Link graph: Change distance scaling algorithm in demand scaler

Fix erratic scaling and increase effect at large setting values

src/linkgraph/demands.cpp

@@ -308,23 +308,26 @@ void DemandCalculator::CalcDemand(LinkGraphJob &job, const std::vector<bool> &re
 			int32_t supply = scaler.EffectiveSupply(job[from_id], job[to_id]);
 			assert(supply > 0);
 
-			/* Scale the distance by mod_dist around max_distance */
-			int32_t distance = this->max_distance - (this->max_distance -
-					(int32_t)DistanceMaxPlusManhattan(job[from_id].XY(), job[to_id].XY())) *
-					this->mod_dist / 100;
+			constexpr int32_t divisor_scale = 16;
+
+			int32_t scaled_distance = this->base_distance;
+			if (this->mod_dist > 0) {
+				const int32_t distance = DistanceMaxPlusManhattan(job[from_id].XY(), job[to_id].XY());
+				/* Scale distance around base_distance by (mod_dist * (100 / 1024)).
+				 * Note that this means that the distance range is always compressed because mod_dist <= 1024. */
+				scaled_distance = this->base_distance + (((distance - this->base_distance) * this->mod_dist) / 1024);
+			}
 
 			/* Scale the accuracy by distance around accuracy / 2 */
-			int32_t divisor = this->accuracy * (this->mod_dist - 50) / 100 +
-					this->accuracy * distance / this->max_distance + 1;
-
-			assert(divisor > 0);
+			const int32_t divisor = divisor_scale + ((this->accuracy * scaled_distance * divisor_scale) / (this->base_distance * 2));
+			assert(divisor >= divisor_scale);
 
 			uint demand_forw = 0;
-			if (divisor <= supply) {
+			if (divisor <= (supply * divisor_scale)) {
 				/* At first only distribute demand if
 				 * effective supply / accuracy divisor >= 1
 				 * Others are too small or too far away to be considered. */
-				demand_forw = supply / divisor;
+				demand_forw = (supply * divisor_scale) / divisor;
 			} else if (++chance > this->accuracy * num_demands * num_supplies) {
 				/* After some trying, if there is still supply left, distribute
 				 * demand also to other nodes. */
@@ -410,7 +413,7 @@ void DemandCalculator::CalcMinimisedDistanceDemand(LinkGraphJob &job, const std:
  * @param job Job to calculate the demands for.
  */
 DemandCalculator::DemandCalculator(LinkGraphJob &job) :
-	max_distance(DistanceMaxPlusManhattan(TileXY(0,0), TileXY(MapMaxX(), MapMaxY())))
+	base_distance(IntSqrt(DistanceMaxPlusManhattan(TileXY(0,0), TileXY(MapMaxX(), MapMaxY()))))
 {
 	const LinkGraphSettings &settings = job.Settings();
 	CargoID cargo = job.Cargo();
@@ -419,8 +422,7 @@ DemandCalculator::DemandCalculator(LinkGraphJob &job) :
 	this->mod_dist = settings.demand_distance;
 	if (this->mod_dist > 100) {
 		/* Increase effect of mod_dist > 100 */
-		int over100 = this->mod_dist - 100;
-		this->mod_dist = 100 + over100 * over100;
+		this->mod_dist = 100 + ((this->mod_dist - 100) * 4);
 	}
 
 	if (settings.GetDistributionType(cargo) == DT_MANUAL) return;

src/linkgraph/demands.h

@@ -15,9 +15,9 @@ public:
 	DemandCalculator(LinkGraphJob &job);
 
 private:
-	int32_t max_distance; ///< Maximum distance possible on the map.
-	int32_t mod_dist;     ///< Distance modifier, determines how much demands decrease with distance.
-	int32_t accuracy;     ///< Accuracy of the calculation.
+	int32_t base_distance; ///< Base distance for scaling purposes.
+	int32_t mod_dist;      ///< Distance modifier, determines how much demands decrease with distance.
+	int32_t accuracy;      ///< Accuracy of the calculation.
 
 	template<class Tscaler>
 	void CalcDemand(LinkGraphJob &job, const std::vector<bool> &reachable_nodes, Tscaler scaler);