Fix: Clear rail vehicle flipped flag if reverse probability callback returns false. (#14281)

This now distinguishes between not-flipped and callback not implemented.

src/articulated_vehicles.cpp

@@ -432,7 +432,10 @@ void AddArticulatedParts(Vehicle *first)
 
 		if (flip_image) v->spritenum++;
 
-		if (v->type == VEH_TRAIN && TestVehicleBuildProbability(v, v->engine_type, BuildProbabilityType::Reversed)) Train::From(v)->flags.Set(VehicleRailFlag::Flipped);
+		if (v->type == VEH_TRAIN) {
+			auto prob = TestVehicleBuildProbability(v, v->engine_type, BuildProbabilityType::Reversed);
+			if (prob.has_value()) Train::From(v)->flags.Set(VehicleRailFlag::Flipped, prob.value());
+		}
 		v->UpdatePosition();
 	}
 }

src/autoreplace_cmd.cpp

@@ -374,7 +374,11 @@ static CommandCost BuildReplacementVehicle(Vehicle *old_veh, Vehicle **new_vehic
 
 	/* Try to reverse the vehicle, but do not care if it fails as the new type might not be reversible */
 	if (new_veh->type == VEH_TRAIN && Train::From(old_veh)->flags.Test(VehicleRailFlag::Flipped)) {
-		Command<CMD_REVERSE_TRAIN_DIRECTION>::Do(DoCommandFlag::Execute, new_veh->index, true);
+		/* Only copy the reverse state if neither old or new vehicle implements reverse-on-build probability callback. */
+		if (!TestVehicleBuildProbability(old_veh, old_veh->engine_type, BuildProbabilityType::Reversed).has_value() &&
+			!TestVehicleBuildProbability(new_veh, new_veh->engine_type, BuildProbabilityType::Reversed).has_value()) {
+			Command<CMD_REVERSE_TRAIN_DIRECTION>::Do(DoCommandFlag::Execute, new_veh->index, true);
+		}
 	}
 
 	return cost;

src/newgrf_engine.cpp

@@ -1235,12 +1235,12 @@ int GetEngineProperty(EngineID engine, PropertyID property, int orig_value, cons
  * Test for vehicle build probablity type.
  * @param v Vehicle whose build probability to test.
  * @param type Build probability type to test for.
- * @returns True iff the probability result says so.
+ * @returns True or false depending on the probability result, or std::nullopt if the callback failed.
  */
-bool TestVehicleBuildProbability(Vehicle *v, EngineID engine, BuildProbabilityType type)
+std::optional<bool> TestVehicleBuildProbability(Vehicle *v, EngineID engine, BuildProbabilityType type)
 {
 	uint16_t p = GetVehicleCallback(CBID_VEHICLE_BUILD_PROBABILITY, to_underlying(type), 0, engine, v);
-	if (p == CALLBACK_FAILED) return false;
+	if (p == CALLBACK_FAILED) return std::nullopt;
 
 	const uint16_t PROBABILITY_RANGE = 100;
 	return p + RandomRange(PROBABILITY_RANGE) >= PROBABILITY_RANGE;

src/newgrf_engine.h

@@ -105,7 +105,7 @@ enum class BuildProbabilityType : uint8_t {
 	Reversed = 0,
 };
 
-bool TestVehicleBuildProbability(Vehicle *v, EngineID engine, BuildProbabilityType type);
+std::optional<bool> TestVehicleBuildProbability(Vehicle *v, EngineID engine, BuildProbabilityType type);
 
 void TriggerVehicleRandomisation(Vehicle *veh, VehicleRandomTrigger trigger);
 

src/train_cmd.cpp

@@ -662,7 +662,8 @@ static CommandCost CmdBuildRailWagon(DoCommandFlags flags, TileIndex tile, const
 
 		v->group_id = DEFAULT_GROUP;
 
-		if (TestVehicleBuildProbability(v, v->engine_type, BuildProbabilityType::Reversed)) v->flags.Set(VehicleRailFlag::Flipped);
+		auto prob = TestVehicleBuildProbability(v, v->engine_type, BuildProbabilityType::Reversed);
+		if (prob.has_value()) v->flags.Set(VehicleRailFlag::Flipped, prob.value());
 		AddArticulatedParts(v);
 
 		v->UpdatePosition();
@@ -731,7 +732,8 @@ static void AddRearEngineToMultiheadedTrain(Train *v)
 	v->SetMultiheaded();
 	u->SetMultiheaded();
 	v->SetNext(u);
-	if (TestVehicleBuildProbability(u, u->engine_type, BuildProbabilityType::Reversed)) u->flags.Set(VehicleRailFlag::Flipped);
+	auto prob = TestVehicleBuildProbability(u, u->engine_type, BuildProbabilityType::Reversed);
+	if (prob.has_value()) u->flags.Set(VehicleRailFlag::Flipped, prob.value());
 	u->UpdatePosition();
 
 	/* Now we need to link the front and rear engines together */
@@ -804,7 +806,8 @@ CommandCost CmdBuildRailVehicle(DoCommandFlags flags, TileIndex tile, const Engi
 		v->SetFrontEngine();
 		v->SetEngine();
 
-		if (TestVehicleBuildProbability(v, v->engine_type, BuildProbabilityType::Reversed)) v->flags.Set(VehicleRailFlag::Flipped);
+		auto prob = TestVehicleBuildProbability(v, v->engine_type, BuildProbabilityType::Reversed);
+		if (prob.has_value()) v->flags.Set(VehicleRailFlag::Flipped, prob.value());
 		v->UpdatePosition();
 
 		if (rvi->railveh_type == RAILVEH_MULTIHEAD) {

src/vehicle_cmd.cpp

@@ -906,7 +906,11 @@ std::tuple<CommandCost, VehicleID> CmdCloneVehicle(DoCommandFlags flags, TileInd
 			w = Vehicle::Get(new_veh_id);
 
 			if (v->type == VEH_TRAIN && Train::From(v)->flags.Test(VehicleRailFlag::Flipped)) {
-				Train::From(w)->flags.Set(VehicleRailFlag::Flipped);
+				/* Only copy the reverse state if neither old or new vehicle implements reverse-on-build probability callback. */
+				if (!TestVehicleBuildProbability(v, v->engine_type, BuildProbabilityType::Reversed).has_value() &&
+					!TestVehicleBuildProbability(w, w->engine_type, BuildProbabilityType::Reversed).has_value()) {
+					Train::From(w)->flags.Set(VehicleRailFlag::Flipped);
+				}
 			}
 
 			if (v->type == VEH_TRAIN && !v->IsFrontEngine()) {