/**
 * Frame when the 'enter tunnel' sound should be played. This is the second
 * frame on a tile, so the sound is played shortly after entering the tunnel
 * tile, while the vehicle is still visible.
 */
static const byte TUNNEL_SOUND_FRAME = 1;

/**
 * Frame when a vehicle should be hidden in a tunnel with a certain direction.
 * This differs per direction, because of visibility / bounding box issues.
 * Note that direction, in this case, is the direction of tunnel or bridge tile.
 * When entering a tunnel, hide the vehicle when it reaches the given frame.
 * When leaving a tunnel, show the vehicle when it is one frame further
 * to the 'outside', i.e. at (TILE_SIZE-1) - (frame) + 1
 * frame counts from the outside towards wormhole so frame (TILE_SIZE-1) is border to wormhole 
 */
extern const byte _tunnel_visibility_frame[DIAGDIR_END] = {TILE_SIZE - (TILE_SIZE / 4), TILE_SIZE / 2, TILE_SIZE / 2, TILE_SIZE - (TILE_SIZE / 4)};

static VehicleEnterTileStatus VehicleEnter_TunnelBridge(Vehicle *v, TileIndex tile, int x, int y)
{
	int z = GetSlopeZ(x, y) - v->z_pos;

	if (abs(z) > 2) return VETSB_CANNOT_ENTER;

	/* Direction into the wormhole */
	const DiagDirection dir = GetTunnelBridgeDirection(tile);
	byte pos = (DiagDirToAxis(dir) == AXIS_X ? x : y) & TILE_UNIT_MASK;
	/* Number of units moved by the vehicle since entering the tile */
	byte frame = (dir == DIAGDIR_NE || dir == DIAGDIR_NW) ? TILE_SIZE - 1 - pos : pos;

	if (IsTunnel(tile)) {
		/* Handle visibility */
		if (v->vehstatus & VS_HIDDEN) {
			if (frame + 1 == _tunnel_visibility_frame[dir] && z == 0) v->vehstatus &= ~VS_HIDDEN;
		} else {
			if (frame == _tunnel_visibility_frame[dir]) v->vehstatus |= VS_HIDDEN;

			if (frame == TUNNEL_SOUND_FRAME &&
					v->type == VEH_TRAIN &&
					v->Previous() == NULL &&
					DirToDiagDir(v->direction) == dir &&
					!PlayVehicleSound(v, VSE_TUNNEL) &&
					RailVehInfo(v->engine_type)->engclass == EC_STEAM
				) {
				SndPlayVehicleFx(SND_05_TRAIN_THROUGH_TUNNEL, v);
			}
		}
	} else { // IsBridge(tile)
		if (v->type != VEH_SHIP) {
			/* modify speed of vehicle */
			uint16 spd = GetBridgeSpec(GetBridgeType(tile))->speed;

			if (v->type == VEH_ROAD) spd *= 2;
			Vehicle *first = v->First();
			first->cur_speed = min(first->cur_speed, spd);
		}
	}

	if (frame == TILE_SIZE - 1) {
		/* If vehicle has wormhole flag leave wormhole and set flag to tell controller
		 * that vehicle is going to leave wormhole. */
		switch (v->type) {
			default: NOT_REACHED();
			case VEH_TRAIN: if (Train::From(v)->track       == TRACK_BIT_WORMHOLE) return VETSB_LEAVING_WORMHOLE; break;
			case VEH_ROAD : if (RoadVehicle::From(v)->state == RVSB_WORMHOLE)      return VETSB_LEAVING_WORMHOLE; break;
			case VEH_SHIP : if (Ship::From(v)->state        == TRACK_BIT_WORMHOLE) return VETSB_LEAVING_WORMHOLE; break;
		}
	}

	return VETSB_CONTINUE;
}