Cache whether ground vehicles currently have 0 slope resistance

src/ground_vehicle.cpp

@@ -112,6 +112,7 @@ void GroundVehicle<T, Type>::CargoChanged()
 		u->gcache.cached_slope_resistance = current_weight * u->GetSlopeSteepness() * 100;
 		u->cur_image_valid_dir = INVALID_DIR;
 	}
+	ClrBit(this->vcache.cached_veh_flags, VCF_GV_ZERO_SLOPE_RESIST);
 
 	/* Store consist weight in cache. */
 	this->gcache.cached_weight = max<uint32>(1, weight);
@@ -332,12 +333,14 @@ void GroundVehicle<T, Type>::UpdateZPositionInWormhole()
 		if (delta != 2) {
 			slope = slope_north;
 			SetBit(this->gv_flags, going_north ? GVF_GOINGUP_BIT : GVF_GOINGDOWN_BIT);
+			ClrBit(this->First()->vcache.cached_veh_flags, VCF_GV_ZERO_SLOPE_RESIST);
 		}
 	} else if ((delta = south_coord - pos_coord) <= 3) {
 		this->z_pos = TILE_HEIGHT * (delta == 3 ? -2 : -1);
 		if (delta != 2) {
 			slope = SLOPE_ELEVATED ^ slope_north;
 			SetBit(this->gv_flags, going_north ? GVF_GOINGDOWN_BIT : GVF_GOINGUP_BIT);
+			ClrBit(this->First()->vcache.cached_veh_flags, VCF_GV_ZERO_SLOPE_RESIST);
 		}
 	}
 

src/ground_vehicle.hpp

@@ -118,8 +118,10 @@ struct GroundVehicle : public SpecializedVehicle<T, Type> {
 	 * Calculates the total slope resistance for this vehicle.
 	 * @return Slope resistance.
 	 */
-	inline int64 GetSlopeResistance() const
+	inline int64 GetSlopeResistance()
 	{
+		if (likely(HasBit(this->vcache.cached_veh_flags, VCF_GV_ZERO_SLOPE_RESIST))) return 0;
+
 		int64 incl = 0;
 
 		for (const T *u = T::From(this); u != NULL; u = u->Next()) {
@@ -129,6 +131,7 @@ struct GroundVehicle : public SpecializedVehicle<T, Type> {
 				incl -= u->gcache.cached_slope_resistance;
 			}
 		}
+		if (incl == 0) SetBit(this->vcache.cached_veh_flags, VCF_GV_ZERO_SLOPE_RESIST);
 
 		return incl;
 	}
@@ -154,6 +157,7 @@ struct GroundVehicle : public SpecializedVehicle<T, Type> {
 
 			if (middle_z != this->z_pos) {
 				SetBit(this->gv_flags, (middle_z > this->z_pos) ? GVF_GOINGUP_BIT : GVF_GOINGDOWN_BIT);
+				ClrBit(this->First()->vcache.cached_veh_flags, VCF_GV_ZERO_SLOPE_RESIST);
 			}
 		}
 	}

src/train_cmd.cpp

@@ -2175,6 +2175,8 @@ void ReverseTrainDirection(Train *v)
 
 	AdvanceWagonsAfterSwap(v);
 
+	ClrBit(v->vcache.cached_veh_flags, VCF_GV_ZERO_SLOPE_RESIST);
+
 	if (IsRailDepotTile(v->tile)) {
 		InvalidateWindowData(WC_VEHICLE_DEPOT, v->tile);
 	}

src/vehicle.cpp

@@ -3706,6 +3706,7 @@ char *Vehicle::DumpVehicleFlags(char *b, const char *last) const
 	dump('a', HasBit(this->vehicle_flags, VF_AUTOMATE_TIMETABLE));
 	b += seprintf(b, last, ", vcf:");
 	dump('l', HasBit(this->vcache.cached_veh_flags, VCF_LAST_VISUAL_EFFECT));
+	dump('z', HasBit(this->vcache.cached_veh_flags, VCF_GV_ZERO_SLOPE_RESIST));
 	if (this->IsGroundVehicle()) {
 		uint16 gv_flags = this->GetGroundVehicleFlags();
 		b += seprintf(b, last, ", gvf:");

src/vehicle_base.h

@@ -135,6 +135,7 @@ enum GroundVehicleSubtypeFlags {
  */
 enum VehicleCacheFlags {
 	VCF_LAST_VISUAL_EFFECT      = 0, ///< Last vehicle in the consist with a visual effect.
+	VCF_GV_ZERO_SLOPE_RESIST    = 1, ///< GrounVehicle: Consist has zero slope resistance (valid only for the first engine), may be false negative.
 };
 
 /** Cached often queried values common to all vehicles. */