /* Some electric engines can be allowed to run on normal rail. It happens to all
                         * existing electric engines when elrails are disabled and then re-enabled */
                        if (HasBit(u->flags, VRF_EL_ENGINE_ALLOWED_NORMAL_RAIL)) {
                                u->railtype = RAILTYPE_RAIL;
                                u->compatible_railtypes |= RAILTYPES_RAIL;
                        }

                        /* max speed is the minimum of the speed limits of all vehicles in the consist */
                        if ((rvi_u->railveh_type != RAILVEH_WAGON || _settings_game.vehicle.wagon_speed_limits) && !UsesWagonOverride(u)) {
                                uint16 speed = GetVehicleProperty(u, PROP_TRAIN_SPEED, rvi_u->max_speed);
                                if (speed != 0) max_speed = std::min(speed, max_speed);
                        }
                }

                uint16 new_cap = e_u->DetermineCapacity(u);
                if (allowed_changes & CCF_CAPACITY) {
                        /* Update vehicle capacity. */
                        if (u->cargo_cap > new_cap) u->cargo.Truncate(new_cap);
                        u->refit_cap = std::min(new_cap, u->refit_cap);
                        u->cargo_cap = new_cap;
                } else {
                        /* Verify capacity hasn't changed. */
                        if (new_cap != u->cargo_cap) ShowNewGrfVehicleError(u->engine_type, STR_NEWGRF_BROKEN, STR_NEWGRF_BROKEN_CAPACITY, GBUG_VEH_CAPACITY, true);
                }
                u->vcache.cached_cargo_age_period = GetVehicleProperty(u, PROP_TRAIN_CARGO_AGE_PERIOD, e_u->info.cargo_age_period);

                /* check the vehicle length (callback) */
                uint16 veh_len = CALLBACK_FAILED;
                if (e_u->GetGRF() != nullptr && e_u->GetGRF()->grf_version >= 8) {
                        /* Use callback 36 */
                        veh_len = GetVehicleProperty(u, PROP_TRAIN_SHORTEN_FACTOR, CALLBACK_FAILED);

                        if (veh_len != CALLBACK_FAILED && veh_len >= VEHICLE_LENGTH) {
                                ErrorUnknownCallbackResult(e_u->GetGRFID(), CBID_VEHICLE_LENGTH, veh_len);
                        }
                } else if (HasBit(e_u->info.callback_mask, CBM_VEHICLE_LENGTH)) {
                        /* Use callback 11 */
                        veh_len = GetVehicleCallback(CBID_VEHICLE_LENGTH, 0, 0, u->engine_type, u);
                }
                if (veh_len == CALLBACK_FAILED) veh_len = rvi_u->shorten_factor;
                veh_len = VEHICLE_LENGTH - Clamp(veh_len, 0, VEHICLE_LENGTH - 1);

                if (allowed_changes & CCF_LENGTH) {
                        /* Update vehicle length. */
                        u->gcache.cached_veh_length = veh_len;
                } else {
                        /* Verify length hasn't changed. */
                        if (veh_len != u->gcache.cached_veh_length) VehicleLengthChanged(u);
                }

                this->gcache.cached_total_length += u->gcache.cached_veh_length;
                this->InvalidateNewGRFCache();
                u->InvalidateNewGRFCache();
        }

        /* store consist weight/max speed in cache */
        this->vcache.cached_max_speed = max_speed;
        this->tcache.cached_tilt = train_can_tilt;
        this->tcache.cached_curve_speed_mod = min_curve_speed_mod;
        this->tcache.cached_max_curve_speed = this->GetCurveSpeedLimit();

        /* recalculate cached weights and power too (we do this *after* the rest, so it is known which wagons are powered and need extra weight added) */
        this->CargoChanged();

        if (this->IsFrontEngine()) {
                this->UpdateAcceleration();
                SetWindowDirty(WC_VEHICLE_DETAILS, this->index);
                InvalidateWindowData(WC_VEHICLE_REFIT, this->index, VIWD_CONSIST_CHANGED);
                InvalidateWindowData(WC_VEHICLE_ORDERS, this->index, VIWD_CONSIST_CHANGED);
                InvalidateNewGRFInspectWindow(GSF_TRAINS, this->index);
        }
}

/**
 * Get the stop location of (the center) of the front vehicle of a train at
 * a platform of a station.
 * @param station_id     the ID of the station where we're stopping
 * @param tile           the tile where the vehicle currently is
 * @param v              the vehicle to get the stop location of
 * @param station_ahead  'return' the amount of 1/16th tiles in front of the train
 * @param station_length 'return' the station length in 1/16th tiles
 * @return the location, calculated from the begin of the station to stop at.
 */
int GetTrainStopLocation(StationID station_id, TileIndex tile, const Train *v, int *station_ahead, int *station_length)
{
        const Station *st = Station::Get(station_id);
        *station_ahead  = st->GetPlatformLength(tile, DirToDiagDir(v->direction)) * TILE_SIZE;
        *station_length = st->GetPlatformLength(tile) * TILE_SIZE;

        /* Default to the middle of the station for stations stops that are not in
         * the order list like intermediate stations when non-stop is disabled */
        OrderStopLocation osl = OSL_PLATFORM_MIDDLE;
        if (v->gcache.cached_total_length >= *station_length) {
                /* The train is longer than the station, make it stop at the far end of the platform */
                osl = OSL_PLATFORM_FAR_END;
        } else if (v->current_order.IsType(OT_GOTO_STATION) && v->current_order.GetDestination() == station_id) {
                osl = v->current_order.GetStopLocation();
        }

        /* The stop location of the FRONT! of the train */
        int stop;
        switch (osl) {
                default: NOT_REACHED();

                case OSL_PLATFORM_NEAR_END:
                        stop = v->gcache.cached_total_length;
                        break;

                case OSL_PLATFORM_MIDDLE:
                        stop = *station_length - (*station_length - v->gcache.cached_total_length) / 2;
                        break;

                case OSL_PLATFORM_FAR_END:
                        stop = *station_length;
                        break;
        }

        /* Subtract half the front vehicle length of the train so we get the real
         * stop location of the train. */
        return stop - (v->gcache.cached_veh_length + 1) / 2;
}


/**
 * Computes train speed limit caused by curves
 * @return imposed speed limit
 */
int Train::GetCurveSpeedLimit() const
{
        assert(this->First() == this);

        static const int absolute_max_speed = UINT16_MAX;
        int max_speed = absolute_max_speed;

        if (_settings_game.vehicle.train_acceleration_model == AM_ORIGINAL) return max_speed;

        int curvecount[2] = {0, 0};

        /* first find the curve speed limit */
        int numcurve = 0;
        int sum = 0;
        int pos = 0;
        int lastpos = -1;
        for (const Vehicle *u = this; u->Next() != nullptr; u = u->Next(), pos++) {
                Direction this_dir = u->direction;
                Direction next_dir = u->Next()->direction;

                DirDiff dirdiff = DirDifference(this_dir, next_dir);
                if (dirdiff == DIRDIFF_SAME) continue;

                if (dirdiff == DIRDIFF_45LEFT) curvecount[0]++;
                if (dirdiff == DIRDIFF_45RIGHT) curvecount[1]++;
                if (dirdiff == DIRDIFF_45LEFT || dirdiff == DIRDIFF_45RIGHT) {
                        if (lastpos != -1) {
                                numcurve++;
                                sum += pos - lastpos;
                                if (pos - lastpos == 1 && max_speed > 88) {
                                        max_speed = 88;
                                }
                        }
                        lastpos = pos;
                }

                /* if we have a 90 degree turn, fix the speed limit to 60 */
                if (dirdiff == DIRDIFF_90LEFT || dirdiff == DIRDIFF_90RIGHT) {
                        max_speed = 61;
                }
        }

        if (numcurve > 0 && max_speed > 88) {
                if (curvecount[0] == 1 && curvecount[1] == 1) {
                        max_speed = absolute_max_speed;
                } else {
                        sum /= numcurve;
                        max_speed = 232 - (13 - Clamp(sum, 1, 12)) * (13 - Clamp(sum, 1, 12));
                }
        }

        if (max_speed != absolute_max_speed) {
                /* Apply the current railtype's curve speed advantage */
                const RailtypeInfo *rti = GetRailTypeInfo(GetRailType(this->tile));
                max_speed += (max_speed / 2) * rti->curve_speed;

                if (this->tcache.cached_tilt) {
                        /* Apply max_speed bonus of 20% for a tilting train */
                        max_speed += max_speed / 5;
                }

                /* Apply max_speed modifier (cached value is fixed-point binary with 8 fractional bits)
                 * and clamp the result to an acceptable range. */
                max_speed += (max_speed * this->tcache.cached_curve_speed_mod) / 256;
                max_speed = Clamp(max_speed, 2, absolute_max_speed);
        }

        return max_speed;
}

/**
 * Calculates the maximum speed of the vehicle under its current conditions.
 * @return Maximum speed of the vehicle.
 */
int Train::GetCurrentMaxSpeed() const
{
        int max_speed = _settings_game.vehicle.train_acceleration_model == AM_ORIGINAL ?
                        this->gcache.cached_max_track_speed :
                        this->tcache.cached_max_curve_speed;

        if (_settings_game.vehicle.train_acceleration_model == AM_REALISTIC && IsRailStationTile(this->tile)) {
                StationID sid = GetStationIndex(this->tile);
                if (this->current_order.ShouldStopAtStation(this, sid)) {
                        int station_ahead;
                        int station_length;
                        int stop_at = GetTrainStopLocation(sid, this->tile, this, &station_ahead, &station_length);

                        /* The distance to go is whatever is still ahead of the train minus the
                         * distance from the train's stop location to the end of the platform */
                        int distance_to_go = station_ahead / TILE_SIZE - (station_length - stop_at) / TILE_SIZE;

                        if (distance_to_go > 0) {
                                int st_max_speed = 120;

                                int delta_v = this->cur_speed / (distance_to_go + 1);
                                if (max_speed > (this->cur_speed - delta_v)) {
                                        st_max_speed = this->cur_speed - (delta_v / 10);
                                }

                                st_max_speed = std::max(st_max_speed, 25 * distance_to_go);
                                max_speed = std::min(max_speed, st_max_speed);
                        }
                }
        }

        for (const Train *u = this; u != nullptr; u = u->Next()) {
                if (_settings_game.vehicle.train_acceleration_model == AM_REALISTIC && u->track == TRACK_BIT_DEPOT) {
                        max_speed = std::min(max_speed, 61);
                        break;
                }

                /* Vehicle is on the middle part of a bridge. */
                if (u->track == TRACK_BIT_WORMHOLE && !(u->vehstatus & VS_HIDDEN)) {
                        max_speed = std::min<int>(max_speed, GetBridgeSpec(GetBridgeType(u->tile))->speed);
                }
        }

        max_speed = std::min<int>(max_speed, this->current_order.GetMaxSpeed());
        return std::min<int>(max_speed, this->gcache.cached_max_track_speed);
}

/** Update acceleration of the train from the cached power and weight. */
void Train::UpdateAcceleration()
{
        assert(this->IsFrontEngine() || this->IsFreeWagon());

        uint power = this->gcache.cached_power;
        uint weight = this->gcache.cached_weight;
        assert(weight != 0);
        this->acceleration = Clamp(power / weight * 4, 1, 255);
}

/**
 * Get the width of a train vehicle image in the GUI.
 * @param offset Additional offset for positioning the sprite; set to nullptr if not needed
 * @return Width in pixels
 */
int Train::GetDisplayImageWidth(Point *offset) const
{
        int reference_width = TRAININFO_DEFAULT_VEHICLE_WIDTH;
        int vehicle_pitch = 0;

        const Engine *e = this->GetEngine();
        if (e->GetGRF() != nullptr && is_custom_sprite(e->u.rail.image_index)) {
                reference_width = e->GetGRF()->traininfo_vehicle_width;
                vehicle_pitch = e->GetGRF()->traininfo_vehicle_pitch;
        }

        if (offset != nullptr) {
                offset->x = ScaleGUITrad(reference_width) / 2;
                offset->y = ScaleGUITrad(vehicle_pitch);
        }
        return ScaleGUITrad(this->gcache.cached_veh_length * reference_width / VEHICLE_LENGTH);
}

static SpriteID GetDefaultTrainSprite(uint8 spritenum, Direction direction)
{
        assert(IsValidImageIndex<VEH_TRAIN>(spritenum));
        return ((direction + _engine_sprite_add[spritenum]) & _engine_sprite_and[spritenum]) + _engine_sprite_base[spritenum];
}

/**
 * Get the sprite to display the train.
 * @param direction Direction of view/travel.
 * @param image_type Visualisation context.
 * @return Sprite to display.
 */
void Train::GetImage(Direction direction, EngineImageType image_type, VehicleSpriteSeq *result) const
{
        uint8 spritenum = this->spritenum;

        if (HasBit(this->flags, VRF_REVERSE_DIRECTION)) direction = ReverseDir(direction);

        if (is_custom_sprite(spritenum)) {
                GetCustomVehicleSprite(this, (Direction)(direction + 4 * IS_CUSTOM_SECONDHEAD_SPRITE(spritenum)), image_type, result);
                if (result->IsValid()) return;

                spritenum = this->GetEngine()->original_image_index;
        }

        assert(IsValidImageIndex<VEH_TRAIN>(spritenum));
        SpriteID sprite = GetDefaultTrainSprite(spritenum, direction);

        if (this->cargo.StoredCount() >= this->cargo_cap / 2U) sprite += _wagon_full_adder[spritenum];

        result->Set(sprite);
}

static void GetRailIcon(EngineID engine, bool rear_head, int &y, EngineImageType image_type, VehicleSpriteSeq *result)
{
        const Engine *e = Engine::Get(engine);
        Direction dir = rear_head ? DIR_E : DIR_W;
        uint8 spritenum = e->u.rail.image_index;

        if (is_custom_sprite(spritenum)) {
                GetCustomVehicleIcon(engine, dir, image_type, result);
                if (result->IsValid()) {
                        if (e->GetGRF() != nullptr) {
                                y += ScaleGUITrad(e->GetGRF()->traininfo_vehicle_pitch);
                        }
                        return;
                }

                spritenum = Engine::Get(engine)->original_image_index;
        }

        if (rear_head) spritenum++;

        result->Set(GetDefaultTrainSprite(spritenum, DIR_W));
}

void DrawTrainEngine(int left, int right, int preferred_x, int y, EngineID engine, PaletteID pal, EngineImageType image_type)
{
        if (RailVehInfo(engine)->railveh_type == RAILVEH_MULTIHEAD) {
                int yf = y;
                int yr = y;

                VehicleSpriteSeq seqf, seqr;
                GetRailIcon(engine, false, yf, image_type, &seqf);
                GetRailIcon(engine, true, yr, image_type, &seqr);

                Rect rectf, rectr;
                seqf.GetBounds(&rectf);
                seqr.GetBounds(&rectr);

                preferred_x = Clamp(preferred_x,
                                left - UnScaleGUI(rectf.left) + ScaleGUITrad(14),
                                right - UnScaleGUI(rectr.right) - ScaleGUITrad(15));

                seqf.Draw(preferred_x - ScaleGUITrad(14), yf, pal, pal == PALETTE_CRASH);
                seqr.Draw(preferred_x + ScaleGUITrad(15), yr, pal, pal == PALETTE_CRASH);
        } else {
                VehicleSpriteSeq seq;
                GetRailIcon(engine, false, y, image_type, &seq);

                Rect rect;
                seq.GetBounds(&rect);
                preferred_x = Clamp(preferred_x,
                                left - UnScaleGUI(rect.left),
                                right - UnScaleGUI(rect.right));

                seq.Draw(preferred_x, y, pal, pal == PALETTE_CRASH);
        }
}

/**
 * Get the size of the sprite of a train sprite heading west, or both heads (used for lists).
 * @param engine The engine to get the sprite from.
 * @param[out] width The width of the sprite.
 * @param[out] height The height of the sprite.
 * @param[out] xoffs Number of pixels to shift the sprite to the right.
 * @param[out] yoffs Number of pixels to shift the sprite downwards.
 * @param image_type Context the sprite is used in.
 */
void GetTrainSpriteSize(EngineID engine, uint &width, uint &height, int &xoffs, int &yoffs, EngineImageType image_type)
{
        int y = 0;

        VehicleSpriteSeq seq;
        GetRailIcon(engine, false, y, image_type, &seq);

        Rect rect;
        seq.GetBounds(&rect);

        width  = UnScaleGUI(rect.right - rect.left + 1);
        height = UnScaleGUI(rect.bottom - rect.top + 1);
        xoffs  = UnScaleGUI(rect.left);
        yoffs  = UnScaleGUI(rect.top);

        if (RailVehInfo(engine)->railveh_type == RAILVEH_MULTIHEAD) {
                GetRailIcon(engine, true, y, image_type, &seq);
                seq.GetBounds(&rect);

                /* Calculate values relative to an imaginary center between the two sprites. */
                width = ScaleGUITrad(TRAININFO_DEFAULT_VEHICLE_WIDTH) + UnScaleGUI(rect.right) - xoffs;
                height = std::max<uint>(height, UnScaleGUI(rect.bottom - rect.top + 1));
                xoffs  = xoffs - ScaleGUITrad(TRAININFO_DEFAULT_VEHICLE_WIDTH) / 2;
                yoffs  = std::min(yoffs, UnScaleGUI(rect.top));
        }
}

/**
 * Build a railroad wagon.
 * @param tile     tile of the depot where rail-vehicle is built.
 * @param flags    type of operation.
 * @param e        the engine to build.
 * @param[out] ret the vehicle that has been built.
 * @return the cost of this operation or an error.
 */
static CommandCost CmdBuildRailWagon(TileIndex tile, DoCommandFlag flags, const Engine *e, Vehicle **ret)
{
        const RailVehicleInfo *rvi = &e->u.rail;

        /* Check that the wagon can drive on the track in question */
        if (!IsCompatibleRail(rvi->railtype, GetRailType(tile))) return CMD_ERROR;

        if (flags & DC_EXEC) {
                Train *v = new Train();
                *ret = v;
                v->spritenum = rvi->image_index;

                v->engine_type = e->index;
                v->gcache.first_engine = INVALID_ENGINE; // needs to be set before first callback

                DiagDirection dir = GetRailDepotDirection(tile);

                v->direction = DiagDirToDir(dir);
                v->tile = tile;

                int x = TileX(tile) * TILE_SIZE | _vehicle_initial_x_fract[dir];
                int y = TileY(tile) * TILE_SIZE | _vehicle_initial_y_fract[dir];

                v->x_pos = x;
                v->y_pos = y;
                v->z_pos = GetSlopePixelZ(x, y);
                v->owner = _current_company;
                v->track = TRACK_BIT_DEPOT;
                v->vehstatus = VS_HIDDEN | VS_DEFPAL;

                v->SetWagon();

                v->SetFreeWagon();
                InvalidateWindowData(WC_VEHICLE_DEPOT, v->tile);

                v->cargo_type = e->GetDefaultCargoType();
                v->cargo_cap = rvi->capacity;
                v->refit_cap = 0;

                v->railtype = rvi->railtype;

                v->date_of_last_service = _date;
                v->build_year = _cur_year;
                v->sprite_cache.sprite_seq.Set(SPR_IMG_QUERY);
                v->random_bits = VehicleRandomBits();

                v->group_id = DEFAULT_GROUP;

                AddArticulatedParts(v);

                _new_vehicle_id = v->index;

                v->UpdatePosition();
                v->First()->ConsistChanged(CCF_ARRANGE);
                UpdateTrainGroupID(v->First());

                CheckConsistencyOfArticulatedVehicle(v);

