(svn r20079) -Codechange [FS#3922]: Add helper functions to deal with the 192-256-magic of vehicle movement.

src/aircraft_cmd.cpp

@@ -734,7 +734,8 @@ static int UpdateAircraftSpeed(Aircraft *v, uint speed_limit = SPEED_LIMIT_NONE,
 	/* Adjust distance moved by plane speed setting */
 	if (_settings_game.vehicle.plane_speed > 1) spd /= _settings_game.vehicle.plane_speed;
 
-	if (!(v->direction & 1)) spd = spd * 3 / 4;
+	/* Convert direction-indepenent speed into direction-dependent speed. (old movement method) */
+	spd = v->GetOldAdvanceSpeed(spd);
 
 	spd += v->progress;
 	v->progress = (byte)spd;

src/roadveh_cmd.cpp

@@ -756,6 +756,13 @@ static void RoadVehArrivesAt(const RoadVehicle *v, Station *st)
 	}
 }
 
+/**
+ * This function looks at the vehicle and updates its speed (cur_speed
+ * and subspeed) variables. Furthermore, it returns the distance that
+ * the vehicle can drive this tick. #Vehicle::GetAdvanceDistance() determines
+ * the distance to drive before moving a step on the map.
+ * @return distance to drive.
+ */
 static int RoadVehAccelerate(RoadVehicle *v)
 {
 	uint oldspeed = v->cur_speed;
@@ -786,8 +793,7 @@ static int RoadVehAccelerate(RoadVehicle *v)
 		}
 	}
 
-	/* Speed is scaled in the same manner as for trains. @see train_cmd.cpp */
-	int scaled_spd = spd * 3 >> 2;
+	int scaled_spd = v->GetAdvanceSpeed(spd);
 
 	scaled_spd += v->progress;
 	v->progress = 0;
@@ -1595,7 +1601,7 @@ static bool RoadVehController(RoadVehicle *v)
 	/* Check how far the vehicle needs to proceed */
 	int j = RoadVehAccelerate(v);
 
-	int adv_spd = (v->direction & 1) ? 192 : 256;
+	int adv_spd = v->GetAdvanceDistance();
 	bool blocked = false;
 	while (j >= adv_spd) {
 		j -= adv_spd;
@@ -1609,8 +1615,8 @@ static bool RoadVehController(RoadVehicle *v)
 		}
 		if (blocked) break;
 
-		/* 192 spd used for going straight, 256 for going diagonally. */
-		adv_spd = (v->direction & 1) ? 192 : 256;
+		/* Determine distance to next map position */
+		adv_spd = v->GetAdvanceDistance();
 
 		/* Test for a collision, but only if another movement will occur. */
 		if (j >= adv_spd && RoadVehCheckTrainCrash(v)) break;

src/ship_cmd.cpp

@@ -346,8 +346,8 @@ static bool ShipAccelerate(Vehicle *v)
 			SetWindowWidgetDirty(WC_VEHICLE_VIEW, v->index, VVW_WIDGET_START_STOP_VEH);
 	}
 
-	/* Decrease somewhat when turning */
-	if (!(v->direction & 1)) spd = spd * 3 / 4;
+	/* Convert direction-indepenent speed into direction-dependent speed. (old movement method) */
+	spd = v->GetOldAdvanceSpeed(spd);
 
 	if (spd == 0) return false;
 	if ((byte)++spd == 0) return true;

src/train_cmd.cpp

@@ -2839,10 +2839,8 @@ void Train::MarkDirty()
 /**
  * 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. This distance is expressed as 256 * n,
- * where n is the number of straight (long) tracks the train can
- * traverse. This means that moving along a straight track costs 256
- * "speed" and a diagonal track costs 192 "speed".
+ * 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()
@@ -2869,18 +2867,7 @@ int Train::UpdateSpeed()
 		this->cur_speed = spd = Clamp(this->cur_speed + ((int)spd >> 8), 0, tempmax);
 	}
 
