Feature: [NewGRF] Maximum curve speed modifier for rail vehicles (#9346)

src/engine_type.h

@@ -59,6 +59,7 @@ struct RailVehicleInfo {
 	byte tractive_effort;           ///< Tractive effort coefficient
 	byte air_drag;                  ///< Coefficient of air drag
 	byte user_def_data;             ///< Property 0x25: "User-defined bit mask" Used only for (very few) NewGRF vehicles
+	int16 curve_speed_mod;          ///< Modifier to maximum speed in curves (fixed-point binary with 8 fractional bits)
 };
 
 /** Information about a ship vehicle. */

src/newgrf.cpp

@@ -1331,6 +1331,10 @@ static ChangeInfoResult RailVehicleChangeInfo(uint engine, int numinfo, int prop
 				break;
 			}
 
+			case PROP_TRAIN_CURVE_SPEED_MOD: // 0x2E Curve speed modifier
+				rvi->curve_speed_mod = buf->ReadWord();
+				break;
+
 			default:
 				ret = CommonVehicleChangeInfo(ei, prop, buf);
 				break;

src/newgrf_engine.cpp

@@ -1177,16 +1177,23 @@ uint16 GetVehicleCallbackParent(CallbackID callback, uint32 param1, uint32 param
 
 
 /* Callback 36 handlers */
-uint GetVehicleProperty(const Vehicle *v, PropertyID property, uint orig_value)
+int GetVehicleProperty(const Vehicle *v, PropertyID property, int orig_value, bool is_signed)
 {
-	return GetEngineProperty(v->engine_type, property, orig_value, v);
+	return GetEngineProperty(v->engine_type, property, orig_value, v, is_signed);
 }
 
 
-uint GetEngineProperty(EngineID engine, PropertyID property, uint orig_value, const Vehicle *v)
+int GetEngineProperty(EngineID engine, PropertyID property, int orig_value, const Vehicle *v, bool is_signed)
 {
 	uint16 callback = GetVehicleCallback(CBID_VEHICLE_MODIFY_PROPERTY, property, 0, engine, v);
-	if (callback != CALLBACK_FAILED) return callback;
+	if (callback != CALLBACK_FAILED) {
+		if (is_signed) {
+			/* Sign extend 15 bit integer */
+			return static_cast<int16>(callback << 1) >> 1;
+		} else {
+			return callback;
+		}
+	}
 
 	return orig_value;
 }

src/newgrf_engine.h

@@ -100,8 +100,8 @@ bool UsesWagonOverride(const Vehicle *v);
 
 /* Handler to Evaluate callback 36. If the callback fails (i.e. most of the
  * time) orig_value is returned */
-uint GetVehicleProperty(const Vehicle *v, PropertyID property, uint orig_value);
-uint GetEngineProperty(EngineID engine, PropertyID property, uint orig_value, const Vehicle *v = nullptr);
+int GetVehicleProperty(const Vehicle *v, PropertyID property, int orig_value, bool is_signed = false);
+int GetEngineProperty(EngineID engine, PropertyID property, int orig_value, const Vehicle *v = nullptr, bool is_signed = false);
 
 enum VehicleTrigger {
 	VEHICLE_TRIGGER_NEW_CARGO     = 0x01,

src/newgrf_properties.h

@@ -28,6 +28,7 @@ enum PropertyID {
 	PROP_TRAIN_SHORTEN_FACTOR                   = 0x21, ///< Shorter vehicles
 	PROP_TRAIN_USER_DATA                        = 0x25, ///< User defined data for vehicle variable 0x42
 	PROP_TRAIN_CARGO_AGE_PERIOD                 = 0x2B, ///< Number of ticks before carried cargo is aged
+	PROP_TRAIN_CURVE_SPEED_MOD                  = 0x2E, ///< Modifier to maximum speed in curves
 
 	PROP_ROADVEH_RUNNING_COST_FACTOR            = 0x09, ///< Yearly runningcost
 	PROP_ROADVEH_CARGO_CAPACITY                 = 0x0F, ///< Capacity

src/table/engines.h

@@ -386,7 +386,7 @@ static const EngineInfo _orig_engine_info[] = {
  * Tractive effort coefficient by default is the same as TTDPatch, 0.30*256=76
  * Air drag value depends on the top speed of the vehicle.
  */
-#define RVI(a, b, c, d, e, f, g, h, i, j, k) { a, b, c, j, d, e, f, g, h, k, i, 0, 0, 0, VE_DEFAULT, 0, 76, 0, 0 }
+#define RVI(a, b, c, d, e, f, g, h, i, j, k) { a, b, c, j, d, e, f, g, h, k, i, 0, 0, 0, VE_DEFAULT, 0, 76, 0, 0, 0 }
 #define M RAILVEH_MULTIHEAD
 #define W RAILVEH_WAGON
 #define G RAILVEH_SINGLEHEAD

src/train.h

@@ -72,6 +72,7 @@ struct TrainCache {
 
 	/* cached values, recalculated on load and each time a vehicle is added to/removed from the consist. */
 	bool cached_tilt;           ///< train can tilt; feature provides a bonus in curves
+	int cached_curve_speed_mod; ///< curve speed modifier of the entire train
 
 	byte user_def_data;         ///< Cached property 0x25. Can be set by Callback 0x36.
 
@@ -312,6 +313,15 @@ protected: // These functions should not be called outside acceleration code.
 		return GetRailTypeInfo(GetRailType(this->tile))->max_speed;
 	}
 
+	/**
+	 * Returns the curve speed modifier of this vehicle.
+	 * @return Current curve speed modifier, in fixed-point binary representation with 8 fractional bits.
+	 */
+	inline int GetCurveSpeedModifier() const
+	{
+		return GetVehicleProperty(this, PROP_TRAIN_CURVE_SPEED_MOD, RailVehInfo(this->engine_type)->curve_speed_mod, true);
+	}
+
 	/**
 	 * Checks if the vehicle is at a tile that can be sloped.
 	 * @return True if the tile can be sloped.

src/train_cmd.cpp

@@ -115,6 +115,7 @@ void Train::ConsistChanged(ConsistChangeFlags allowed_changes)
 	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);
@@ -146,6 +147,7 @@ void Train::ConsistChanged(ConsistChangeFlags allowed_changes)
 		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);
@@ -229,6 +231,7 @@ void Train::ConsistChanged(ConsistChangeFlags allowed_changes)
 	/* 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) */
@@ -357,6 +360,11 @@ int Train::GetCurveSpeedLimit() const
 			/* 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) >> 8;
+		max_speed = Clamp(max_speed, 2, absolute_max_speed);
 	}
 
 	return max_speed;