                /* Try to connect the vehicle to one of free chains of wagons. */
                for (Train *w : Train::Iterate()) {
                        if (w->tile == tile &&              ///< Same depot
                                        w->IsFreeWagon() &&             ///< A free wagon chain
                                        w->engine_type == e->index &&   ///< Same type
                                        w->First() != v &&              ///< Don't connect to ourself
                                        !(w->vehstatus & VS_CRASHED)) { ///< Not crashed/flooded
                                if (DoCommand(0, v->index | 1 << 20, w->Last()->index, DC_EXEC, CMD_MOVE_RAIL_VEHICLE).Succeeded()) {
                                        break;
                                }
                        }
                }
        }

        return CommandCost();
}

/** Move all free vehicles in the depot to the train */
static void NormalizeTrainVehInDepot(const Train *u)
{
        for (const Train *v : Train::Iterate()) {
                if (v->IsFreeWagon() && v->tile == u->tile &&
                                v->track == TRACK_BIT_DEPOT) {
                        if (DoCommand(0, v->index | 1 << 20, u->index, DC_EXEC,
                                        CMD_MOVE_RAIL_VEHICLE).Failed())
                                break;
                }
        }
}

static void AddRearEngineToMultiheadedTrain(Train *v)
{
        Train *u = new Train();
        v->value >>= 1;
        u->value = v->value;
        u->direction = v->direction;
        u->owner = v->owner;
        u->tile = v->tile;
        u->x_pos = v->x_pos;
        u->y_pos = v->y_pos;
        u->z_pos = v->z_pos;
        u->track = TRACK_BIT_DEPOT;
        u->vehstatus = v->vehstatus & ~VS_STOPPED;
        u->spritenum = v->spritenum + 1;
        u->cargo_type = v->cargo_type;
        u->cargo_subtype = v->cargo_subtype;
        u->cargo_cap = v->cargo_cap;
        u->refit_cap = v->refit_cap;
        u->railtype = v->railtype;
        u->engine_type = v->engine_type;
        u->date_of_last_service = v->date_of_last_service;
        u->build_year = v->build_year;
        u->sprite_cache.sprite_seq.Set(SPR_IMG_QUERY);
        u->random_bits = VehicleRandomBits();
        v->SetMultiheaded();
        u->SetMultiheaded();
        v->SetNext(u);
        u->UpdatePosition();

        /* Now we need to link the front and rear engines together */
        v->other_multiheaded_part = u;
        u->other_multiheaded_part = v;
}

/**
 * Build a railroad vehicle.
 * @param tile     tile of the depot where rail-vehicle is built.
 * @param flags    type of operation.
 * @param e        the engine to build.
 * @param data     bit 0 prevents any free cars from being added to the train.
 * @param[out] ret the vehicle that has been built.
 * @return the cost of this operation or an error.
 */
CommandCost CmdBuildRailVehicle(TileIndex tile, DoCommandFlag flags, const Engine *e, uint16 data, Vehicle **ret)
{
        const RailVehicleInfo *rvi = &e->u.rail;

        if (rvi->railveh_type == RAILVEH_WAGON) return CmdBuildRailWagon(tile, flags, e, ret);

        /* Check if depot and new engine uses the same kind of tracks *
         * We need to see if the engine got power on the tile to avoid electric engines in non-electric depots */
        if (!HasPowerOnRail(rvi->railtype, GetRailType(tile))) return CMD_ERROR;

        if (flags & DC_EXEC) {
                DiagDirection dir = GetRailDepotDirection(tile);
                int x = TileX(tile) * TILE_SIZE + _vehicle_initial_x_fract[dir];
                int y = TileY(tile) * TILE_SIZE + _vehicle_initial_y_fract[dir];

                Train *v = new Train();
                *ret = v;
                v->direction = DiagDirToDir(dir);
                v->tile = tile;
                v->owner = _current_company;
                v->x_pos = x;
                v->y_pos = y;
                v->z_pos = GetSlopePixelZ(x, y);
                v->track = TRACK_BIT_DEPOT;
                v->vehstatus = VS_HIDDEN | VS_STOPPED | VS_DEFPAL;
                v->spritenum = rvi->image_index;
                v->cargo_type = e->GetDefaultCargoType();
                v->cargo_cap = rvi->capacity;
                v->refit_cap = 0;
                v->last_station_visited = INVALID_STATION;
                v->last_loading_station = INVALID_STATION;

                v->engine_type = e->index;
                v->gcache.first_engine = INVALID_ENGINE; // needs to be set before first callback

                v->reliability = e->reliability;
                v->reliability_spd_dec = e->reliability_spd_dec;
                v->max_age = e->GetLifeLengthInDays();

                v->railtype = rvi->railtype;
                _new_vehicle_id = v->index;

                v->SetServiceInterval(Company::Get(_current_company)->settings.vehicle.servint_trains);
                v->date_of_last_service = _date;
                v->build_year = _cur_year;
                v->sprite_cache.sprite_seq.Set(SPR_IMG_QUERY);
                v->random_bits = VehicleRandomBits();

                if (e->flags & ENGINE_EXCLUSIVE_PREVIEW) SetBit(v->vehicle_flags, VF_BUILT_AS_PROTOTYPE);
                v->SetServiceIntervalIsPercent(Company::Get(_current_company)->settings.vehicle.servint_ispercent);

                v->group_id = DEFAULT_GROUP;

                v->SetFrontEngine();
                v->SetEngine();

                v->UpdatePosition();

                if (rvi->railveh_type == RAILVEH_MULTIHEAD) {
                        AddRearEngineToMultiheadedTrain(v);
                } else {
                        AddArticulatedParts(v);
                }

                v->ConsistChanged(CCF_ARRANGE);
                UpdateTrainGroupID(v);

                if (!HasBit(data, 0) && !(flags & DC_AUTOREPLACE)) { // check if the cars should be added to the new vehicle
                        NormalizeTrainVehInDepot(v);
                }

                CheckConsistencyOfArticulatedVehicle(v);
        }

        return CommandCost();
}

static Train *FindGoodVehiclePos(const Train *src)
{
        EngineID eng = src->engine_type;
        TileIndex tile = src->tile;

        for (Train *dst : Train::Iterate()) {
                if (dst->IsFreeWagon() && dst->tile == tile && !(dst->vehstatus & VS_CRASHED)) {
                        /* check so all vehicles in the line have the same engine. */
                        Train *t = dst;
                        while (t->engine_type == eng) {
                                t = t->Next();
                                if (t == nullptr) return dst;
                        }
                }
        }

        return nullptr;
}

/** Helper type for lists/vectors of trains */
typedef std::vector<Train *> TrainList;

/**
 * Make a backup of a train into a train list.
 * @param list to make the backup in
 * @param t    the train to make the backup of
 */
static void MakeTrainBackup(TrainList &list, Train *t)
{
        for (; t != nullptr; t = t->Next()) list.push_back(t);
}

/**
 * Restore the train from the backup list.
 * @param list the train to restore.
 */
static void RestoreTrainBackup(TrainList &list)
{
        /* No train, nothing to do. */
        if (list.size() == 0) return;

        Train *prev = nullptr;
        /* Iterate over the list and rebuild it. */
        for (Train *t : list) {
                if (prev != nullptr) {
                        prev->SetNext(t);
                } else if (t->Previous() != nullptr) {
                        /* Make sure the head of the train is always the first in the chain. */
                        t->Previous()->SetNext(nullptr);
                }
                prev = t;
        }
}

/**
 * Remove the given wagon from its consist.
 * @param part the part of the train to remove.
 * @param chain whether to remove the whole chain.
 */
static void RemoveFromConsist(Train *part, bool chain = false)
{
        Train *tail = chain ? part->Last() : part->GetLastEnginePart();

        /* Unlink at the front, but make it point to the next
         * vehicle after the to be remove part. */
        if (part->Previous() != nullptr) part->Previous()->SetNext(tail->Next());

        /* Unlink at the back */
        tail->SetNext(nullptr);
}

/**
 * Inserts a chain into the train at dst.
 * @param dst   the place where to append after.
 * @param chain the chain to actually add.
 */
static void InsertInConsist(Train *dst, Train *chain)
{
        /* We do not want to add something in the middle of an articulated part. */
        assert(dst != nullptr && (dst->Next() == nullptr || !dst->Next()->IsArticulatedPart()));

        chain->Last()->SetNext(dst->Next());
        dst->SetNext(chain);
}

/**
 * Normalise the dual heads in the train, i.e. if one is
 * missing move that one to this train.
 * @param t the train to normalise.
 */
static void NormaliseDualHeads(Train *t)
{
        for (; t != nullptr; t = t->GetNextVehicle()) {
                if (!t->IsMultiheaded() || !t->IsEngine()) continue;

                /* Make sure that there are no free cars before next engine */
                Train *u;
                for (u = t; u->Next() != nullptr && !u->Next()->IsEngine(); u = u->Next()) {}

                if (u == t->other_multiheaded_part) continue;

                /* Remove the part from the 'wrong' train */
                RemoveFromConsist(t->other_multiheaded_part);
                /* And add it to the 'right' train */
                InsertInConsist(u, t->other_multiheaded_part);
        }
}

/**
 * Normalise the sub types of the parts in this chain.
 * @param chain the chain to normalise.
 */
static void NormaliseSubtypes(Train *chain)
{
        /* Nothing to do */
        if (chain == nullptr) return;

        /* We must be the first in the chain. */
        assert(chain->Previous() == nullptr);

        /* Set the appropriate bits for the first in the chain. */
        if (chain->IsWagon()) {
                chain->SetFreeWagon();
        } else {
                assert(chain->IsEngine());
                chain->SetFrontEngine();
        }

        /* Now clear the bits for the rest of the chain */
        for (Train *t = chain->Next(); t != nullptr; t = t->Next()) {
                t->ClearFreeWagon();
                t->ClearFrontEngine();
        }
}

/**
 * Check/validate whether we may actually build a new train.
 * @note All vehicles are/were 'heads' of their chains.
 * @param original_dst The original destination chain.
 * @param dst          The destination chain after constructing the train.
 * @param original_src The original source chain.
 * @param src          The source chain after constructing the train.
 * @return possible error of this command.
 */
static CommandCost CheckNewTrain(Train *original_dst, Train *dst, Train *original_src, Train *src)
{
        /* Just add 'new' engines and subtract the original ones.
         * If that's less than or equal to 0 we can be sure we did
         * not add any engines (read: trains) along the way. */
        if ((src          != nullptr && src->IsEngine()          ? 1 : 0) +
                        (dst          != nullptr && dst->IsEngine()          ? 1 : 0) -
                        (original_src != nullptr && original_src->IsEngine() ? 1 : 0) -
                        (original_dst != nullptr && original_dst->IsEngine() ? 1 : 0) <= 0) {
                return CommandCost();
        }

        /* Get a free unit number and check whether it's within the bounds.
         * There will always be a maximum of one new train. */
        if (GetFreeUnitNumber(VEH_TRAIN) <= _settings_game.vehicle.max_trains) return CommandCost();

        return_cmd_error(STR_ERROR_TOO_MANY_VEHICLES_IN_GAME);
}

/**
 * Check whether the train parts can be attached.
 * @param t the train to check
 * @return possible error of this command.
 */
static CommandCost CheckTrainAttachment(Train *t)
{
        /* No multi-part train, no need to check. */
        if (t == nullptr || t->Next() == nullptr) return CommandCost();

        /* The maximum length for a train. For each part we decrease this by one
         * and if the result is negative the train is simply too long. */
        int allowed_len = _settings_game.vehicle.max_train_length * TILE_SIZE - t->gcache.cached_veh_length;

        /* For free-wagon chains, check if they are within the max_train_length limit. */
        if (!t->IsEngine()) {
                t = t->Next();
                while (t != nullptr) {
                        allowed_len -= t->gcache.cached_veh_length;

                        t = t->Next();
                }

                if (allowed_len < 0) return_cmd_error(STR_ERROR_TRAIN_TOO_LONG);
                return CommandCost();
        }

        Train *head = t;
        Train *prev = t;

        /* Break the prev -> t link so it always holds within the loop. */
        t = t->Next();
        prev->SetNext(nullptr);

        /* Make sure the cache is cleared. */
        head->InvalidateNewGRFCache();

        while (t != nullptr) {
                allowed_len -= t->gcache.cached_veh_length;

                Train *next = t->Next();

                /* Unlink the to-be-added piece; it is already unlinked from the previous
                 * part due to the fact that the prev -> t link is broken. */
                t->SetNext(nullptr);

                /* Don't check callback for articulated or rear dual headed parts */
                if (!t->IsArticulatedPart() && !t->IsRearDualheaded()) {
                        /* Back up and clear the first_engine data to avoid using wagon override group */
                        EngineID first_engine = t->gcache.first_engine;
                        t->gcache.first_engine = INVALID_ENGINE;

                        /* We don't want the cache to interfere. head's cache is cleared before
                         * the loop and after each callback does not need to be cleared here. */
                        t->InvalidateNewGRFCache();

                        uint16 callback = GetVehicleCallbackParent(CBID_TRAIN_ALLOW_WAGON_ATTACH, 0, 0, head->engine_type, t, head);

                        /* Restore original first_engine data */
                        t->gcache.first_engine = first_engine;

                        /* We do not want to remember any cached variables from the test run */
                        t->InvalidateNewGRFCache();
                        head->InvalidateNewGRFCache();

                        if (callback != CALLBACK_FAILED) {
                                /* A failing callback means everything is okay */
                                StringID error = STR_NULL;

                                if (head->GetGRF()->grf_version < 8) {
                                        if (callback == 0xFD) error = STR_ERROR_INCOMPATIBLE_RAIL_TYPES;
                                        if (callback  < 0xFD) error = GetGRFStringID(head->GetGRFID(), 0xD000 + callback);
                                        if (callback >= 0x100) ErrorUnknownCallbackResult(head->GetGRFID(), CBID_TRAIN_ALLOW_WAGON_ATTACH, callback);
                                } else {
                                        if (callback < 0x400) {
                                                error = GetGRFStringID(head->GetGRFID(), 0xD000 + callback);
                                        } else {
                                                switch (callback) {
                                                        case 0x400: // allow if railtypes match (always the case for OpenTTD)
                                                        case 0x401: // allow
                                                                break;

                                                        default:    // unknown reason -> disallow
                                                        case 0x402: // disallow attaching
                                                                error = STR_ERROR_INCOMPATIBLE_RAIL_TYPES;
                                                                break;
                                                }
                                        }
                                }

                                if (error != STR_NULL) return_cmd_error(error);
                        }
                }

                /* And link it to the new part. */
                prev->SetNext(t);
                prev = t;
                t = next;
        }

        if (allowed_len < 0) return_cmd_error(STR_ERROR_TRAIN_TOO_LONG);
        return CommandCost();
}

/**
 * Validate whether we are going to create valid trains.
 * @note All vehicles are/were 'heads' of their chains.
 * @param original_dst The original destination chain.
 * @param dst          The destination chain after constructing the train.
 * @param original_src The original source chain.
 * @param src          The source chain after constructing the train.
 * @param check_limit  Whether to check the vehicle limit.
 * @return possible error of this command.
 */
static CommandCost ValidateTrains(Train *original_dst, Train *dst, Train *original_src, Train *src, bool check_limit)
{
        /* Check whether we may actually construct the trains. */
        CommandCost ret = CheckTrainAttachment(src);
        if (ret.Failed()) return ret;
        ret = CheckTrainAttachment(dst);
        if (ret.Failed()) return ret;

        /* Check whether we need to build a new train. */
        return check_limit ? CheckNewTrain(original_dst, dst, original_src, src) : CommandCost();
}

/**
 * Arrange the trains in the wanted way.
 * @param dst_head   The destination chain of the to be moved vehicle.
 * @param dst        The destination for the to be moved vehicle.
 * @param src_head   The source chain of the to be moved vehicle.
 * @param src        The to be moved vehicle.
 * @param move_chain Whether to move all vehicles after src or not.
 */
static void ArrangeTrains(Train **dst_head, Train *dst, Train **src_head, Train *src, bool move_chain)
{
        /* First determine the front of the two resulting trains */
        if (*src_head == *dst_head) {
                /* If we aren't moving part(s) to a new train, we are just moving the
                 * front back and there is not destination head. */
                *dst_head = nullptr;
        } else if (*dst_head == nullptr) {
                /* If we are moving to a new train the head of the move train would become
                 * the head of the new vehicle. */
                *dst_head = src;
        }

        if (src == *src_head) {
                /* If we are moving the front of a train then we are, in effect, creating
                 * a new head for the train. Point to that. Unless we are moving the whole
                 * train in which case there is not 'source' train anymore.
                 * In case we are a multiheaded part we want the complete thing to come
                 * with us, so src->GetNextUnit(), however... when we are e.g. a wagon
                 * that is followed by a rear multihead we do not want to include that. */
                *src_head = move_chain ? nullptr :
                                (src->IsMultiheaded() ? src->GetNextUnit() : src->GetNextVehicle());
        }

        /* Now it's just simply removing the part that we are going to move from the
         * source train and *if* the destination is a not a new train add the chain
         * at the destination location. */
        RemoveFromConsist(src, move_chain);
        if (*dst_head != src) InsertInConsist(dst, src);

        /* Now normalise the dual heads, that is move the dual heads around in such
         * a way that the head and rear of a dual head are in the same train */
        NormaliseDualHeads(*src_head);
        NormaliseDualHeads(*dst_head);
}

/**
 * Normalise the head of the train again, i.e. that is tell the world that
 * we have changed and update all kinds of variables.
 * @param head the train to update.
 */
static void NormaliseTrainHead(Train *head)
{
        /* Not much to do! */
        if (head == nullptr) return;

        /* Tell the 'world' the train changed. */
        head->ConsistChanged(CCF_ARRANGE);
        UpdateTrainGroupID(head);

        /* Not a front engine, i.e. a free wagon chain. No need to do more. */
        if (!head->IsFrontEngine()) return;

        /* Update the refit button and window */
        InvalidateWindowData(WC_VEHICLE_REFIT, head->index, VIWD_CONSIST_CHANGED);
        SetWindowWidgetDirty(WC_VEHICLE_VIEW, head->index, WID_VV_REFIT);

        /* If we don't have a unit number yet, set one. */
        if (head->unitnumber != 0) return;
        head->unitnumber = GetFreeUnitNumber(VEH_TRAIN);
}

/**
 * Move a rail vehicle around inside the depot.
 * @param tile unused
 * @param flags type of operation
 *              Note: DC_AUTOREPLACE is set when autoreplace tries to undo its modifications or moves vehicles to temporary locations inside the depot.
 * @param p1 various bitstuffed elements
 * - p1 (bit  0 - 19) source vehicle index
 * - p1 (bit      20) move all vehicles following the source vehicle
 * @param p2 what wagon to put the source wagon AFTER, XXX - INVALID_VEHICLE to make a new line
 * @param text unused
 * @return the cost of this operation or an error
 */
CommandCost CmdMoveRailVehicle(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32 p2, const std::string &text)
{
        VehicleID s = GB(p1, 0, 20);
        VehicleID d = GB(p2, 0, 20);
        bool move_chain = HasBit(p1, 20);

        Train *src = Train::GetIfValid(s);
        if (src == nullptr) return CMD_ERROR;

        CommandCost ret = CheckOwnership(src->owner);
        if (ret.Failed()) return ret;

        /* Do not allow moving crashed vehicles inside the depot, it is likely to cause asserts later */
        if (src->vehstatus & VS_CRASHED) return CMD_ERROR;

        /* if nothing is selected as destination, try and find a matching vehicle to drag to. */
        Train *dst;
        if (d == INVALID_VEHICLE) {
                dst = (src->IsEngine() || (flags & DC_AUTOREPLACE)) ? nullptr : FindGoodVehiclePos(src);
        } else {
                dst = Train::GetIfValid(d);
                if (dst == nullptr) return CMD_ERROR;