-	/* Scale speed by 3/4. Previously this was only done when the train was
-	 * facing diagonally and would apply to however many moves the train made
-	 * regardless the of direction actually moved in. Now it is always scaled,
-	 * 256 spd is used to go straight and 192 is used to go diagonally
-	 * (3/4 of 256). This results in the same effect, but without the error the
-	 * previous method caused.
-	 *
-	 * The scaling is done in this direction and not by multiplying the amount
-	 * to be subtracted by 4/3 so that the leftover speed can be saved in a
-	 * byte in this->progress.
-	 */
-	int scaled_spd = spd * 3 >> 2;
+	int scaled_spd = this->GetAdvanceSpeed(spd);
 
 	scaled_spd += this->progress;
 	this->progress = 0; // set later in TrainLocoHandler or TrainController
@@ -3885,7 +3872,7 @@ static bool TrainLocoHandler(Train *v, bool mode)
 		SetWindowDirty(WC_VEHICLE_VIEW, v->index);
 	}
 
-	int adv_spd = (v->direction & 1) ? 192 : 256;
+	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) SetLastSpeed(v, v->cur_speed);
@@ -3897,8 +3884,8 @@ static bool TrainLocoHandler(Train *v, bool mode)
 			TrainController(v, NULL);
 			/* Don't continue to move if the train crashed. */
 			if (CheckTrainCollision(v)) break;
-			/* 192 spd used for going straight, 256 for going diagonally. */
-			adv_spd = (v->direction & 1) ? 192 : 256;
+			/* Determine distance to next map position */
+			adv_spd = v->GetAdvanceDistance();
 
 			/* No more moving this tick */
 			if (j < adv_spd || v->cur_speed == 0) break;

src/vehicle_base.h

@@ -226,6 +226,53 @@ public:
 	 */
 	virtual void UpdateDeltaXY(Direction direction) {}
 
+	/**
+	 * Determines the effective direction-specific vehicle movement speed.
+	 *
+	 * This method belongs to the old vehicle movement method:
+	 * A vehicle moves a step every 256 progress units.
+	 * The vehicle speed is scaled by 3/4 when moving in X or Y direction due to the longer distance.
+	 *
+	 * However, this method is slightly wrong in corners, as the leftover progress is not scaled correctly
+	 * when changing movement direction. #GetAdvanceSpeed() and #GetAdvanceDistance() are better wrt. this.
+	 *
+	 * @param speed Direction-independent unscaled speed.
+	 * @return speed scaled by movement direction. 256 units are required for each movement step.
+	 */
+	FORCEINLINE uint GetOldAdvanceSpeed(uint speed)
+	{
+		return (this->direction & 1) ? speed : speed * 3 / 4;
+	}
+
+	/**
+	 * Determines the effective vehicle movement speed.
+	 *
+	 * Together with #GetAdvanceDistance() this function is a replacement for #GetOldAdvanceSpeed().
+	 *
+	 * A vehicle progresses independent of it's movement direction.
+	 * However different amounts of "progress" are needed for moving a step in a specific direction.
+	 * That way the leftover progress does not need any adaption when changing movement direction.
+	 *
+	 * @param speed Direction-independent unscaled speed.
+	 * @return speed, scaled to match #GetAdvanceDistance().
+	 */
+	static FORCEINLINE uint GetAdvanceSpeed(uint speed)
+	{
+		return speed * 3 / 4;
+	}
+
+	/**
+	 * Determines the vehicle "progress" needed for moving a step.
+	 *
+	 * Together with #GetAdvanceSpeed() this function is a replacement for #GetOldAdvanceSpeed().
+	 *
+	 * @return distance to drive for a movement step on the map.
+	 */
+	FORCEINLINE uint GetAdvanceDistance()
+	{
+		return (this->direction & 1) ? 192 : 256;
+	}
+
 	/**
 	 * Sets the expense type associated to this vehicle type
 	 * @param income whether this is income or (running) expenses of the vehicle