                CommandCost ret = CheckOwnership(dst->owner);
                if (ret.Failed()) return ret;

                /* Do not allow appending to crashed vehicles, too */
                if (dst->vehstatus & VS_CRASHED) return CMD_ERROR;
        }

        /* if an articulated part is being handled, deal with its parent vehicle */
        src = src->GetFirstEnginePart();
        if (dst != nullptr) {
                dst = dst->GetFirstEnginePart();
        }

        /* don't move the same vehicle.. */
        if (src == dst) return CommandCost();

        /* locate the head of the two chains */
        Train *src_head = src->First();
        Train *dst_head;
        if (dst != nullptr) {
                dst_head = dst->First();
                if (dst_head->tile != src_head->tile) return CMD_ERROR;
                /* Now deal with articulated part of destination wagon */
                dst = dst->GetLastEnginePart();
        } else {
                dst_head = nullptr;
        }

        if (src->IsRearDualheaded()) return_cmd_error(STR_ERROR_REAR_ENGINE_FOLLOW_FRONT);

        /* When moving all wagons, we can't have the same src_head and dst_head */
        if (move_chain && src_head == dst_head) return CommandCost();

        /* When moving a multiheaded part to be place after itself, bail out. */
        if (!move_chain && dst != nullptr && dst->IsRearDualheaded() && src == dst->other_multiheaded_part) return CommandCost();

        /* Check if all vehicles in the source train are stopped inside a depot. */
        if (!src_head->IsStoppedInDepot()) return_cmd_error(STR_ERROR_TRAINS_CAN_ONLY_BE_ALTERED_INSIDE_A_DEPOT);

        /* Check if all vehicles in the destination train are stopped inside a depot. */
        if (dst_head != nullptr && !dst_head->IsStoppedInDepot()) return_cmd_error(STR_ERROR_TRAINS_CAN_ONLY_BE_ALTERED_INSIDE_A_DEPOT);

        /* First make a backup of the order of the trains. That way we can do
         * whatever we want with the order and later on easily revert. */
        TrainList original_src;
        TrainList original_dst;

        MakeTrainBackup(original_src, src_head);
        MakeTrainBackup(original_dst, dst_head);

        /* Also make backup of the original heads as ArrangeTrains can change them.
         * For the destination head we do not care if it is the same as the source
         * head because in that case it's just a copy. */
        Train *original_src_head = src_head;
        Train *original_dst_head = (dst_head == src_head ? nullptr : dst_head);

        /* We want this information from before the rearrangement, but execute this after the validation.
         * original_src_head can't be nullptr; src is by definition != nullptr, so src_head can't be nullptr as
         * src->GetFirst() always yields non-nullptr, so eventually original_src_head != nullptr as well. */
        bool original_src_head_front_engine = original_src_head->IsFrontEngine();
        bool original_dst_head_front_engine = original_dst_head != nullptr && original_dst_head->IsFrontEngine();

        /* (Re)arrange the trains in the wanted arrangement. */
        ArrangeTrains(&dst_head, dst, &src_head, src, move_chain);

        if ((flags & DC_AUTOREPLACE) == 0) {
                /* If the autoreplace flag is set we do not need to test for the validity
                 * because we are going to revert the train to its original state. As we
                 * assume the original state was correct autoreplace can skip this. */
                CommandCost ret = ValidateTrains(original_dst_head, dst_head, original_src_head, src_head, true);
                if (ret.Failed()) {
                        /* Restore the train we had. */
                        RestoreTrainBackup(original_src);
                        RestoreTrainBackup(original_dst);
                        return ret;
                }
        }

        /* do it? */
        if (flags & DC_EXEC) {
                /* Remove old heads from the statistics */
                if (original_src_head_front_engine) GroupStatistics::CountVehicle(original_src_head, -1);
                if (original_dst_head_front_engine) GroupStatistics::CountVehicle(original_dst_head, -1);

                /* First normalise the sub types of the chains. */
                NormaliseSubtypes(src_head);
                NormaliseSubtypes(dst_head);

                /* There are 14 different cases:
                 *  1) front engine gets moved to a new train, it stays a front engine.
                 *     a) the 'next' part is a wagon that becomes a free wagon chain.
                 *     b) the 'next' part is an engine that becomes a front engine.
                 *     c) there is no 'next' part, nothing else happens
                 *  2) front engine gets moved to another train, it is not a front engine anymore
                 *     a) the 'next' part is a wagon that becomes a free wagon chain.
                 *     b) the 'next' part is an engine that becomes a front engine.
                 *     c) there is no 'next' part, nothing else happens
                 *  3) front engine gets moved to later in the current train, it is not a front engine anymore.
                 *     a) the 'next' part is a wagon that becomes a free wagon chain.
                 *     b) the 'next' part is an engine that becomes a front engine.
                 *  4) free wagon gets moved
                 *     a) the 'next' part is a wagon that becomes a free wagon chain.
                 *     b) the 'next' part is an engine that becomes a front engine.
                 *     c) there is no 'next' part, nothing else happens
                 *  5) non front engine gets moved and becomes a new train, nothing else happens
                 *  6) non front engine gets moved within a train / to another train, nothing happens
                 *  7) wagon gets moved, nothing happens
                 */
                if (src == original_src_head && src->IsEngine() && !src->IsFrontEngine()) {
                        /* Cases #2 and #3: the front engine gets trashed. */
                        CloseWindowById(WC_VEHICLE_VIEW, src->index);
                        CloseWindowById(WC_VEHICLE_ORDERS, src->index);
                        CloseWindowById(WC_VEHICLE_REFIT, src->index);
                        CloseWindowById(WC_VEHICLE_DETAILS, src->index);
                        CloseWindowById(WC_VEHICLE_TIMETABLE, src->index);
                        DeleteNewGRFInspectWindow(GSF_TRAINS, src->index);
                        SetWindowDirty(WC_COMPANY, _current_company);

                        /* Delete orders, group stuff and the unit number as we're not the
                         * front of any vehicle anymore. */
                        DeleteVehicleOrders(src);
                        RemoveVehicleFromGroup(src);
                        src->unitnumber = 0;
                }

                /* We weren't a front engine but are becoming one. So
                 * we should be put in the default group. */
                if (original_src_head != src && dst_head == src) {
                        SetTrainGroupID(src, DEFAULT_GROUP);
                        SetWindowDirty(WC_COMPANY, _current_company);
                }

                /* Add new heads to statistics */
                if (src_head != nullptr && src_head->IsFrontEngine()) GroupStatistics::CountVehicle(src_head, 1);
                if (dst_head != nullptr && dst_head->IsFrontEngine()) GroupStatistics::CountVehicle(dst_head, 1);

                /* Handle 'new engine' part of cases #1b, #2b, #3b, #4b and #5 in NormaliseTrainHead. */
                NormaliseTrainHead(src_head);
                NormaliseTrainHead(dst_head);

                if ((flags & DC_NO_CARGO_CAP_CHECK) == 0) {
                        CheckCargoCapacity(src_head);
                        CheckCargoCapacity(dst_head);
                }

                if (src_head != nullptr) src_head->First()->MarkDirty();
                if (dst_head != nullptr) dst_head->First()->MarkDirty();

                /* We are undoubtedly changing something in the depot and train list. */
                InvalidateWindowData(WC_VEHICLE_DEPOT, src->tile);
                InvalidateWindowClassesData(WC_TRAINS_LIST, 0);
        } else {
                /* We don't want to execute what we're just tried. */
                RestoreTrainBackup(original_src);
                RestoreTrainBackup(original_dst);
        }

        return CommandCost();
}

/**
 * Sell a (single) train wagon/engine.
 * @param flags type of operation
 * @param t     the train wagon to sell
 * @param data  the selling mode
 * - data = 0: only sell the single dragged wagon/engine (and any belonging rear-engines)
 * - data = 1: sell the vehicle and all vehicles following it in the chain
 *             if the wagon is dragged, don't delete the possibly belonging rear-engine to some front
 * @param user  the user for the order backup.
 * @return the cost of this operation or an error
 */
CommandCost CmdSellRailWagon(DoCommandFlag flags, Vehicle *t, uint16 data, uint32 user)
{
        /* Sell a chain of vehicles or not? */
        bool sell_chain = HasBit(data, 0);

        Train *v = Train::From(t)->GetFirstEnginePart();
        Train *first = v->First();

        if (v->IsRearDualheaded()) return_cmd_error(STR_ERROR_REAR_ENGINE_FOLLOW_FRONT);

        /* First make a backup of the order of the train. That way we can do
         * whatever we want with the order and later on easily revert. */
        TrainList original;
        MakeTrainBackup(original, first);

        /* We need to keep track of the new head and the head of what we're going to sell. */
        Train *new_head = first;
        Train *sell_head = nullptr;

        /* Split the train in the wanted way. */
        ArrangeTrains(&sell_head, nullptr, &new_head, v, sell_chain);

        /* We don't need to validate the second train; it's going to be sold. */
        CommandCost ret = ValidateTrains(nullptr, nullptr, first, new_head, (flags & DC_AUTOREPLACE) == 0);
        if (ret.Failed()) {
                /* Restore the train we had. */
                RestoreTrainBackup(original);
                return ret;
        }

        if (first->orders.list == nullptr && !OrderList::CanAllocateItem()) {
                /* Restore the train we had. */
                RestoreTrainBackup(original);
                return_cmd_error(STR_ERROR_NO_MORE_SPACE_FOR_ORDERS);
        }

        CommandCost cost(EXPENSES_NEW_VEHICLES);
        for (Train *part = sell_head; part != nullptr; part = part->Next()) cost.AddCost(-part->value);

        /* do it? */
        if (flags & DC_EXEC) {
                /* First normalise the sub types of the chain. */
                NormaliseSubtypes(new_head);

                if (v == first && v->IsEngine() && !sell_chain && new_head != nullptr && new_head->IsFrontEngine()) {
                        /* We are selling the front engine. In this case we want to
                         * 'give' the order, unit number and such to the new head. */
                        new_head->orders.list = first->orders.list;
                        new_head->AddToShared(first);
                        DeleteVehicleOrders(first);

                        /* Copy other important data from the front engine */
                        new_head->CopyVehicleConfigAndStatistics(first);
                        GroupStatistics::CountVehicle(new_head, 1); // after copying over the profit
                } else if (v->IsPrimaryVehicle() && data & (MAKE_ORDER_BACKUP_FLAG >> 20)) {
                        OrderBackup::Backup(v, user);
                }

                /* We need to update the information about the train. */
                NormaliseTrainHead(new_head);

                /* We are undoubtedly changing something in the depot and train list. */
                InvalidateWindowData(WC_VEHICLE_DEPOT, v->tile);
                InvalidateWindowClassesData(WC_TRAINS_LIST, 0);

                /* Actually delete the sold 'goods' */
                delete sell_head;
        } else {
                /* We don't want to execute what we're just tried. */
                RestoreTrainBackup(original);
        }

        return cost;
}

void Train::UpdateDeltaXY()
{
        /* Set common defaults. */
        this->x_offs    = -1;
        this->y_offs    = -1;
        this->x_extent  =  3;
        this->y_extent  =  3;
        this->z_extent  =  6;
        this->x_bb_offs =  0;
        this->y_bb_offs =  0;

        if (!IsDiagonalDirection(this->direction)) {
                static const int _sign_table[] =
                {
                        /* x, y */
                        -1, -1, // DIR_N
                        -1,  1, // DIR_E
                         1,  1, // DIR_S
                         1, -1, // DIR_W
                };

                int half_shorten = (VEHICLE_LENGTH - this->gcache.cached_veh_length) / 2;

                /* For all straight directions, move the bound box to the centre of the vehicle, but keep the size. */
                this->x_offs -= half_shorten * _sign_table[this->direction];
                this->y_offs -= half_shorten * _sign_table[this->direction + 1];
                this->x_extent += this->x_bb_offs = half_shorten * _sign_table[direction];
                this->y_extent += this->y_bb_offs = half_shorten * _sign_table[direction + 1];
        } else {
                switch (this->direction) {
                                /* Shorten southern corner of the bounding box according the vehicle length
                                 * and center the bounding box on the vehicle. */
                        case DIR_NE:
                                this->x_offs    = 1 - (this->gcache.cached_veh_length + 1) / 2;
                                this->x_extent  = this->gcache.cached_veh_length - 1;
                                this->x_bb_offs = -1;
                                break;

                        case DIR_NW:
                                this->y_offs    = 1 - (this->gcache.cached_veh_length + 1) / 2;
                                this->y_extent  = this->gcache.cached_veh_length - 1;
                                this->y_bb_offs = -1;
                                break;

                                /* Move northern corner of the bounding box down according to vehicle length
                                 * and center the bounding box on the vehicle. */
                        case DIR_SW:
                                this->x_offs    = 1 + (this->gcache.cached_veh_length + 1) / 2 - VEHICLE_LENGTH;
                                this->x_extent  = VEHICLE_LENGTH - 1;
                                this->x_bb_offs = VEHICLE_LENGTH - this->gcache.cached_veh_length - 1;
                                break;

                        case DIR_SE:
                                this->y_offs    = 1 + (this->gcache.cached_veh_length + 1) / 2 - VEHICLE_LENGTH;
                                this->y_extent  = VEHICLE_LENGTH - 1;
                                this->y_bb_offs = VEHICLE_LENGTH - this->gcache.cached_veh_length - 1;
                                break;

                        default:
                                NOT_REACHED();
                }
        }
}

/**
 * Mark a train as stuck and stop it if it isn't stopped right now.
 * @param v %Train to mark as being stuck.
 */
static void MarkTrainAsStuck(Train *v)
{
        if (!HasBit(v->flags, VRF_TRAIN_STUCK)) {
                /* It is the first time the problem occurred, set the "train stuck" flag. */
                SetBit(v->flags, VRF_TRAIN_STUCK);

                v->wait_counter = 0;

                /* Stop train */
                v->cur_speed = 0;
                v->subspeed = 0;
                v->SetLastSpeed();

                SetWindowWidgetDirty(WC_VEHICLE_VIEW, v->index, WID_VV_START_STOP);
        }
}

/**
 * Swap the two up/down flags in two ways:
 * - Swap values of \a swap_flag1 and \a swap_flag2, and
 * - If going up previously (#GVF_GOINGUP_BIT set), the #GVF_GOINGDOWN_BIT is set, and vice versa.
 * @param[in,out] swap_flag1 First train flag.
 * @param[in,out] swap_flag2 Second train flag.
 */
static void SwapTrainFlags(uint16 *swap_flag1, uint16 *swap_flag2)
{
        uint16 flag1 = *swap_flag1;
        uint16 flag2 = *swap_flag2;

        /* Clear the flags */
        ClrBit(*swap_flag1, GVF_GOINGUP_BIT);
        ClrBit(*swap_flag1, GVF_GOINGDOWN_BIT);
        ClrBit(*swap_flag2, GVF_GOINGUP_BIT);
        ClrBit(*swap_flag2, GVF_GOINGDOWN_BIT);

        /* Reverse the rail-flags (if needed) */
        if (HasBit(flag1, GVF_GOINGUP_BIT)) {
                SetBit(*swap_flag2, GVF_GOINGDOWN_BIT);
        } else if (HasBit(flag1, GVF_GOINGDOWN_BIT)) {
                SetBit(*swap_flag2, GVF_GOINGUP_BIT);
        }
        if (HasBit(flag2, GVF_GOINGUP_BIT)) {
                SetBit(*swap_flag1, GVF_GOINGDOWN_BIT);
        } else if (HasBit(flag2, GVF_GOINGDOWN_BIT)) {
                SetBit(*swap_flag1, GVF_GOINGUP_BIT);
        }
}

/**
 * Updates some variables after swapping the vehicle.
 * @param v swapped vehicle
 */
static void UpdateStatusAfterSwap(Train *v)
{
        /* Reverse the direction. */
        if (v->track != TRACK_BIT_DEPOT) v->direction = ReverseDir(v->direction);

        /* Call the proper EnterTile function unless we are in a wormhole. */
        if (v->track != TRACK_BIT_WORMHOLE) {
                VehicleEnterTile(v, v->tile, v->x_pos, v->y_pos);
        } else {
                /* VehicleEnter_TunnelBridge() sets TRACK_BIT_WORMHOLE when the vehicle
                 * is on the last bit of the bridge head (frame == TILE_SIZE - 1).
                 * If we were swapped with such a vehicle, we have set TRACK_BIT_WORMHOLE,
                 * when we shouldn't have. Check if this is the case. */
                TileIndex vt = TileVirtXY(v->x_pos, v->y_pos);
                if (IsTileType(vt, MP_TUNNELBRIDGE)) {
                        VehicleEnterTile(v, vt, v->x_pos, v->y_pos);
                        if (v->track != TRACK_BIT_WORMHOLE && IsBridgeTile(v->tile)) {
                                /* We have just left the wormhole, possibly set the
                                 * "goingdown" bit. UpdateInclination() can be used
                                 * because we are at the border of the tile. */
                                v->UpdatePosition();
                                v->UpdateInclination(true, true);
                                return;
                        }
                }
        }

        v->UpdatePosition();
        v->UpdateViewport(true, true);
}

/**
 * Swap vehicles \a l and \a r in consist \a v, and reverse their direction.
 * @param v Consist to change.
 * @param l %Vehicle index in the consist of the first vehicle.
 * @param r %Vehicle index in the consist of the second vehicle.
 */
void ReverseTrainSwapVeh(Train *v, int l, int r)
{
        Train *a, *b;

        /* locate vehicles to swap */
        for (a = v; l != 0; l--) a = a->Next();
        for (b = v; r != 0; r--) b = b->Next();

        if (a != b) {
                /* swap the hidden bits */
                {
                        uint16 tmp = (a->vehstatus & ~VS_HIDDEN) | (b->vehstatus & VS_HIDDEN);
                        b->vehstatus = (b->vehstatus & ~VS_HIDDEN) | (a->vehstatus & VS_HIDDEN);
                        a->vehstatus = tmp;
                }

                Swap(a->track, b->track);
                Swap(a->direction, b->direction);
                Swap(a->x_pos, b->x_pos);
                Swap(a->y_pos, b->y_pos);
                Swap(a->tile,  b->tile);
                Swap(a->z_pos, b->z_pos);

                SwapTrainFlags(&a->gv_flags, &b->gv_flags);

                UpdateStatusAfterSwap(a);
                UpdateStatusAfterSwap(b);
        } else {
                /* Swap GVF_GOINGUP_BIT/GVF_GOINGDOWN_BIT.
                 * This is a little bit redundant way, a->gv_flags will
                 * be (re)set twice, but it reduces code duplication */
                SwapTrainFlags(&a->gv_flags, &a->gv_flags);
                UpdateStatusAfterSwap(a);
        }
}


/**
 * Check if the vehicle is a train
 * @param v vehicle on tile
 * @return v if it is a train, nullptr otherwise
 */
static Vehicle *TrainOnTileEnum(Vehicle *v, void *)
{
        return (v->type == VEH_TRAIN) ? v : nullptr;
}


/**
 * Checks if a train is approaching a rail-road crossing
 * @param v vehicle on tile
 * @param data tile with crossing we are testing
 * @return v if it is approaching a crossing, nullptr otherwise
 */
static Vehicle *TrainApproachingCrossingEnum(Vehicle *v, void *data)
{
        if (v->type != VEH_TRAIN || (v->vehstatus & VS_CRASHED)) return nullptr;

        Train *t = Train::From(v);
        if (!t->IsFrontEngine()) return nullptr;

        TileIndex tile = *(TileIndex *)data;

        if (TrainApproachingCrossingTile(t) != tile) return nullptr;

        return t;
}


/**
 * Finds a vehicle approaching rail-road crossing
 * @param tile tile to test
 * @return true if a vehicle is approaching the crossing
 * @pre tile is a rail-road crossing
 */
static bool TrainApproachingCrossing(TileIndex tile)
{
        assert(IsLevelCrossingTile(tile));

        DiagDirection dir = AxisToDiagDir(GetCrossingRailAxis(tile));
        TileIndex tile_from = tile + TileOffsByDiagDir(dir);

        if (HasVehicleOnPos(tile_from, &tile, &TrainApproachingCrossingEnum)) return true;

        dir = ReverseDiagDir(dir);
        tile_from = tile + TileOffsByDiagDir(dir);

        return HasVehicleOnPos(tile_from, &tile, &TrainApproachingCrossingEnum);
}


/**
 * Sets correct crossing state
 * @param tile tile to update
 * @param sound should we play sound?
 * @pre tile is a rail-road crossing
 */
void UpdateLevelCrossing(TileIndex tile, bool sound)
{
        assert(IsLevelCrossingTile(tile));

        /* reserved || train on crossing || train approaching crossing */
        bool new_state = HasCrossingReservation(tile) || HasVehicleOnPos(tile, nullptr, &TrainOnTileEnum) || TrainApproachingCrossing(tile);

        if (new_state != IsCrossingBarred(tile)) {
                if (new_state && sound) {
                        if (_settings_client.sound.ambient) SndPlayTileFx(SND_0E_LEVEL_CROSSING, tile);
                }
                SetCrossingBarred(tile, new_state);
                MarkTileDirtyByTile(tile);
        }
}


/**
 * Bars crossing and plays ding-ding sound if not barred already
 * @param tile tile with crossing
 * @pre tile is a rail-road crossing
 */
static inline void MaybeBarCrossingWithSound(TileIndex tile)
{
        if (!IsCrossingBarred(tile)) {
                BarCrossing(tile);
                if (_settings_client.sound.ambient) SndPlayTileFx(SND_0E_LEVEL_CROSSING, tile);
                MarkTileDirtyByTile(tile);
        }
}


/**
 * Advances wagons for train reversing, needed for variable length wagons.
 * This one is called before the train is reversed.
 * @param v First vehicle in chain
 */
static void AdvanceWagonsBeforeSwap(Train *v)
{
        Train *base = v;
        Train *first = base; // first vehicle to move
        Train *last = v->Last(); // last vehicle to move
        uint length = CountVehiclesInChain(v);

        while (length > 2) {
                last = last->Previous();
                first = first->Next();

                int differential = base->CalcNextVehicleOffset() - last->CalcNextVehicleOffset();

                /* do not update images now
                 * negative differential will be handled in AdvanceWagonsAfterSwap() */
                for (int i = 0; i < differential; i++) TrainController(first, last->Next());

                base = first; // == base->Next()
                length -= 2;
        }
}


/**
 * Advances wagons for train reversing, needed for variable length wagons.
 * This one is called after the train is reversed.
 * @param v First vehicle in chain
 */
static void AdvanceWagonsAfterSwap(Train *v)
{
        /* first of all, fix the situation when the train was entering a depot */
        Train *dep = v; // last vehicle in front of just left depot
        while (dep->Next() != nullptr && (dep->track == TRACK_BIT_DEPOT || dep->Next()->track != TRACK_BIT_DEPOT)) {
                dep = dep->Next(); // find first vehicle outside of a depot, with next vehicle inside a depot
        }

        Train *leave = dep->Next(); // first vehicle in a depot we are leaving now

        if (leave != nullptr) {
                /* 'pull' next wagon out of the depot, so we won't miss it (it could stay in depot forever) */
                int d = TicksToLeaveDepot(dep);

                if (d <= 0) {
                        leave->vehstatus &= ~VS_HIDDEN; // move it out of the depot
                        leave->track = TrackToTrackBits(GetRailDepotTrack(leave->tile));
                        for (int i = 0; i >= d; i--) TrainController(leave, nullptr); // maybe move it, and maybe let another wagon leave
                }
        } else {
                dep = nullptr; // no vehicle in a depot, so no vehicle leaving a depot
        }

        Train *base = v;
        Train *first = base; // first vehicle to move
        Train *last = v->Last(); // last vehicle to move
        uint length = CountVehiclesInChain(v);

        /* We have to make sure all wagons that leave a depot because of train reversing are moved correctly
         * they have already correct spacing, so we have to make sure they are moved how they should */
        bool nomove = (dep == nullptr); // If there is no vehicle leaving a depot, limit the number of wagons moved immediately.

        while (length > 2) {
                /* we reached vehicle (originally) in front of a depot, stop now
                 * (we would move wagons that are already moved with new wagon length). */
                if (base == dep) break;

                /* the last wagon was that one leaving a depot, so do not move it anymore */
                if (last == dep) nomove = true;

                last = last->Previous();
                first = first->Next();

                int differential = last->CalcNextVehicleOffset() - base->CalcNextVehicleOffset();

                /* do not update images now */
                for (int i = 0; i < differential; i++) TrainController(first, (nomove ? last->Next() : nullptr));

                base = first; // == base->Next()
                length -= 2;
        }
}

static bool IsWholeTrainInsideDepot(const Train *v)
{
        for (const Train *u = v; u != nullptr; u = u->Next()) {
                if (u->track != TRACK_BIT_DEPOT || u->tile != v->tile) return false;
        }
        return true;
}

/**
 * Turn a train around.
 * @param v %Train to turn around.
 */
void ReverseTrainDirection(Train *v)
{
        if (IsRailDepotTile(v->tile)) {
                if (IsWholeTrainInsideDepot(v)) return;
                InvalidateWindowData(WC_VEHICLE_DEPOT, v->tile);
        }

        /* Clear path reservation in front if train is not stuck. */
        if (!HasBit(v->flags, VRF_TRAIN_STUCK)) FreeTrainTrackReservation(v);

        /* Check if we were approaching a rail/road-crossing */
        TileIndex crossing = TrainApproachingCrossingTile(v);

        /* count number of vehicles */
        int r = CountVehiclesInChain(v) - 1;  // number of vehicles - 1

        AdvanceWagonsBeforeSwap(v);

        /* swap start<>end, start+1<>end-1, ... */
        int l = 0;
        do {
                ReverseTrainSwapVeh(v, l++, r--);
        } while (l <= r);

        AdvanceWagonsAfterSwap(v);

        if (IsRailDepotTile(v->tile)) {
                InvalidateWindowData(WC_VEHICLE_DEPOT, v->tile);
        }

        ToggleBit(v->flags, VRF_TOGGLE_REVERSE);

        ClrBit(v->flags, VRF_REVERSING);

        /* recalculate cached data */
        v->ConsistChanged(CCF_TRACK);

        /* update all images */
        for (Train *u = v; u != nullptr; u = u->Next()) u->UpdateViewport(false, false);

        /* update crossing we were approaching */
        if (crossing != INVALID_TILE) UpdateLevelCrossing(crossing);

        /* maybe we are approaching crossing now, after reversal */
        crossing = TrainApproachingCrossingTile(v);
        if (crossing != INVALID_TILE) MaybeBarCrossingWithSound(crossing);

        /* If we are inside a depot after reversing, don't bother with path reserving. */
        if (v->track == TRACK_BIT_DEPOT) {
                /* Can't be stuck here as inside a depot is always a safe tile. */
                if (HasBit(v->flags, VRF_TRAIN_STUCK)) SetWindowWidgetDirty(WC_VEHICLE_VIEW, v->index, WID_VV_START_STOP);
                ClrBit(v->flags, VRF_TRAIN_STUCK);
                return;
        }

        /* VehicleExitDir does not always produce the desired dir for depots and
         * tunnels/bridges that is needed for UpdateSignalsOnSegment. */
        DiagDirection dir = VehicleExitDir(v->direction, v->track);
        if (IsRailDepotTile(v->tile) || IsTileType(v->tile, MP_TUNNELBRIDGE)) dir = INVALID_DIAGDIR;

        if (UpdateSignalsOnSegment(v->tile, dir, v->owner) == SIGSEG_PBS || _settings_game.pf.reserve_paths) {
                /* If we are currently on a tile with conventional signals, we can't treat the
                 * current tile as a safe tile or we would enter a PBS block without a reservation. */
                bool first_tile_okay = !(IsTileType(v->tile, MP_RAILWAY) &&
                        HasSignalOnTrackdir(v->tile, v->GetVehicleTrackdir()) &&
                        !IsPbsSignal(GetSignalType(v->tile, FindFirstTrack(v->track))));

                /* If we are on a depot tile facing outwards, do not treat the current tile as safe. */
                if (IsRailDepotTile(v->tile) && TrackdirToExitdir(v->GetVehicleTrackdir()) == GetRailDepotDirection(v->tile)) first_tile_okay = false;

                if (IsRailStationTile(v->tile)) SetRailStationPlatformReservation(v->tile, TrackdirToExitdir(v->GetVehicleTrackdir()), true);
                if (TryPathReserve(v, false, first_tile_okay)) {
                        /* Do a look-ahead now in case our current tile was already a safe tile. */
                        CheckNextTrainTile(v);
                } else if (v->current_order.GetType() != OT_LOADING) {
                        /* Do not wait for a way out when we're still loading */
                        MarkTrainAsStuck(v);
                }
        } else if (HasBit(v->flags, VRF_TRAIN_STUCK)) {
                /* A train not inside a PBS block can't be stuck. */
                ClrBit(v->flags, VRF_TRAIN_STUCK);
                v->wait_counter = 0;
        }
}

/**
 * Reverse train.
 * @param tile unused
 * @param flags type of operation
 * @param p1 train to reverse
 * @param p2 if true, reverse a unit in a train (needs to be in a depot)
 * @param text unused
 * @return the cost of this operation or an error
 */
CommandCost CmdReverseTrainDirection(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32 p2, const std::string &text)
{
        Train *v = Train::GetIfValid(p1);
        if (v == nullptr) return CMD_ERROR;

        CommandCost ret = CheckOwnership(v->owner);
        if (ret.Failed()) return ret;

        if (p2 != 0) {
                /* turn a single unit around */

                if (v->IsMultiheaded() || HasBit(EngInfo(v->engine_type)->callback_mask, CBM_VEHICLE_ARTIC_ENGINE)) {
                        return_cmd_error(STR_ERROR_CAN_T_REVERSE_DIRECTION_RAIL_VEHICLE_MULTIPLE_UNITS);
                }
                if (!HasBit(EngInfo(v->engine_type)->misc_flags, EF_RAIL_FLIPS)) return CMD_ERROR;

                Train *front = v->First();
                /* make sure the vehicle is stopped in the depot */
                if (!front->IsStoppedInDepot()) {
                        return_cmd_error(STR_ERROR_TRAINS_CAN_ONLY_BE_ALTERED_INSIDE_A_DEPOT);
                }

                if (flags & DC_EXEC) {
                        ToggleBit(v->flags, VRF_REVERSE_DIRECTION);

                        front->ConsistChanged(CCF_ARRANGE);
                        SetWindowDirty(WC_VEHICLE_DEPOT, front->tile);
                        SetWindowDirty(WC_VEHICLE_DETAILS, front->index);
                        SetWindowDirty(WC_VEHICLE_VIEW, front->index);
                        SetWindowClassesDirty(WC_TRAINS_LIST);
                }
        } else {
                /* turn the whole train around */
                if ((v->vehstatus & VS_CRASHED) || v->breakdown_ctr != 0) return CMD_ERROR;

                if (flags & DC_EXEC) {
                        /* Properly leave the station if we are loading and won't be loading anymore */
                        if (v->current_order.IsType(OT_LOADING)) {
                                const Vehicle *last = v;
                                while (last->Next() != nullptr) last = last->Next();

                                /* not a station || different station --> leave the station */
                                if (!IsTileType(last->tile, MP_STATION) || GetStationIndex(last->tile) != GetStationIndex(v->tile)) {
                                        v->LeaveStation();
                                }
                        }

                        /* We cancel any 'skip signal at dangers' here */
                        v->force_proceed = TFP_NONE;
                        SetWindowDirty(WC_VEHICLE_VIEW, v->index);

                        if (_settings_game.vehicle.train_acceleration_model != AM_ORIGINAL && v->cur_speed != 0) {
                                ToggleBit(v->flags, VRF_REVERSING);
                        } else {
                                v->cur_speed = 0;
                                v->SetLastSpeed();
                                HideFillingPercent(&v->fill_percent_te_id);
                                ReverseTrainDirection(v);
                        }
                }
        }
        return CommandCost();
}

/**
 * Force a train through a red signal
 * @param tile unused
 * @param flags type of operation
 * @param p1 train to ignore the red signal
 * @param p2 unused
 * @param text unused
 * @return the cost of this operation or an error
 */
CommandCost CmdForceTrainProceed(TileIndex tile, DoCommandFlag flags, uint32 p1, uint32 p2, const std::string &text)
{
        Train *t = Train::GetIfValid(p1);
        if (t == nullptr) return CMD_ERROR;

        if (!t->IsPrimaryVehicle()) return CMD_ERROR;

        CommandCost ret = CheckOwnership(t->owner);
        if (ret.Failed()) return ret;


        if (flags & DC_EXEC) {
                /* If we are forced to proceed, cancel that order.
                 * If we are marked stuck we would want to force the train
                 * to proceed to the next signal. In the other cases we
                 * would like to pass the signal at danger and run till the
                 * next signal we encounter. */
                t->force_proceed = t->force_proceed == TFP_SIGNAL ? TFP_NONE : HasBit(t->flags, VRF_TRAIN_STUCK) || t->IsChainInDepot() ? TFP_STUCK : TFP_SIGNAL;
                SetWindowDirty(WC_VEHICLE_VIEW, t->index);
        }

        return CommandCost();
}

/**
 * Try to find a depot nearby.
 * @param v %Train that wants a depot.
 * @param max_distance Maximal search distance.
 * @return Information where the closest train depot is located.
 * @pre The given vehicle must not be crashed!
 */
static FindDepotData FindClosestTrainDepot(Train *v, int max_distance)
{
        assert(!(v->vehstatus & VS_CRASHED));

        if (IsRailDepotTile(v->tile)) return FindDepotData(v->tile, 0);

        PBSTileInfo origin = FollowTrainReservation(v);
        if (IsRailDepotTile(origin.tile)) return FindDepotData(origin.tile, 0);

        switch (_settings_game.pf.pathfinder_for_trains) {
                case VPF_NPF: return NPFTrainFindNearestDepot(v, max_distance);
                case VPF_YAPF: return YapfTrainFindNearestDepot(v, max_distance);

                default: NOT_REACHED();
        }
}

/**
 * Locate the closest depot for this consist, and return the information to the caller.
 * @param[out] location    If not \c nullptr and a depot is found, store its location in the given address.
 * @param[out] destination If not \c nullptr and a depot is found, store its index in the given address.
 * @param[out] reverse     If not \c nullptr and a depot is found, store reversal information in the given address.
 * @return A depot has been found.
 */
bool Train::FindClosestDepot(TileIndex *location, DestinationID *destination, bool *reverse)
{
        FindDepotData tfdd = FindClosestTrainDepot(this, 0);
        if (tfdd.best_length == UINT_MAX) return false;

        if (location    != nullptr) *location    = tfdd.tile;
        if (destination != nullptr) *destination = GetDepotIndex(tfdd.tile);
        if (reverse     != nullptr) *reverse     = tfdd.reverse;

        return true;
}

/** Play a sound for a train leaving the station. */
void Train::PlayLeaveStationSound() const
{
        static const SoundFx sfx[] = {
                SND_04_DEPARTURE_STEAM,
                SND_0A_DEPARTURE_TRAIN,
                SND_0A_DEPARTURE_TRAIN,
                SND_47_DEPARTURE_MONORAIL,
                SND_41_DEPARTURE_MAGLEV
        };

        if (PlayVehicleSound(this, VSE_START)) return;

        EngineID engtype = this->engine_type;
        SndPlayVehicleFx(sfx[RailVehInfo(engtype)->engclass], this);
}

/**
 * Check if the train is on the last reserved tile and try to extend the path then.
 * @param v %Train that needs its path extended.
 */
static void CheckNextTrainTile(Train *v)
{
        /* Don't do any look-ahead if path_backoff_interval is 255. */
        if (_settings_game.pf.path_backoff_interval == 255) return;

        /* Exit if we are inside a depot. */
        if (v->track == TRACK_BIT_DEPOT) return;

        switch (v->current_order.GetType()) {
                /* Exit if we reached our destination depot. */
                case OT_GOTO_DEPOT:
                        if (v->tile == v->dest_tile) return;
                        break;

                case OT_GOTO_WAYPOINT:
                        /* If we reached our waypoint, make sure we see that. */
                        if (IsRailWaypointTile(v->tile) && GetStationIndex(v->tile) == v->current_order.GetDestination()) ProcessOrders(v);
                        break;

                case OT_NOTHING:
                case OT_LEAVESTATION:
                case OT_LOADING:
                        /* Exit if the current order doesn't have a destination, but the train has orders. */
                        if (v->GetNumOrders() > 0) return;
                        break;

                default:
                        break;
        }
        /* Exit if we are on a station tile and are going to stop. */
        if (IsRailStationTile(v->tile) && v->current_order.ShouldStopAtStation(v, GetStationIndex(v->tile))) return;

        Trackdir td = v->GetVehicleTrackdir();

        /* On a tile with a red non-pbs signal, don't look ahead. */
        if (IsTileType(v->tile, MP_RAILWAY) && HasSignalOnTrackdir(v->tile, td) &&
                        !IsPbsSignal(GetSignalType(v->tile, TrackdirToTrack(td))) &&
                        GetSignalStateByTrackdir(v->tile, td) == SIGNAL_STATE_RED) return;

        CFollowTrackRail ft(v);
        if (!ft.Follow(v->tile, td)) return;

        if (!HasReservedTracks(ft.m_new_tile, TrackdirBitsToTrackBits(ft.m_new_td_bits))) {
                /* Next tile is not reserved. */
                if (KillFirstBit(ft.m_new_td_bits) == TRACKDIR_BIT_NONE) {
                        if (HasPbsSignalOnTrackdir(ft.m_new_tile, FindFirstTrackdir(ft.m_new_td_bits))) {
                                /* If the next tile is a PBS signal, try to make a reservation. */
                                TrackBits tracks = TrackdirBitsToTrackBits(ft.m_new_td_bits);
                                if (Rail90DegTurnDisallowed(GetTileRailType(ft.m_old_tile), GetTileRailType(ft.m_new_tile))) {
                                        tracks &= ~TrackCrossesTracks(TrackdirToTrack(ft.m_old_td));
                                }
                                ChooseTrainTrack(v, ft.m_new_tile, ft.m_exitdir, tracks, false, nullptr, false);
                        }
                }
        }
}

/**
 * Will the train stay in the depot the next tick?
 * @param v %Train to check.
 * @return True if it stays in the depot, false otherwise.
 */
static bool CheckTrainStayInDepot(Train *v)
{
        /* bail out if not all wagons are in the same depot or not in a depot at all */
        for (const Train *u = v; u != nullptr; u = u->Next()) {
                if (u->track != TRACK_BIT_DEPOT || u->tile != v->tile) return false;
        }

        /* if the train got no power, then keep it in the depot */
        if (v->gcache.cached_power == 0) {
                v->vehstatus |= VS_STOPPED;
                SetWindowDirty(WC_VEHICLE_DEPOT, v->tile);
                return true;
        }

        SigSegState seg_state;

        if (v->force_proceed == TFP_NONE) {
                /* force proceed was not pressed */
                if (++v->wait_counter < 37) {
                        SetWindowClassesDirty(WC_TRAINS_LIST);
                        return true;
                }

                v->wait_counter = 0;

                seg_state = _settings_game.pf.reserve_paths ? SIGSEG_PBS : UpdateSignalsOnSegment(v->tile, INVALID_DIAGDIR, v->owner);
                if (seg_state == SIGSEG_FULL || HasDepotReservation(v->tile)) {
                        /* Full and no PBS signal in block or depot reserved, can't exit. */
                        SetWindowClassesDirty(WC_TRAINS_LIST);
                        return true;
                }
        } else {
                seg_state = _settings_game.pf.reserve_paths ? SIGSEG_PBS : UpdateSignalsOnSegment(v->tile, INVALID_DIAGDIR, v->owner);
        }

        /* We are leaving a depot, but have to go to the exact same one; re-enter. */
        if (v->current_order.IsType(OT_GOTO_DEPOT) && v->tile == v->dest_tile) {
                /* Service when depot has no reservation. */
                if (!HasDepotReservation(v->tile)) VehicleEnterDepot(v);
                return true;
        }

        /* Only leave when we can reserve a path to our destination. */
        if (seg_state == SIGSEG_PBS && !TryPathReserve(v) && v->force_proceed == TFP_NONE) {
                /* No path and no force proceed. */
                SetWindowClassesDirty(WC_TRAINS_LIST);
                MarkTrainAsStuck(v);
                return true;
        }

        SetDepotReservation(v->tile, true);
        if (_settings_client.gui.show_track_reservation) MarkTileDirtyByTile(v->tile);

        VehicleServiceInDepot(v);
        SetWindowClassesDirty(WC_TRAINS_LIST);
        v->PlayLeaveStationSound();

        v->track = TRACK_BIT_X;
        if (v->direction & 2) v->track = TRACK_BIT_Y;

        v->vehstatus &= ~VS_HIDDEN;
        v->cur_speed = 0;

        v->UpdateViewport(true, true);
        v->UpdatePosition();
        UpdateSignalsOnSegment(v->tile, INVALID_DIAGDIR, v->owner);
        v->UpdateAcceleration();
        InvalidateWindowData(WC_VEHICLE_DEPOT, v->tile);

        return false;
}

/**
 * Clear the reservation of \a tile that was just left by a wagon on \a track_dir.
 * @param v %Train owning the reservation.
 * @param tile Tile with reservation to clear.
 * @param track_dir Track direction to clear.
 */
static void ClearPathReservation(const Train *v, TileIndex tile, Trackdir track_dir)
{
        DiagDirection dir = TrackdirToExitdir(track_dir);

        if (IsTileType(tile, MP_TUNNELBRIDGE)) {
                /* Are we just leaving a tunnel/bridge? */
                if (GetTunnelBridgeDirection(tile) == ReverseDiagDir(dir)) {
                        TileIndex end = GetOtherTunnelBridgeEnd(tile);

                        if (TunnelBridgeIsFree(tile, end, v).Succeeded()) {
                                /* Free the reservation only if no other train is on the tiles. */
                                SetTunnelBridgeReservation(tile, false);
                                SetTunnelBridgeReservation(end, false);

                                if (_settings_client.gui.show_track_reservation) {
                                        if (IsBridge(tile)) {
                                                MarkBridgeDirty(tile);
                                        } else {
                                                MarkTileDirtyByTile(tile);
                                                MarkTileDirtyByTile(end);
                                        }
                                }
                        }
                }
        } else if (IsRailStationTile(tile)) {
                TileIndex new_tile = TileAddByDiagDir(tile, dir);
                /* If the new tile is not a further tile of the same station, we
                 * clear the reservation for the whole platform. */
                if (!IsCompatibleTrainStationTile(new_tile, tile)) {
                        SetRailStationPlatformReservation(tile, ReverseDiagDir(dir), false);
                }
        } else {
                /* Any other tile */
                UnreserveRailTrack(tile, TrackdirToTrack(track_dir));
        }
}

/**
 * Free the reserved path in front of a vehicle.
 * @param v %Train owning the reserved path.
 */
void FreeTrainTrackReservation(const Train *v)
{
        assert(v->IsFrontEngine());

        TileIndex tile = v->tile;
        Trackdir  td = v->GetVehicleTrackdir();
        bool      free_tile = !(IsRailStationTile(v->tile) || IsTileType(v->tile, MP_TUNNELBRIDGE));
        StationID station_id = IsRailStationTile(v->tile) ? GetStationIndex(v->tile) : INVALID_STATION;

        /* Can't be holding a reservation if we enter a depot. */
        if (IsRailDepotTile(tile) && TrackdirToExitdir(td) != GetRailDepotDirection(tile)) return;
        if (v->track == TRACK_BIT_DEPOT) {
                /* Front engine is in a depot. We enter if some part is not in the depot. */
                for (const Train *u = v; u != nullptr; u = u->Next()) {
                        if (u->track != TRACK_BIT_DEPOT || u->tile != v->tile) return;
                }
        }
        /* Don't free reservation if it's not ours. */
        if (TracksOverlap(GetReservedTrackbits(tile) | TrackToTrackBits(TrackdirToTrack(td)))) return;

        CFollowTrackRail ft(v, GetRailTypeInfo(v->railtype)->compatible_railtypes);
        while (ft.Follow(tile, td)) {
                tile = ft.m_new_tile;
                TrackdirBits bits = ft.m_new_td_bits & TrackBitsToTrackdirBits(GetReservedTrackbits(tile));
                td = RemoveFirstTrackdir(&bits);
                assert(bits == TRACKDIR_BIT_NONE);

                if (!IsValidTrackdir(td)) break;

                if (IsTileType(tile, MP_RAILWAY)) {
                        if (HasSignalOnTrackdir(tile, td) && !IsPbsSignal(GetSignalType(tile, TrackdirToTrack(td)))) {
                                /* Conventional signal along trackdir: remove reservation and stop. */
                                UnreserveRailTrack(tile, TrackdirToTrack(td));
                                break;
                        }
                        if (HasPbsSignalOnTrackdir(tile, td)) {
                                if (GetSignalStateByTrackdir(tile, td) == SIGNAL_STATE_RED) {
                                        /* Red PBS signal? Can't be our reservation, would be green then. */
                                        break;
                                } else {
                                        /* Turn the signal back to red. */
                                        SetSignalStateByTrackdir(tile, td, SIGNAL_STATE_RED);
                                        MarkTileDirtyByTile(tile);
                                }
                        } else if (HasSignalOnTrackdir(tile, ReverseTrackdir(td)) && IsOnewaySignal(tile, TrackdirToTrack(td))) {
                                break;
                        }
                }

                /* Don't free first station/bridge/tunnel if we are on it. */
                if (free_tile || (!(ft.m_is_station && GetStationIndex(ft.m_new_tile) == station_id) && !ft.m_is_tunnel && !ft.m_is_bridge)) ClearPathReservation(v, tile, td);

                free_tile = true;
        }
}

static const byte _initial_tile_subcoord[6][4][3] = {
{{ 15, 8, 1 }, { 0, 0, 0 }, { 0, 8, 5 }, { 0,  0, 0 }},
{{  0, 0, 0 }, { 8, 0, 3 }, { 0, 0, 0 }, { 8, 15, 7 }},
{{  0, 0, 0 }, { 7, 0, 2 }, { 0, 7, 6 }, { 0,  0, 0 }},
{{ 15, 8, 2 }, { 0, 0, 0 }, { 0, 0, 0 }, { 8, 15, 6 }},
{{ 15, 7, 0 }, { 8, 0, 4 }, { 0, 0, 0 }, { 0,  0, 0 }},
{{  0, 0, 0 }, { 0, 0, 0 }, { 0, 8, 4 }, { 7, 15, 0 }},
};

/**
 * Perform pathfinding for a train.
 *
 * @param v The train
 * @param tile The tile the train is about to enter
 * @param enterdir Diagonal direction the train is coming from
 * @param tracks Usable tracks on the new tile
 * @param[out] path_found Whether a path has been found or not.
 * @param do_track_reservation Path reservation is requested
 * @param[out] dest State and destination of the requested path
 * @return The best track the train should follow
 */
static Track DoTrainPathfind(const Train *v, TileIndex tile, DiagDirection enterdir, TrackBits tracks, bool &path_found, bool do_track_reservation, PBSTileInfo *dest)
{
        switch (_settings_game.pf.pathfinder_for_trains) {
                case VPF_NPF: return NPFTrainChooseTrack(v, path_found, do_track_reservation, dest);
                case VPF_YAPF: return YapfTrainChooseTrack(v, tile, enterdir, tracks, path_found, do_track_reservation, dest);

                default: NOT_REACHED();
        }
}

/**
 * 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.
 */
static PBSTileInfo ExtendTrainReservation(const Train *v, TrackBits *new_tracks, DiagDirection *enterdir)
{
        PBSTileInfo origin = FollowTrainReservation(v);

        CFollowTrackRail ft(v);

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

                if (Rail90DegTurnDisallowed(GetTileRailType(ft.m_old_tile), GetTileRailType(ft.m_new_tile))) {
                        ft.m_new_td_bits &= ~TrackdirCrossesTrackdirs(ft.m_old_td);
                        if (ft.m_new_td_bits == TRACKDIR_BIT_NONE) break;
                }

                /* Station, depot or waypoint are a possible target. */
                bool target_seen = ft.m_is_station || (IsTileType(ft.m_new_tile, MP_RAILWAY) && !IsPlainRail(ft.m_new_tile));
                if (target_seen || KillFirstBit(ft.m_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.m_new_tile, TrackdirBitsToTrackBits(TrackdirReachesTrackdirs(ft.m_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.m_tiles_skipped != 0) ft.m_new_tile -= TileOffsByDiagDir(ft.m_exitdir) * ft.m_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.m_new_td_bits);
                        if (enterdir != nullptr) *enterdir = ft.m_exitdir;
                        return PBSTileInfo(ft.m_new_tile, ft.m_old_td, false);
                }

                tile = ft.m_new_tile;
                cur_td = FindFirstTrackdir(ft.m_new_td_bits);

                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;
                        /* Safe position is all good, path valid and okay. */
                        return PBSTileInfo(tile, cur_td, true);
                }

                if (!TryReserveRailTrack(tile, TrackdirToTrack(cur_td))) break;
        }

        if (ft.m_err == CFollowTrackRail::EC_OWNER || ft.m_err == CFollowTrackRail::EC_NO_WAY) {
                /* End of line, path valid and okay. */
                return PBSTileInfo(ft.m_old_tile, ft.m_old_td, true);
        }

        /* Sorry, can't reserve path, back out. */
        tile = origin.tile;
        cur_td = origin.trackdir;
        TileIndex stopped = ft.m_old_tile;
        Trackdir  stopped_td = ft.m_old_td;
        while (tile != stopped || cur_td != stopped_td) {
                if (!ft.Follow(tile, cur_td)) break;

                if (Rail90DegTurnDisallowed(GetTileRailType(ft.m_old_tile), GetTileRailType(ft.m_new_tile))) {
                        ft.m_new_td_bits &= ~TrackdirCrossesTrackdirs(ft.m_old_td);
                        assert(ft.m_new_td_bits != TRACKDIR_BIT_NONE);
                }
                assert(KillFirstBit(ft.m_new_td_bits) == TRACKDIR_BIT_NONE);

                tile = ft.m_new_tile;
                cur_td = FindFirstTrackdir(ft.m_new_td_bits);

                UnreserveRailTrack(tile, TrackdirToTrack(cur_td));
        }

        /* Path invalid. */
        return PBSTileInfo();
}

/**
 * Try to reserve any path to a safe tile, ignoring the vehicle's destination.
 * Safe tiles are tiles in front of a signal, depots and station tiles at end of line.
 *
 * @param v The vehicle.
 * @param tile The tile the search should start from.
 * @param td The trackdir the search should start from.
 * @param override_railtype Whether all physically compatible railtypes should be followed.
 * @return True if a path to a safe stopping tile could be reserved.
 */
static bool TryReserveSafeTrack(const Train *v, TileIndex tile, Trackdir td, bool override_railtype)
{
        switch (_settings_game.pf.pathfinder_for_trains) {
                case VPF_NPF: return NPFTrainFindNearestSafeTile(v, tile, td, override_railtype);
                case VPF_YAPF: return YapfTrainFindNearestSafeTile(v, tile, td, override_railtype);

                default: NOT_REACHED();
        }
}

/** This class will save the current order of a vehicle and restore it on destruction. */
class VehicleOrderSaver {
private:
        Train          *v;
        Order          old_order;
        TileIndex      old_dest_tile;
        StationID      old_last_station_visited;
        VehicleOrderID index;
        bool           suppress_implicit_orders;

public:
        VehicleOrderSaver(Train *_v) :
                v(_v),
                old_order(_v->current_order),
                old_dest_tile(_v->dest_tile),
                old_last_station_visited(_v->last_station_visited),
                index(_v->cur_real_order_index),
                suppress_implicit_orders(HasBit(_v->gv_flags, GVF_SUPPRESS_IMPLICIT_ORDERS))
        {
        }

        ~VehicleOrderSaver()
        {
                this->v->current_order = this->old_order;
                this->v->dest_tile = this->old_dest_tile;
                this->v->last_station_visited = this->old_last_station_visited;
                SB(this->v->gv_flags, GVF_SUPPRESS_IMPLICIT_ORDERS, 1, suppress_implicit_orders ? 1: 0);
        }

        /**
         * Set the current vehicle order to the next order in the order list.
         * @param skip_first Shall the first (i.e. active) order be skipped?
         * @return True if a suitable next order could be found.
         */
        bool SwitchToNextOrder(bool skip_first)
        {
                if (this->v->GetNumOrders() == 0) return false;

                if (skip_first) ++this->index;

                int depth = 0;

                do {
                        /* Wrap around. */
                        if (this->index >= this->v->GetNumOrders()) this->index = 0;

                        Order *order = this->v->GetOrder(this->index);
                        assert(order != nullptr);

                        switch (order->GetType()) {
                                case OT_GOTO_DEPOT:
                                        /* Skip service in depot orders when the train doesn't need service. */
                                        if ((order->GetDepotOrderType() & ODTFB_SERVICE) && !this->v->NeedsServicing()) break;
                                        FALLTHROUGH;
                                case OT_GOTO_STATION:
                                case OT_GOTO_WAYPOINT:
                                        this->v->current_order = *order;
                                        return UpdateOrderDest(this->v, order, 0, true);
                                case OT_CONDITIONAL: {
                                        VehicleOrderID next = ProcessConditionalOrder(order, this->v);
                                        if (next != INVALID_VEH_ORDER_ID) {
                                                depth++;
                                                this->index = next;
                                                /* Don't increment next, so no break here. */
                                                continue;
                                        }
                                        break;
                                }
                                default:
                                        break;
                        }
                        /* Don't increment inside the while because otherwise conditional
                         * orders can lead to an infinite loop. */
                        ++this->index;
                        depth++;
                } while (this->index != this->v->cur_real_order_index && depth < this->v->GetNumOrders());

                return false;
        }
};

/* choose a track */
static Track ChooseTrainTrack(Train *v, TileIndex tile, DiagDirection enterdir, TrackBits tracks, bool force_res, bool *got_reservation, bool mark_stuck)
{
        Track best_track = INVALID_TRACK;
        bool do_track_reservation = _settings_game.pf.reserve_paths || force_res;
        bool changed_signal = false;

        assert((tracks & ~TRACK_BIT_MASK) == 0);

        if (got_reservation != nullptr) *got_reservation = false;

        /* Don't use tracks here as the setting to forbid 90 deg turns might have been switched between reservation and now. */
        TrackBits res_tracks = (TrackBits)(GetReservedTrackbits(tile) & DiagdirReachesTracks(enterdir));
        /* Do we have a suitable reserved track? */
        if (res_tracks != TRACK_BIT_NONE) return FindFirstTrack(res_tracks);

        /* Quick return in case only one possible track is available */
        if (KillFirstBit(tracks) == TRACK_BIT_NONE) {
                Track track = FindFirstTrack(tracks);
                /* We need to check for signals only here, as a junction tile can't have signals. */
                if (track != INVALID_TRACK && HasPbsSignalOnTrackdir(tile, TrackEnterdirToTrackdir(track, enterdir))) {
                        do_track_reservation = true;
                        changed_signal = true;
                        SetSignalStateByTrackdir(tile, TrackEnterdirToTrackdir(track, enterdir), SIGNAL_STATE_GREEN);
                } else if (!do_track_reservation) {
                        return track;
                }
                best_track = track;
        }

        PBSTileInfo   res_dest(tile, INVALID_TRACKDIR, false);
        DiagDirection dest_enterdir = enterdir;
        if (do_track_reservation) {
                res_dest = ExtendTrainReservation(v, &tracks, &dest_enterdir);
                if (res_dest.tile == INVALID_TILE) {
                        /* Reservation failed? */
                        if (mark_stuck) MarkTrainAsStuck(v);
                        if (changed_signal) SetSignalStateByTrackdir(tile, TrackEnterdirToTrackdir(best_track, enterdir), SIGNAL_STATE_RED);
                        return FindFirstTrack(tracks);
                }
                if (res_dest.okay) {
                        /* Got a valid reservation that ends at a safe target, quick exit. */
                        if (got_reservation != nullptr) *got_reservation = true;
                        if (changed_signal) MarkTileDirtyByTile(tile);
                        TryReserveRailTrack(v->tile, TrackdirToTrack(v->GetVehicleTrackdir()));
                        return best_track;
                }

                /* Check if the train needs service here, so it has a chance to always find a depot.
                 * Also check if the current order is a service order so we don't reserve a path to
                 * the destination but instead to the next one if service isn't needed. */
                CheckIfTrainNeedsService(v);
                if (v->current_order.IsType(OT_DUMMY) || v->current_order.IsType(OT_CONDITIONAL) || v->current_order.IsType(OT_GOTO_DEPOT)) ProcessOrders(v);
        }

        /* Save the current train order. The destructor will restore the old order on function exit. */
        VehicleOrderSaver orders(v);

        /* If the current tile is the destination of the current order and
         * a reservation was requested, advance to the next order.
         * Don't advance on a depot order as depots are always safe end points
         * for a path and no look-ahead is necessary. This also avoids a
         * problem with depot orders not part of the order list when the
         * order list itself is empty. */
        if (v->current_order.IsType(OT_LEAVESTATION)) {
                orders.SwitchToNextOrder(false);
        } else if (v->current_order.IsType(OT_LOADING) || (!v->current_order.IsType(OT_GOTO_DEPOT) && (
                        v->current_order.IsType(OT_GOTO_STATION) ?
                        IsRailStationTile(v->tile) && v->current_order.GetDestination() == GetStationIndex(v->tile) :
                        v->tile == v->dest_tile))) {
                orders.SwitchToNextOrder(true);
        }

        if (res_dest.tile != INVALID_TILE && !res_dest.okay) {
                /* Pathfinders are able to tell that route was only 'guessed'. */
                bool      path_found = true;
                TileIndex new_tile = res_dest.tile;

                Track next_track = DoTrainPathfind(v, new_tile, dest_enterdir, tracks, path_found, do_track_reservation, &res_dest);
                if (new_tile == tile) best_track = next_track;
                v->HandlePathfindingResult(path_found);
        }

        /* No track reservation requested -> finished. */
        if (!do_track_reservation) return best_track;

        /* A path was found, but could not be reserved. */
        if (res_dest.tile != INVALID_TILE && !res_dest.okay) {
                if (mark_stuck) MarkTrainAsStuck(v);
                FreeTrainTrackReservation(v);
                return best_track;
        }

        /* No possible reservation target found, we are probably lost. */
        if (res_dest.tile == INVALID_TILE) {
                /* Try to find any safe destination. */
                PBSTileInfo origin = FollowTrainReservation(v);
                if (TryReserveSafeTrack(v, origin.tile, origin.trackdir, false)) {
                        TrackBits res = GetReservedTrackbits(tile) & DiagdirReachesTracks(enterdir);
                        best_track = FindFirstTrack(res);
                        TryReserveRailTrack(v->tile, TrackdirToTrack(v->GetVehicleTrackdir()));
                        if (got_reservation != nullptr) *got_reservation = true;
                        if (changed_signal) MarkTileDirtyByTile(tile);
                } else {
                        FreeTrainTrackReservation(v);
                        if (mark_stuck) MarkTrainAsStuck(v);
                }
                return best_track;
        }

        if (got_reservation != nullptr) *got_reservation = true;

        /* Reservation target found and free, check if it is safe. */
        while (!IsSafeWaitingPosition(v, res_dest.tile, res_dest.trackdir, true, _settings_game.pf.forbid_90_deg)) {
                /* Extend reservation until we have found a safe position. */
                DiagDirection exitdir = TrackdirToExitdir(res_dest.trackdir);
                TileIndex     next_tile = TileAddByDiagDir(res_dest.tile, exitdir);
                TrackBits     reachable = TrackdirBitsToTrackBits((TrackdirBits)(GetTileTrackStatus(next_tile, TRANSPORT_RAIL, 0))) & DiagdirReachesTracks(exitdir);
                if (Rail90DegTurnDisallowed(GetTileRailType(res_dest.tile), GetTileRailType(next_tile))) {
                        reachable &= ~TrackCrossesTracks(TrackdirToTrack(res_dest.trackdir));
                }

                /* Get next order with destination. */
                if (orders.SwitchToNextOrder(true)) {
                        PBSTileInfo cur_dest;
                        bool path_found;
                        DoTrainPathfind(v, next_tile, exitdir, reachable, path_found, true, &cur_dest);
                        if (cur_dest.tile != INVALID_TILE) {
                                res_dest = cur_dest;
                                if (res_dest.okay) continue;
                                /* Path found, but could not be reserved. */
                                FreeTrainTrackReservation(v);
                                if (mark_stuck) MarkTrainAsStuck(v);
                                if (got_reservation != nullptr) *got_reservation = false;
                                changed_signal = false;
                                break;
                        }
                }
                /* No order or no safe position found, try any position. */
                if (!TryReserveSafeTrack(v, res_dest.tile, res_dest.trackdir, true)) {
                        FreeTrainTrackReservation(v);
                        if (mark_stuck) MarkTrainAsStuck(v);
                        if (got_reservation != nullptr) *got_reservation = false;
                        changed_signal = false;
                }
                break;
        }

        TryReserveRailTrack(v->tile, TrackdirToTrack(v->GetVehicleTrackdir()));

        if (changed_signal) MarkTileDirtyByTile(tile);

        return best_track;
}

/**
 * Try to reserve a path to a safe position.
 *
 * @param v The vehicle
 * @param mark_as_stuck Should the train be marked as stuck on a failed reservation?
 * @param first_tile_okay True if no path should be reserved if the current tile is a safe position.
 * @return True if a path could be reserved.
 */
bool TryPathReserve(Train *v, bool mark_as_stuck, bool first_tile_okay)
{
        assert(v->IsFrontEngine());

        /* We have to handle depots specially as the track follower won't look
         * at the depot tile itself but starts from the next tile. If we are still
         * inside the depot, a depot reservation can never be ours. */
        if (v->track == TRACK_BIT_DEPOT) {
                if (HasDepotReservation(v->tile)) {
                        if (mark_as_stuck) MarkTrainAsStuck(v);
                        return false;
                } else {
                        /* Depot not reserved, but the next tile might be. */
                        TileIndex next_tile = TileAddByDiagDir(v->tile, GetRailDepotDirection(v->tile));
                        if (HasReservedTracks(next_tile, DiagdirReachesTracks(GetRailDepotDirection(v->tile)))) return false;
                }
        }

        Vehicle *other_train = nullptr;
        PBSTileInfo origin = FollowTrainReservation(v, &other_train);
        /* The path we are driving on is already blocked by some other train.
         * This can only happen in certain situations when mixing path and
         * block signals or when changing tracks and/or signals.
         * Exit here as doing any further reservations will probably just
         * make matters worse. */
        if (other_train != nullptr && other_train->index != v->index) {
                if (mark_as_stuck) MarkTrainAsStuck(v);
                return false;
        }
        /* If we have a reserved path and the path ends at a safe tile, we are finished already. */
        if (origin.okay && (v->tile != origin.tile || first_tile_okay)) {
                /* Can't be stuck then. */
                if (HasBit(v->flags, VRF_TRAIN_STUCK)) SetWindowWidgetDirty(WC_VEHICLE_VIEW, v->index, WID_VV_START_STOP);
                ClrBit(v->flags, VRF_TRAIN_STUCK);
                return true;
        }

        /* If we are in a depot, tentatively reserve the depot. */
        if (v->track == TRACK_BIT_DEPOT) {
                SetDepotReservation(v->tile, true);
                if (_settings_client.gui.show_track_reservation) MarkTileDirtyByTile(v->tile);
        }

        DiagDirection exitdir = TrackdirToExitdir(origin.trackdir);
        TileIndex     new_tile = TileAddByDiagDir(origin.tile, exitdir);
        TrackBits     reachable = TrackdirBitsToTrackBits(TrackStatusToTrackdirBits(GetTileTrackStatus(new_tile, TRANSPORT_RAIL, 0)) & DiagdirReachesTrackdirs(exitdir));

        if (Rail90DegTurnDisallowed(GetTileRailType(origin.tile), GetTileRailType(new_tile))) reachable &= ~TrackCrossesTracks(TrackdirToTrack(origin.trackdir));

        bool res_made = false;
        ChooseTrainTrack(v, new_tile, exitdir, reachable, true, &res_made, mark_as_stuck);

        if (!res_made) {
                /* Free the depot reservation as well. */
                if (v->track == TRACK_BIT_DEPOT) SetDepotReservation(v->tile, false);
                return false;
        }

        if (HasBit(v->flags, VRF_TRAIN_STUCK)) {
                v->wait_counter = 0;
                SetWindowWidgetDirty(WC_VEHICLE_VIEW, v->index, WID_VV_START_STOP);
        }
        ClrBit(v->flags, VRF_TRAIN_STUCK);
        return true;
}


static bool CheckReverseTrain(const Train *v)
{
        if (_settings_game.difficulty.line_reverse_mode != 0 ||
                        v->track == TRACK_BIT_DEPOT || v->track == TRACK_BIT_WORMHOLE ||
                        !(v->direction & 1)) {
                return false;
        }

        assert(v->track != TRACK_BIT_NONE);

        switch (_settings_game.pf.pathfinder_for_trains) {
                case VPF_NPF: return NPFTrainCheckReverse(v);
                case VPF_YAPF: return YapfTrainCheckReverse(v);

                default: NOT_REACHED();
        }
}

/**
 * Get the location of the next station to visit.
 * @param station Next station to visit.
 * @return Location of the new station.
 */
TileIndex Train::GetOrderStationLocation(StationID station)
{
        if (station == this->last_station_visited) this->last_station_visited = INVALID_STATION;

        const Station *st = Station::Get(station);
        if (!(st->facilities & FACIL_TRAIN)) {
                /* The destination station has no trainstation tiles. */
                this->IncrementRealOrderIndex();
                return 0;
        }

        return st->xy;
}

/** Goods at the consist have changed, update the graphics, cargo, and acceleration. */
void Train::MarkDirty()
{
        Train *v = this;
        do {
                v->colourmap = PAL_NONE;
                v->UpdateViewport(true, false);
        } while ((v = v->Next()) != nullptr);

        /* need to update acceleration and cached values since the goods on the train changed. */
        this->CargoChanged();
        this->UpdateAcceleration();
}

/**
 * This function looks at the vehicle and updates its speed (cur_speed
 * and subspeed) variables. Furthermore, it returns the distance that
 * the train can drive this tick. #Vehicle::GetAdvanceDistance() determines
 * the distance to drive before moving a step on the map.
 * @return distance to drive.
 */
int Train::UpdateSpeed()
{
        switch (_settings_game.vehicle.train_acceleration_model) {
                default: NOT_REACHED();
                case AM_ORIGINAL:
                        return this->DoUpdateSpeed(this->acceleration * (this->GetAccelerationStatus() == AS_BRAKE ? -4 : 2), 0, this->GetCurrentMaxSpeed());

                case AM_REALISTIC:
                        return this->DoUpdateSpeed(this->GetAcceleration(), this->GetAccelerationStatus() == AS_BRAKE ? 0 : 2, this->GetCurrentMaxSpeed());
        }
}

/**
 * Trains enters a station, send out a news item if it is the first train, and start loading.
 * @param v Train that entered the station.
 * @param station Station visited.
 */
static void TrainEnterStation(Train *v, StationID station)
{
        v->last_station_visited = station;

        /* check if a train ever visited this station before */
        Station *st = Station::Get(station);
        if (!(st->had_vehicle_of_type & HVOT_TRAIN)) {
                st->had_vehicle_of_type |= HVOT_TRAIN;
                SetDParam(0, st->index);
                AddVehicleNewsItem(
                        STR_NEWS_FIRST_TRAIN_ARRIVAL,
                        v->owner == _local_company ? NT_ARRIVAL_COMPANY : NT_ARRIVAL_OTHER,
                        v->index,
                        st->index
                );
                AI::NewEvent(v->owner, new ScriptEventStationFirstVehicle(st->index, v->index));
                Game::NewEvent(new ScriptEventStationFirstVehicle(st->index, v->index));
        }

        v->force_proceed = TFP_NONE;
        SetWindowDirty(WC_VEHICLE_VIEW, v->index);

        v->BeginLoading();

        TriggerStationRandomisation(st, v->tile, SRT_TRAIN_ARRIVES);
        TriggerStationAnimation(st, v->tile, SAT_TRAIN_ARRIVES);
}

/* Check if the vehicle is compatible with the specified tile */
static inline bool CheckCompatibleRail(const Train *v, TileIndex tile)
{
        return IsTileOwner(tile, v->owner) &&
                        (!v->IsFrontEngine() || HasBit(v->compatible_railtypes, GetRailType(tile)));
}

/** Data structure for storing engine speed changes of an acceleration type. */
struct AccelerationSlowdownParams {
        byte small_turn; ///< Speed change due to a small turn.
        byte large_turn; ///< Speed change due to a large turn.
        byte z_up;       ///< Fraction to remove when moving up.
        byte z_down;     ///< Fraction to add when moving down.
};

/** Speed update fractions for each acceleration type. */
static const AccelerationSlowdownParams _accel_slowdown[] = {
        /* normal accel */
        {256 / 4, 256 / 2, 256 / 4, 2}, ///< normal
        {256 / 4, 256 / 2, 256 / 4, 2}, ///< monorail
        {0,       256 / 2, 256 / 4, 2}, ///< maglev
};

/**
 * Modify the speed of the vehicle due to a change in altitude.
 * @param v %Train to update.
 * @param old_z Previous height.
 */
static inline void AffectSpeedByZChange(Train *v, int old_z)
{
        if (old_z == v->z_pos || _settings_game.vehicle.train_acceleration_model != AM_ORIGINAL) return;

        const AccelerationSlowdownParams *asp = &_accel_slowdown[GetRailTypeInfo(v->railtype)->acceleration_type];

        if (old_z < v->z_pos) {
                v->cur_speed -= (v->cur_speed * asp->z_up >> 8);
        } else {
                uint16 spd = v->cur_speed + asp->z_down;
                if (spd <= v->gcache.cached_max_track_speed) v->cur_speed = spd;
        }
}

static bool TrainMovedChangeSignals(TileIndex tile, DiagDirection dir)
{
        if (IsTileType(tile, MP_RAILWAY) &&
                        GetRailTileType(tile) == RAIL_TILE_SIGNALS) {
                TrackdirBits tracks = TrackBitsToTrackdirBits(GetTrackBits(tile)) & DiagdirReachesTrackdirs(dir);
                Trackdir trackdir = FindFirstTrackdir(tracks);
                if (UpdateSignalsOnSegment(tile,  TrackdirToExitdir(trackdir), GetTileOwner(tile)) == SIGSEG_PBS && HasSignalOnTrackdir(tile, trackdir)) {
                        /* A PBS block with a non-PBS signal facing us? */
                        if (!IsPbsSignal(GetSignalType(tile, TrackdirToTrack(trackdir)))) return true;
                }
        }
        return false;
}

/** Tries to reserve track under whole train consist. */
void Train::ReserveTrackUnderConsist() const
{
        for (const Train *u = this; u != nullptr; u = u->Next()) {
                switch (u->track) {
                        case TRACK_BIT_WORMHOLE:
                                TryReserveRailTrack(u->tile, DiagDirToDiagTrack(GetTunnelBridgeDirection(u->tile)));
                                break;
                        case TRACK_BIT_DEPOT:
                                break;
                        default:
                                TryReserveRailTrack(u->tile, TrackBitsToTrack(u->track));
                                break;
                }
        }
}

/**
 * The train vehicle crashed!
 * Update its status and other parts around it.
 * @param flooded Crash was caused by flooding.
 * @return Number of people killed.
 */
uint Train::Crash(bool flooded)
{
        uint pass = 0;
        if (this->IsFrontEngine()) {
                pass += 2; // driver

                /* Remove the reserved path in front of the train if it is not stuck.
                 * Also clear all reserved tracks the train is currently on. */
                if (!HasBit(this->flags, VRF_TRAIN_STUCK)) FreeTrainTrackReservation(this);
                for (const Train *v = this; v != nullptr; v = v->Next()) {
                        ClearPathReservation(v, v->tile, v->GetVehicleTrackdir());
                        if (IsTileType(v->tile, MP_TUNNELBRIDGE)) {
                                /* ClearPathReservation will not free the wormhole exit
                                 * if the train has just entered the wormhole. */
                                SetTunnelBridgeReservation(GetOtherTunnelBridgeEnd(v->tile), false);
                        }
                }

                /* we may need to update crossing we were approaching,
                 * but must be updated after the train has been marked crashed */
                TileIndex crossing = TrainApproachingCrossingTile(this);
                if (crossing != INVALID_TILE) UpdateLevelCrossing(crossing);

                /* Remove the loading indicators (if any) */
                HideFillingPercent(&this->fill_percent_te_id);
        }

        pass += this->GroundVehicleBase::Crash(flooded);

        this->crash_anim_pos = flooded ? 4000 : 1; // max 4440, disappear pretty fast when flooded
        return pass;
}

/**
 * Marks train as crashed and creates an AI event.
 * Doesn't do anything if the train is crashed already.
 * @param v first vehicle of chain
 * @return number of victims (including 2 drivers; zero if train was already crashed)
 */
static uint TrainCrashed(Train *v)
{
        uint num = 0;

        /* do not crash train twice */
        if (!(v->vehstatus & VS_CRASHED)) {
                num = v->Crash();
                AI::NewEvent(v->owner, new ScriptEventVehicleCrashed(v->index, v->tile, ScriptEventVehicleCrashed::CRASH_TRAIN));
                Game::NewEvent(new ScriptEventVehicleCrashed(v->index, v->tile, ScriptEventVehicleCrashed::CRASH_TRAIN));
        }

        /* Try to re-reserve track under already crashed train too.
         * Crash() clears the reservation! */
        v->ReserveTrackUnderConsist();

        return num;
}

/** Temporary data storage for testing collisions. */
struct TrainCollideChecker {
        Train *v; ///< %Vehicle we are testing for collision.
        uint num; ///< Total number of victims if train collided.
};

/**
 * Collision test function.
 * @param v %Train vehicle to test collision with.
 * @param data %Train being examined.
 * @return \c nullptr (always continue search)
 */
static Vehicle *FindTrainCollideEnum(Vehicle *v, void *data)
{
        TrainCollideChecker *tcc = (TrainCollideChecker*)data;

        /* not a train or in depot */
        if (v->type != VEH_TRAIN || Train::From(v)->track == TRACK_BIT_DEPOT) return nullptr;

        /* do not crash into trains of another company. */
        if (v->owner != tcc->v->owner) return nullptr;

        /* get first vehicle now to make most usual checks faster */
        Train *coll = Train::From(v)->First();

        /* can't collide with own wagons */
        if (coll == tcc->v) return nullptr;

        int x_diff = v->x_pos - tcc->v->x_pos;
        int y_diff = v->y_pos - tcc->v->y_pos;

        /* Do fast calculation to check whether trains are not in close vicinity
         * and quickly reject trains distant enough for any collision.
         * Differences are shifted by 7, mapping range [-7 .. 8] into [0 .. 15]
         * Differences are then ORed and then we check for any higher bits */
        uint hash = (y_diff + 7) | (x_diff + 7);
        if (hash & ~15) return nullptr;

        /* Slower check using multiplication */
        int min_diff = (Train::From(v)->gcache.cached_veh_length + 1) / 2 + (tcc->v->gcache.cached_veh_length + 1) / 2 - 1;
        if (x_diff * x_diff + y_diff * y_diff > min_diff * min_diff) return nullptr;

        /* Happens when there is a train under bridge next to bridge head */
        if (abs(v->z_pos - tcc->v->z_pos) > 5) return nullptr;

        /* crash both trains */
        tcc->num += TrainCrashed(tcc->v);
        tcc->num += TrainCrashed(coll);

        return nullptr; // continue searching
}

/**
 * Checks whether the specified train has a collision with another vehicle. If
 * so, destroys this vehicle, and the other vehicle if its subtype has TS_Front.
 * Reports the incident in a flashy news item, modifies station ratings and
 * plays a sound.
 * @param v %Train to test.
 */
static bool CheckTrainCollision(Train *v)
{
        /* can't collide in depot */
        if (v->track == TRACK_BIT_DEPOT) return false;

        assert(v->track == TRACK_BIT_WORMHOLE || TileVirtXY(v->x_pos, v->y_pos) == v->tile);

        TrainCollideChecker tcc;
        tcc.v = v;
        tcc.num = 0;

        /* find colliding vehicles */
        if (v->track == TRACK_BIT_WORMHOLE) {
                FindVehicleOnPos(v->tile, &tcc, FindTrainCollideEnum);
                FindVehicleOnPos(GetOtherTunnelBridgeEnd(v->tile), &tcc, FindTrainCollideEnum);
        } else {
                FindVehicleOnPosXY(v->x_pos, v->y_pos, &tcc, FindTrainCollideEnum);
        }

        /* any dead -> no crash */
        if (tcc.num == 0) return false;

        SetDParam(0, tcc.num);
        AddTileNewsItem(STR_NEWS_TRAIN_CRASH, NT_ACCIDENT, v->tile);

        ModifyStationRatingAround(v->tile, v->owner, -160, 30);
        if (_settings_client.sound.disaster) SndPlayVehicleFx(SND_13_TRAIN_COLLISION, v);
        return true;
}

static Vehicle *CheckTrainAtSignal(Vehicle *v, void *data)
{
        if (v->type != VEH_TRAIN || (v->vehstatus & VS_CRASHED)) return nullptr;

        Train *t = Train::From(v);
        DiagDirection exitdir = *(DiagDirection *)data;

        /* not front engine of a train, inside wormhole or depot, crashed */
        if (!t->IsFrontEngine() || !(t->track & TRACK_BIT_MASK)) return nullptr;

        if (t->cur_speed > 5 || VehicleExitDir(t->direction, t->track) != exitdir) return nullptr;

        return t;
}

/**
 * Move a vehicle chain one movement stop forwards.
 * @param v First vehicle to move.
 * @param nomove Stop moving this and all following vehicles.
 * @param reverse Set to false to not execute the vehicle reversing. This does not change any other logic.
 * @return True if the vehicle could be moved forward, false otherwise.
 */
bool TrainController(Train *v, Vehicle *nomove, bool reverse)
{
        Train *first = v->First();
        Train *prev;
        bool direction_changed = false; // has direction of any part changed?

        /* For every vehicle after and including the given vehicle */
        for (prev = v->Previous(); v != nomove; prev = v, v = v->Next()) {
                DiagDirection enterdir = DIAGDIR_BEGIN;
                bool update_signals_crossing = false; // will we update signals or crossing state?

                GetNewVehiclePosResult gp = GetNewVehiclePos(v);
                if (v->track != TRACK_BIT_WORMHOLE) {
                        /* Not inside tunnel */
                        if (gp.old_tile == gp.new_tile) {
                                /* Staying in the old tile */
                                if (v->track == TRACK_BIT_DEPOT) {
                                        /* Inside depot */
                                        gp.x = v->x_pos;
                                        gp.y = v->y_pos;
                                } else {
                                        /* Not inside depot */

                                        /* Reverse when we are at the end of the track already, do not move to the new position */
                                        if (v->IsFrontEngine() && !TrainCheckIfLineEnds(v, reverse)) return false;

                                        uint32 r = VehicleEnterTile(v, gp.new_tile, gp.x, gp.y);
                                        if (HasBit(r, VETS_CANNOT_ENTER)) {
                                                goto invalid_rail;
                                        }
                                        if (HasBit(r, VETS_ENTERED_STATION)) {
                                                /* The new position is the end of the platform */
                                                TrainEnterStation(v, r >> VETS_STATION_ID_OFFSET);
                                        }
                                }
                        } else {
                                /* A new tile is about to be entered. */

                                /* Determine what direction we're entering the new tile from */
                                enterdir = DiagdirBetweenTiles(gp.old_tile, gp.new_tile);
                                assert(IsValidDiagDirection(enterdir));

                                /* Get the status of the tracks in the new tile and mask
                                 * away the bits that aren't reachable. */
                                TrackStatus ts = GetTileTrackStatus(gp.new_tile, TRANSPORT_RAIL, 0, ReverseDiagDir(enterdir));
                                TrackdirBits reachable_trackdirs = DiagdirReachesTrackdirs(enterdir);

                                TrackdirBits trackdirbits = TrackStatusToTrackdirBits(ts) & reachable_trackdirs;
                                TrackBits red_signals = TrackdirBitsToTrackBits(TrackStatusToRedSignals(ts) & reachable_trackdirs);

                                TrackBits bits = TrackdirBitsToTrackBits(trackdirbits);
                                if (Rail90DegTurnDisallowed(GetTileRailType(gp.old_tile), GetTileRailType(gp.new_tile)) && prev == nullptr) {
                                        /* We allow wagons to make 90 deg turns, because forbid_90_deg
                                         * can be switched on halfway a turn */
                                        bits &= ~TrackCrossesTracks(FindFirstTrack(v->track));
                                }

                                if (bits == TRACK_BIT_NONE) goto invalid_rail;

                                /* Check if the new tile constrains tracks that are compatible
                                 * with the current train, if not, bail out. */
                                if (!CheckCompatibleRail(v, gp.new_tile)) goto invalid_rail;

                                TrackBits chosen_track;
                                if (prev == nullptr) {
                                        /* Currently the locomotive is active. Determine which one of the
                                         * available tracks to choose */
                                        chosen_track = TrackToTrackBits(ChooseTrainTrack(v, gp.new_tile, enterdir, bits, false, nullptr, true));
                                        assert(chosen_track & (bits | GetReservedTrackbits(gp.new_tile)));

                                        if (v->force_proceed != TFP_NONE && IsPlainRailTile(gp.new_tile) && HasSignals(gp.new_tile)) {
                                                /* For each signal we find decrease the counter by one.
                                                 * We start at two, so the first signal we pass decreases
                                                 * this to one, then if we reach the next signal it is
                                                 * decreased to zero and we won't pass that new signal. */
                                                Trackdir dir = FindFirstTrackdir(trackdirbits);
                                                if (HasSignalOnTrackdir(gp.new_tile, dir) ||
                                                                (HasSignalOnTrackdir(gp.new_tile, ReverseTrackdir(dir)) &&
                                                                GetSignalType(gp.new_tile, TrackdirToTrack(dir)) != SIGTYPE_PBS)) {
                                                        /* However, we do not want to be stopped by PBS signals
                                                         * entered via the back. */
                                                        v->force_proceed = (v->force_proceed == TFP_SIGNAL) ? TFP_STUCK : TFP_NONE;
                                                        SetWindowDirty(WC_VEHICLE_VIEW, v->index);
                                                }
                                        }

                                        /* Check if it's a red signal and that force proceed is not clicked. */
                                        if ((red_signals & chosen_track) && v->force_proceed == TFP_NONE) {
                                                /* In front of a red signal */
                                                Trackdir i = FindFirstTrackdir(trackdirbits);

                                                /* Don't handle stuck trains here. */
                                                if (HasBit(v->flags, VRF_TRAIN_STUCK)) return false;

                                                if (!HasSignalOnTrackdir(gp.new_tile, ReverseTrackdir(i))) {
                                                        v->cur_speed = 0;
                                                        v->subspeed = 0;
                                                        v->progress = 255; // make sure that every bit of acceleration will hit the signal again, so speed stays 0.
                                                        if (!_settings_game.pf.reverse_at_signals || ++v->wait_counter < _settings_game.pf.wait_oneway_signal * DAY_TICKS * 2) return false;
                                                } else if (HasSignalOnTrackdir(gp.new_tile, i)) {
                                                        v->cur_speed = 0;
                                                        v->subspeed = 0;
                                                        v->progress = 255; // make sure that every bit of acceleration will hit the signal again, so speed stays 0.
                                                        if (!_settings_game.pf.reverse_at_signals || ++v->wait_counter < _settings_game.pf.wait_twoway_signal * DAY_TICKS * 2) {
                                                                DiagDirection exitdir = TrackdirToExitdir(i);
                                                                TileIndex o_tile = TileAddByDiagDir(gp.new_tile, exitdir);

                                                                exitdir = ReverseDiagDir(exitdir);

                                                                /* check if a train is waiting on the other side */
                                                                if (!HasVehicleOnPos(o_tile, &exitdir, &CheckTrainAtSignal)) return false;
                                                        }
                                                }

                                                /* If we would reverse but are currently in a PBS block and
                                                 * reversing of stuck trains is disabled, don't reverse.
                                                 * This does not apply if the reason for reversing is a one-way
                                                 * signal blocking us, because a train would then be stuck forever. */
                                                if (!_settings_game.pf.reverse_at_signals && !HasOnewaySignalBlockingTrackdir(gp.new_tile, i) &&
                                                                UpdateSignalsOnSegment(v->tile, enterdir, v->owner) == SIGSEG_PBS) {
                                                        v->wait_counter = 0;
                                                        return false;
                                                }
                                                goto reverse_train_direction;
                                        } else {
                                                TryReserveRailTrack(gp.new_tile, TrackBitsToTrack(chosen_track), false);
                                        }
                                } else {
                                        /* The wagon is active, simply follow the prev vehicle. */
                                        if (prev->tile == gp.new_tile) {
                                                /* Choose the same track as prev */
                                                if (prev->track == TRACK_BIT_WORMHOLE) {
                                                        /* Vehicles entering tunnels enter the wormhole earlier than for bridges.
                                                         * However, just choose the track into the wormhole. */
                                                        assert(IsTunnel(prev->tile));
                                                        chosen_track = bits;
                                                } else {
                                                        chosen_track = prev->track;
                                                }
                                        } else {
                                                /* Choose the track that leads to the tile where prev is.
                                                 * This case is active if 'prev' is already on the second next tile, when 'v' just enters the next tile.
                                                 * I.e. when the tile between them has only space for a single vehicle like
                                                 *  1) horizontal/vertical track tiles and
                                                 *  2) some orientations of tunnel entries, where the vehicle is already inside the wormhole at 8/16 from the tile edge.
                                                 *     Is also the train just reversing, the wagon inside the tunnel is 'on' the tile of the opposite tunnel entry.
                                                 */
                                                static const TrackBits _connecting_track[DIAGDIR_END][DIAGDIR_END] = {
                                                        {TRACK_BIT_X,     TRACK_BIT_LOWER, TRACK_BIT_NONE,  TRACK_BIT_LEFT },
                                                        {TRACK_BIT_UPPER, TRACK_BIT_Y,     TRACK_BIT_LEFT,  TRACK_BIT_NONE },
                                                        {TRACK_BIT_NONE,  TRACK_BIT_RIGHT, TRACK_BIT_X,     TRACK_BIT_UPPER},
                                                        {TRACK_BIT_RIGHT, TRACK_BIT_NONE,  TRACK_BIT_LOWER, TRACK_BIT_Y    }
                                                };
                                                DiagDirection exitdir = DiagdirBetweenTiles(gp.new_tile, prev->tile);
                                                assert(IsValidDiagDirection(exitdir));
                                                chosen_track = _connecting_track[enterdir][exitdir];
                                        }
                                        chosen_track &= bits;
                                }

                                /* Make sure chosen track is a valid track */
                                assert(
                                                chosen_track == TRACK_BIT_X     || chosen_track == TRACK_BIT_Y ||
                                                chosen_track == TRACK_BIT_UPPER || chosen_track == TRACK_BIT_LOWER ||
                                                chosen_track == TRACK_BIT_LEFT  || chosen_track == TRACK_BIT_RIGHT);

                                /* Update XY to reflect the entrance to the new tile, and select the direction to use */
                                const byte *b = _initial_tile_subcoord[FIND_FIRST_BIT(chosen_track)][enterdir];
                                gp.x = (gp.x & ~0xF) | b[0];
                                gp.y = (gp.y & ~0xF) | b[1];
                                Direction chosen_dir = (Direction)b[2];

                                /* Call the landscape function and tell it that the vehicle entered the tile */
                                uint32 r = VehicleEnterTile(v, gp.new_tile, gp.x, gp.y);
                                if (HasBit(r, VETS_CANNOT_ENTER)) {
                                        goto invalid_rail;
                                }

                                if (!HasBit(r, VETS_ENTERED_WORMHOLE)) {
                                        Track track = FindFirstTrack(chosen_track);
                                        Trackdir tdir = TrackDirectionToTrackdir(track, chosen_dir);
                                        if (v->IsFrontEngine() && HasPbsSignalOnTrackdir(gp.new_tile, tdir)) {
                                                SetSignalStateByTrackdir(gp.new_tile, tdir, SIGNAL_STATE_RED);
                                                MarkTileDirtyByTile(gp.new_tile);
                                        }

                                        /* Clear any track reservation when the last vehicle leaves the tile */
                                        if (v->Next() == nullptr) ClearPathReservation(v, v->tile, v->GetVehicleTrackdir());

                                        v->tile = gp.new_tile;

                                        if (GetTileRailType(gp.new_tile) != GetTileRailType(gp.old_tile)) {
                                                v->First()->ConsistChanged(CCF_TRACK);
                                        }

                                        v->track = chosen_track;
                                        assert(v->track);
                                }

                                /* We need to update signal status, but after the vehicle position hash
                                 * has been updated by UpdateInclination() */
                                update_signals_crossing = true;

                                if (chosen_dir != v->direction) {
                                        if (prev == nullptr && _settings_game.vehicle.train_acceleration_model == AM_ORIGINAL) {
                                                const AccelerationSlowdownParams *asp = &_accel_slowdown[GetRailTypeInfo(v->railtype)->acceleration_type];
                                                DirDiff diff = DirDifference(v->direction, chosen_dir);
                                                v->cur_speed -= (diff == DIRDIFF_45RIGHT || diff == DIRDIFF_45LEFT ? asp->small_turn : asp->large_turn) * v->cur_speed >> 8;
                                        }
                                        direction_changed = true;
                                        v->direction = chosen_dir;
                                }

                                if (v->IsFrontEngine()) {
                                        v->wait_counter = 0;

                                        /* If we are approaching a crossing that is reserved, play the sound now. */
                                        TileIndex crossing = TrainApproachingCrossingTile(v);
                                        if (crossing != INVALID_TILE && HasCrossingReservation(crossing) && _settings_client.sound.ambient) SndPlayTileFx(SND_0E_LEVEL_CROSSING, crossing);

                                        /* Always try to extend the reservation when entering a tile. */
                                        CheckNextTrainTile(v);
                                }

                                if (HasBit(r, VETS_ENTERED_STATION)) {
                                        /* The new position is the location where we want to stop */
                                        TrainEnterStation(v, r >> VETS_STATION_ID_OFFSET);
                                }
                        }
                } else {
                        if (IsTileType(gp.new_tile, MP_TUNNELBRIDGE) && HasBit(VehicleEnterTile(v, gp.new_tile, gp.x, gp.y), VETS_ENTERED_WORMHOLE)) {
                                /* Perform look-ahead on tunnel exit. */
                                if (v->IsFrontEngine()) {
                                        TryReserveRailTrack(gp.new_tile, DiagDirToDiagTrack(GetTunnelBridgeDirection(gp.new_tile)));
                                        CheckNextTrainTile(v);
                                }
                                /* Prevent v->UpdateInclination() being called with wrong parameters.
                                 * This could happen if the train was reversed inside the tunnel/bridge. */
                                if (gp.old_tile == gp.new_tile) {
                                        gp.old_tile = GetOtherTunnelBridgeEnd(gp.old_tile);
                                }
                        } else {
                                v->x_pos = gp.x;
                                v->y_pos = gp.y;
                                v->UpdatePosition();
                                if ((v->vehstatus & VS_HIDDEN) == 0) v->Vehicle::UpdateViewport(true);
                                continue;
                        }
                }

                /* update image of train, as well as delta XY */
                v->UpdateDeltaXY();

                v->x_pos = gp.x;
                v->y_pos = gp.y;
                v->UpdatePosition();

                /* update the Z position of the vehicle */
                int old_z = v->UpdateInclination(gp.new_tile != gp.old_tile, false);

                if (prev == nullptr) {
                        /* This is the first vehicle in the train */
                        AffectSpeedByZChange(v, old_z);
                }

                if (update_signals_crossing) {
                        if (v->IsFrontEngine()) {
                                if (TrainMovedChangeSignals(gp.new_tile, enterdir)) {
                                        /* We are entering a block with PBS signals right now, but
                                         * not through a PBS signal. This means we don't have a
                                         * reservation right now. As a conventional signal will only
                                         * ever be green if no other train is in the block, getting
                                         * a path should always be possible. If the player built
                                         * such a strange network that it is not possible, the train
                                         * will be marked as stuck and the player has to deal with
                                         * the problem. */
                                        if ((!HasReservedTracks(gp.new_tile, v->track) &&
                                                        !TryReserveRailTrack(gp.new_tile, FindFirstTrack(v->track))) ||
                                                        !TryPathReserve(v)) {
                                                MarkTrainAsStuck(v);
                                        }
                                }
                        }

                        /* Signals can only change when the first
                         * (above) or the last vehicle moves. */
                        if (v->Next() == nullptr) {
                                TrainMovedChangeSignals(gp.old_tile, ReverseDiagDir(enterdir));
                                if (IsLevelCrossingTile(gp.old_tile)) UpdateLevelCrossing(gp.old_tile);
                        }
                }

                /* Do not check on every tick to save some computing time. */
                if (v->IsFrontEngine() && v->tick_counter % _settings_game.pf.path_backoff_interval == 0) CheckNextTrainTile(v);
        }

        if (direction_changed) first->tcache.cached_max_curve_speed = first->GetCurveSpeedLimit();

        return true;

invalid_rail:
        /* We've reached end of line?? */
        if (prev != nullptr) error("Disconnecting train");

reverse_train_direction:
        if (reverse) {
                v->wait_counter = 0;
                v->cur_speed = 0;
                v->subspeed = 0;
                ReverseTrainDirection(v);
        }

        return false;
}

/**
 * Collect trackbits of all crashed train vehicles on a tile
 * @param v Vehicle passed from Find/HasVehicleOnPos()
 * @param data trackdirbits for the result
 * @return nullptr to iterate over all vehicles on the tile.
 */
static Vehicle *CollectTrackbitsFromCrashedVehiclesEnum(Vehicle *v, void *data)
{
        TrackBits *trackbits = (TrackBits *)data;

        if (v->type == VEH_TRAIN && (v->vehstatus & VS_CRASHED) != 0) {
                TrackBits train_tbits = Train::From(v)->track;
                if (train_tbits == TRACK_BIT_WORMHOLE) {
                        /* Vehicle is inside a wormhole, v->track contains no useful value then. */
                        *trackbits |= DiagDirToDiagTrackBits(GetTunnelBridgeDirection(v->tile));
                } else if (train_tbits != TRACK_BIT_DEPOT) {
                        *trackbits |= train_tbits;
                }
        }

        return nullptr;
}

/**
 * Deletes/Clears the last wagon of a crashed train. It takes the engine of the
 * train, then goes to the last wagon and deletes that. Each call to this function
 * will remove the last wagon of a crashed train. If this wagon was on a crossing,
 * or inside a tunnel/bridge, recalculate the signals as they might need updating
 * @param v the Vehicle of which last wagon is to be removed
 */
static void DeleteLastWagon(Train *v)
{
        Train *first = v->First();

        /* Go to the last wagon and delete the link pointing there
         * *u is then the one-before-last wagon, and *v the last
         * one which will physically be removed */
        Train *u = v;
        for (; v->Next() != nullptr; v = v->Next()) u = v;
        u->SetNext(nullptr);

        if (first != v) {
                /* Recalculate cached train properties */
                first->ConsistChanged(CCF_ARRANGE);
                /* Update the depot window if the first vehicle is in depot -
                 * if v == first, then it is updated in PreDestructor() */
                if (first->track == TRACK_BIT_DEPOT) {
                        SetWindowDirty(WC_VEHICLE_DEPOT, first->tile);
                }
                v->last_station_visited = first->last_station_visited; // for PreDestructor
        }

        /* 'v' shouldn't be accessed after it has been deleted */
        TrackBits trackbits = v->track;
        TileIndex tile = v->tile;
        Owner owner = v->owner;

        delete v;
        v = nullptr; // make sure nobody will try to read 'v' anymore

        if (trackbits == TRACK_BIT_WORMHOLE) {
                /* Vehicle is inside a wormhole, v->track contains no useful value then. */
                trackbits = DiagDirToDiagTrackBits(GetTunnelBridgeDirection(tile));
        }

        Track track = TrackBitsToTrack(trackbits);
        if (HasReservedTracks(tile, trackbits)) {
                UnreserveRailTrack(tile, track);

                /* If there are still crashed vehicles on the tile, give the track reservation to them */
                TrackBits remaining_trackbits = TRACK_BIT_NONE;
                FindVehicleOnPos(tile, &remaining_trackbits, CollectTrackbitsFromCrashedVehiclesEnum);

                /* It is important that these two are the first in the loop, as reservation cannot deal with every trackbit combination */
                assert(TRACK_BEGIN == TRACK_X && TRACK_Y == TRACK_BEGIN + 1);
                for (Track t : SetTrackBitIterator(remaining_trackbits)) TryReserveRailTrack(tile, t);
        }

        /* check if the wagon was on a road/rail-crossing */
        if (IsLevelCrossingTile(tile)) UpdateLevelCrossing(tile);

        /* Update signals */
        if (IsTileType(tile, MP_TUNNELBRIDGE) || IsRailDepotTile(tile)) {
                UpdateSignalsOnSegment(tile, INVALID_DIAGDIR, owner);
        } else {
                SetSignalsOnBothDir(tile, track, owner);
        }
}

/**
 * Rotate all vehicles of a (crashed) train chain randomly to animate the crash.
 * @param v First crashed vehicle.
 */
static void ChangeTrainDirRandomly(Train *v)
{
        static const DirDiff delta[] = {
                DIRDIFF_45LEFT, DIRDIFF_SAME, DIRDIFF_SAME, DIRDIFF_45RIGHT
        };

        do {
                /* We don't need to twist around vehicles if they're not visible */
                if (!(v->vehstatus & VS_HIDDEN)) {
                        v->direction = ChangeDir(v->direction, delta[GB(Random(), 0, 2)]);
                        /* Refrain from updating the z position of the vehicle when on
                         * a bridge, because UpdateInclination() will put the vehicle under
                         * the bridge in that case */
                        if (v->track != TRACK_BIT_WORMHOLE) {
                                v->UpdatePosition();
                                v->UpdateInclination(false, true);
                        } else {
                                v->UpdateViewport(false, true);
                        }
                }
        } while ((v = v->Next()) != nullptr);
}

/**
 * Handle a crashed train.
 * @param v First train vehicle.
 * @return %Vehicle chain still exists.
 */
static bool HandleCrashedTrain(Train *v)
{
        int state = ++v->crash_anim_pos;

        if (state == 4 && !(v->vehstatus & VS_HIDDEN)) {
                CreateEffectVehicleRel(v, 4, 4, 8, EV_EXPLOSION_LARGE);
        }

        uint32 r;
        if (state <= 200 && Chance16R(1, 7, r)) {
                int index = (r * 10 >> 16);

                Vehicle *u = v;
                do {
                        if (--index < 0) {
                                r = Random();

                                CreateEffectVehicleRel(u,
                                        GB(r,  8, 3) + 2,
                                        GB(r, 16, 3) + 2,
                                        GB(r,  0, 3) + 5,
                                        EV_EXPLOSION_SMALL);
                                break;
                        }
                } while ((u = u->Next()) != nullptr);
        }

        if (state <= 240 && !(v->tick_counter & 3)) ChangeTrainDirRandomly(v);

        if (state >= 4440 && !(v->tick_counter & 0x1F)) {
                bool ret = v->Next() != nullptr;
                DeleteLastWagon(v);
                return ret;
        }

        return true;
}

/** Maximum speeds for train that is broken down or approaching line end */
static const uint16 _breakdown_speeds[16] = {
        225, 210, 195, 180, 165, 150, 135, 120, 105, 90, 75, 60, 45, 30, 15, 15
};


/**
 * Train is approaching line end, slow down and possibly reverse
 *
 * @param v front train engine
 * @param signal not line end, just a red signal
 * @param reverse Set to false to not execute the vehicle reversing. This does not change any other logic.
 * @return true iff we did NOT have to reverse
 */
static bool TrainApproachingLineEnd(Train *v, bool signal, bool reverse)
{
        /* Calc position within the current tile */
        uint x = v->x_pos & 0xF;
        uint y = v->y_pos & 0xF;

        /* for diagonal directions, 'x' will be 0..15 -
         * for other directions, it will be 1, 3, 5, ..., 15 */
        switch (v->direction) {
                case DIR_N : x = ~x + ~y + 25; break;
                case DIR_NW: x = y;            FALLTHROUGH;
                case DIR_NE: x = ~x + 16;      break;
                case DIR_E : x = ~x + y + 9;   break;
                case DIR_SE: x = y;            break;
                case DIR_S : x = x + y - 7;    break;
                case DIR_W : x = ~y + x + 9;   break;
                default: break;
        }

        /* Do not reverse when approaching red signal. Make sure the vehicle's front
         * does not cross the tile boundary when we do reverse, but as the vehicle's
         * location is based on their center, use half a vehicle's length as offset.
         * Multiply the half-length by two for straight directions to compensate that
         * we only get odd x offsets there. */
        if (!signal && x + (v->gcache.cached_veh_length + 1) / 2 * (IsDiagonalDirection(v->direction) ? 1 : 2) >= TILE_SIZE) {
                /* we are too near the tile end, reverse now */
                v->cur_speed = 0;
                if (reverse) ReverseTrainDirection(v);
                return false;
        }

        /* slow down */
        v->vehstatus |= VS_TRAIN_SLOWING;
        uint16 break_speed = _breakdown_speeds[x & 0xF];
        if (break_speed < v->cur_speed) v->cur_speed = break_speed;

        return true;
}


/**
 * Determines whether train would like to leave the tile
 * @param v train to test
 * @return true iff vehicle is NOT entering or inside a depot or tunnel/bridge
 */
static bool TrainCanLeaveTile(const Train *v)
{
        /* Exit if inside a tunnel/bridge or a depot */
        if (v->track == TRACK_BIT_WORMHOLE || v->track == TRACK_BIT_DEPOT) return false;

        TileIndex tile = v->tile;

        /* entering a tunnel/bridge? */
        if (IsTileType(tile, MP_TUNNELBRIDGE)) {
                DiagDirection dir = GetTunnelBridgeDirection(tile);
                if (DiagDirToDir(dir) == v->direction) return false;
        }

        /* entering a depot? */
        if (IsRailDepotTile(tile)) {
                DiagDirection dir = ReverseDiagDir(GetRailDepotDirection(tile));
                if (DiagDirToDir(dir) == v->direction) return false;
        }

        return true;
}


/**
 * Determines whether train is approaching a rail-road crossing
 *   (thus making it barred)
 * @param v front engine of train
 * @return TileIndex of crossing the train is approaching, else INVALID_TILE
 * @pre v in non-crashed front engine
 */
static TileIndex TrainApproachingCrossingTile(const Train *v)
{
        assert(v->IsFrontEngine());
        assert(!(v->vehstatus & VS_CRASHED));

        if (!TrainCanLeaveTile(v)) return INVALID_TILE;

        DiagDirection dir = VehicleExitDir(v->direction, v->track);
        TileIndex tile = v->tile + TileOffsByDiagDir(dir);

        /* not a crossing || wrong axis || unusable rail (wrong type or owner) */
        if (!IsLevelCrossingTile(tile) || DiagDirToAxis(dir) == GetCrossingRoadAxis(tile) ||
                        !CheckCompatibleRail(v, tile)) {
                return INVALID_TILE;
        }

        return tile;
}


/**
 * Checks for line end. Also, bars crossing at next tile if needed
 *
 * @param v vehicle we are checking
 * @param reverse Set to false to not execute the vehicle reversing. This does not change any other logic.
 * @return true iff we did NOT have to reverse
 */
static bool TrainCheckIfLineEnds(Train *v, bool reverse)
{
        /* First, handle broken down train */

        int t = v->breakdown_ctr;
        if (t > 1) {
                v->vehstatus |= VS_TRAIN_SLOWING;

                uint16 break_speed = _breakdown_speeds[GB(~t, 4, 4)];
                if (break_speed < v->cur_speed) v->cur_speed = break_speed;
        } else {
                v->vehstatus &= ~VS_TRAIN_SLOWING;
        }

        if (!TrainCanLeaveTile(v)) return true;

        /* Determine the non-diagonal direction in which we will exit this tile */
        DiagDirection dir = VehicleExitDir(v->direction, v->track);
        /* Calculate next tile */
        TileIndex tile = v->tile + TileOffsByDiagDir(dir);

        /* Determine the track status on the next tile */
        TrackStatus ts = GetTileTrackStatus(tile, TRANSPORT_RAIL, 0, ReverseDiagDir(dir));
        TrackdirBits reachable_trackdirs = DiagdirReachesTrackdirs(dir);

        TrackdirBits trackdirbits = TrackStatusToTrackdirBits(ts) & reachable_trackdirs;
        TrackdirBits red_signals = TrackStatusToRedSignals(ts) & reachable_trackdirs;

        /* We are sure the train is not entering a depot, it is detected above */

        /* mask unreachable track bits if we are forbidden to do 90deg turns */
        TrackBits bits = TrackdirBitsToTrackBits(trackdirbits);
        if (Rail90DegTurnDisallowed(GetTileRailType(v->tile), GetTileRailType(tile))) {
                bits &= ~TrackCrossesTracks(FindFirstTrack(v->track));
        }

        /* no suitable trackbits at all || unusable rail (wrong type or owner) */
        if (bits == TRACK_BIT_NONE || !CheckCompatibleRail(v, tile)) {
                return TrainApproachingLineEnd(v, false, reverse);
        }

        /* approaching red signal */
        if ((trackdirbits & red_signals) != 0) return TrainApproachingLineEnd(v, true, reverse);

        /* approaching a rail/road crossing? then make it red */
        if (IsLevelCrossingTile(tile)) MaybeBarCrossingWithSound(tile);

        return true;
}


static bool TrainLocoHandler(Train *v, bool mode)
{
        /* train has crashed? */
        if (v->vehstatus & VS_CRASHED) {
                return mode ? true : HandleCrashedTrain(v); // 'this' can be deleted here
        }

        if (v->force_proceed != TFP_NONE) {
                ClrBit(v->flags, VRF_TRAIN_STUCK);
                SetWindowWidgetDirty(WC_VEHICLE_VIEW, v->index, WID_VV_START_STOP);
        }

        /* train is broken down? */
        if (v->HandleBreakdown()) return true;

        if (HasBit(v->flags, VRF_REVERSING) && v->cur_speed == 0) {
                ReverseTrainDirection(v);
        }

        /* exit if train is stopped */
        if ((v->vehstatus & VS_STOPPED) && v->cur_speed == 0) return true;

        bool valid_order = !v->current_order.IsType(OT_NOTHING) && v->current_order.GetType() != OT_CONDITIONAL;
        if (ProcessOrders(v) && CheckReverseTrain(v)) {
                v->wait_counter = 0;
                v->cur_speed = 0;
                v->subspeed = 0;
                ClrBit(v->flags, VRF_LEAVING_STATION);
                ReverseTrainDirection(v);
                return true;
        } else if (HasBit(v->flags, VRF_LEAVING_STATION)) {
                /* Try to reserve a path when leaving the station as we
                 * might not be marked as wanting a reservation, e.g.
                 * when an overlength train gets turned around in a station. */
                DiagDirection dir = VehicleExitDir(v->direction, v->track);
                if (IsRailDepotTile(v->tile) || IsTileType(v->tile, MP_TUNNELBRIDGE)) dir = INVALID_DIAGDIR;

                if (UpdateSignalsOnSegment(v->tile, dir, v->owner) == SIGSEG_PBS || _settings_game.pf.reserve_paths) {
                        TryPathReserve(v, true, true);
                }
                ClrBit(v->flags, VRF_LEAVING_STATION);
        }

        v->HandleLoading(mode);

        if (v->current_order.IsType(OT_LOADING)) return true;

        if (CheckTrainStayInDepot(v)) return true;

        if (!mode) v->ShowVisualEffect();

        /* We had no order but have an order now, do look ahead. */
        if (!valid_order && !v->current_order.IsType(OT_NOTHING)) {
                CheckNextTrainTile(v);
        }

        /* Handle stuck trains. */
        if (!mode && HasBit(v->flags, VRF_TRAIN_STUCK)) {
                ++v->wait_counter;

                /* Should we try reversing this tick if still stuck? */
                bool turn_around = v->wait_counter % (_settings_game.pf.wait_for_pbs_path * DAY_TICKS) == 0 && _settings_game.pf.reverse_at_signals;

                if (!turn_around && v->wait_counter % _settings_game.pf.path_backoff_interval != 0 && v->force_proceed == TFP_NONE) return true;
                if (!TryPathReserve(v)) {
                        /* Still stuck. */
                        if (turn_around) ReverseTrainDirection(v);

                        if (HasBit(v->flags, VRF_TRAIN_STUCK) && v->wait_counter > 2 * _settings_game.pf.wait_for_pbs_path * DAY_TICKS) {
                                /* Show message to player. */
                                if (_settings_client.gui.lost_vehicle_warn && v->owner == _local_company) {
                                        SetDParam(0, v->index);
                                        AddVehicleAdviceNewsItem(STR_NEWS_TRAIN_IS_STUCK, v->index);
                                }
                                v->wait_counter = 0;
                        }
                        /* Exit if force proceed not pressed, else reset stuck flag anyway. */
                        if (v->force_proceed == TFP_NONE) return true;
                        ClrBit(v->flags, VRF_TRAIN_STUCK);
                        v->wait_counter = 0;
                        SetWindowWidgetDirty(WC_VEHICLE_VIEW, v->index, WID_VV_START_STOP);
                }
        }

        if (v->current_order.IsType(OT_LEAVESTATION)) {
                v->current_order.Free();
                SetWindowWidgetDirty(WC_VEHICLE_VIEW, v->index, WID_VV_START_STOP);
                return true;
        }

        int j = v->UpdateSpeed();

        /* we need to invalidate the widget if we are stopping from 'Stopping 0 km/h' to 'Stopped' */
        if (v->cur_speed == 0 && (v->vehstatus & VS_STOPPED)) {
                /* If we manually stopped, we're not force-proceeding anymore. */
                v->force_proceed = TFP_NONE;
                SetWindowDirty(WC_VEHICLE_VIEW, v->index);
        }

        int adv_spd = v->GetAdvanceDistance();
        if (j < adv_spd) {
                /* if the vehicle has speed 0, update the last_speed field. */
                if (v->cur_speed == 0) v->SetLastSpeed();
        } else {
                TrainCheckIfLineEnds(v);
                /* Loop until the train has finished moving. */
                for (;;) {
                        j -= adv_spd;
                        TrainController(v, nullptr);
                        /* Don't continue to move if the train crashed. */
                        if (CheckTrainCollision(v)) break;
                        /* Determine distance to next map position */
                        adv_spd = v->GetAdvanceDistance();

                        /* No more moving this tick */
                        if (j < adv_spd || v->cur_speed == 0) break;

                        OrderType order_type = v->current_order.GetType();
                        /* Do not skip waypoints (incl. 'via' stations) when passing through at full speed. */
                        if ((order_type == OT_GOTO_WAYPOINT || order_type == OT_GOTO_STATION) &&
                                                (v->current_order.GetNonStopType() & ONSF_NO_STOP_AT_DESTINATION_STATION) &&
                                                IsTileType(v->tile, MP_STATION) &&
                                                v->current_order.GetDestination() == GetStationIndex(v->tile)) {
                                ProcessOrders(v);
                        }
                }
                v->SetLastSpeed();
        }

        for (Train *u = v; u != nullptr; u = u->Next()) {
                if ((u->vehstatus & VS_HIDDEN) != 0) continue;

                u->UpdateViewport(false, false);
        }

        if (v->progress == 0) v->progress = j; // Save unused spd for next time, if TrainController didn't set progress

        return true;
}

/**
 * Get running cost for the train consist.
 * @return Yearly running costs.
 */
Money Train::GetRunningCost() const
{
        Money cost = 0;
        const Train *v = this;

        do {
                const Engine *e = v->GetEngine();
                if (e->u.rail.running_cost_class == INVALID_PRICE) continue;

                uint cost_factor = GetVehicleProperty(v, PROP_TRAIN_RUNNING_COST_FACTOR, e->u.rail.running_cost);
                if (cost_factor == 0) continue;

                /* Halve running cost for multiheaded parts */
                if (v->IsMultiheaded()) cost_factor /= 2;

                cost += GetPrice(e->u.rail.running_cost_class, cost_factor, e->GetGRF());
        } while ((v = v->GetNextVehicle()) != nullptr);

        return cost;
}

/**
 * Update train vehicle data for a tick.
 * @return True if the vehicle still exists, false if it has ceased to exist (front of consists only).
 */
bool Train::Tick()
{
        PerformanceAccumulator framerate(PFE_GL_TRAINS);

        this->tick_counter++;

        if (this->IsFrontEngine()) {
                if (!(this->vehstatus & VS_STOPPED) || this->cur_speed > 0) this->running_ticks++;

                this->current_order_time++;

                if (!TrainLocoHandler(this, false)) return false;

                return TrainLocoHandler(this, true);
        } else if (this->IsFreeWagon() && (this->vehstatus & VS_CRASHED)) {
                /* Delete flooded standalone wagon chain */
                if (++this->crash_anim_pos >= 4400) {
                        delete this;
                        return false;
                }
        }

        return true;
}

/**
 * Check whether a train needs service, and if so, find a depot or service it.
 * @return v %Train to check.
 */
static void CheckIfTrainNeedsService(Train *v)
{
        if (Company::Get(v->owner)->settings.vehicle.servint_trains == 0 || !v->NeedsAutomaticServicing()) return;
        if (v->IsChainInDepot()) {
                VehicleServiceInDepot(v);
                return;
        }

        uint max_penalty;
        switch (_settings_game.pf.pathfinder_for_trains) {
                case VPF_NPF:  max_penalty = _settings_game.pf.npf.maximum_go_to_depot_penalty;  break;
                case VPF_YAPF: max_penalty = _settings_game.pf.yapf.maximum_go_to_depot_penalty; break;
                default: NOT_REACHED();
        }

        FindDepotData tfdd = FindClosestTrainDepot(v, max_penalty);
        /* Only go to the depot if it is not too far out of our way. */
        if (tfdd.best_length == UINT_MAX || tfdd.best_length > max_penalty) {
                if (v->current_order.IsType(OT_GOTO_DEPOT)) {
                        /* If we were already heading for a depot but it has
                         * suddenly moved farther away, we continue our normal
                         * schedule? */
                        v->current_order.MakeDummy();
                        SetWindowWidgetDirty(WC_VEHICLE_VIEW, v->index, WID_VV_START_STOP);
                }
                return;
        }

        DepotID depot = GetDepotIndex(tfdd.tile);

        if (v->current_order.IsType(OT_GOTO_DEPOT) &&
                        v->current_order.GetDestination() != depot &&
                        !Chance16(3, 16)) {
                return;
        }

        SetBit(v->gv_flags, GVF_SUPPRESS_IMPLICIT_ORDERS);
        v->current_order.MakeGoToDepot(depot, ODTFB_SERVICE);
        v->dest_tile = tfdd.tile;
        SetWindowWidgetDirty(WC_VEHICLE_VIEW, v->index, WID_VV_START_STOP);
}

/** Update day counters of the train vehicle. */
void Train::OnNewDay()
{
        AgeVehicle(this);

        if ((++this->day_counter & 7) == 0) DecreaseVehicleValue(this);

        if (this->IsFrontEngine()) {
                CheckVehicleBreakdown(this);

                CheckIfTrainNeedsService(this);

                CheckOrders(this);

                /* update destination */
                if (this->current_order.IsType(OT_GOTO_STATION)) {
                        TileIndex tile = Station::Get(this->current_order.GetDestination())->train_station.tile;
                        if (tile != INVALID_TILE) this->dest_tile = tile;
                }

                if (this->running_ticks != 0) {
                        /* running costs */
                        CommandCost cost(EXPENSES_TRAIN_RUN, this->GetRunningCost() * this->running_ticks / (DAYS_IN_YEAR  * DAY_TICKS));

                        this->profit_this_year -= cost.GetCost();
                        this->running_ticks = 0;

                        SubtractMoneyFromCompanyFract(this->owner, cost);

                        SetWindowDirty(WC_VEHICLE_DETAILS, this->index);
                        SetWindowClassesDirty(WC_TRAINS_LIST);
                }
        }
}

/**
 * Get the tracks of the train vehicle.
 * @return Current tracks of the vehicle.
 */
Trackdir Train::GetVehicleTrackdir() const
{
        if (this->vehstatus & VS_CRASHED) return INVALID_TRACKDIR;

        if (this->track == TRACK_BIT_DEPOT) {
                /* We'll assume the train is facing outwards */
                return DiagDirToDiagTrackdir(GetRailDepotDirection(this->tile)); // Train in depot
        }

        if (this->track == TRACK_BIT_WORMHOLE) {
                /* train in tunnel or on bridge, so just use its direction and assume a diagonal track */
                return DiagDirToDiagTrackdir(DirToDiagDir(this->direction));
        }

        return TrackDirectionToTrackdir(FindFirstTrack(this->track), this->direction);
}