reduce train gold after each city (#3400)

## Description:

Now that cities snap to existing rails, it's possible to line up dozens
of cities in a row, producing way too much gold. This PR reduces the
gold after each stop to prevent that. Gold only starts decreasing after
the 3rd city.

## Please complete the following:

- [x] I have added screenshots for all UI updates
- [x] I process any text displayed to the user through translateText()
and I've added it to the en.json file
- [x] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Please put your Discord username so you can be contacted if a bug or
regression is found:

evan

src/core/configuration/Config.ts

@@ -132,7 +132,10 @@ export interface Config {
     tradeShipSpawnRejections: number,
     numTradeShips: number,
   ): number;
-  trainGold(rel: "self" | "team" | "ally" | "other"): Gold;
+  trainGold(
+    rel: "self" | "team" | "ally" | "other",
+    citiesVisited: number,
+  ): Gold;
   trainSpawnRate(numPlayerFactories: number): number;
   trainStationMinRange(): number;
   trainStationMaxRange(): number;

src/core/configuration/DefaultConfig.ts

@@ -20,7 +20,7 @@ import { PlayerView } from "../game/GameView";
 import { UserSettings } from "../game/UserSettings";
 import { GameConfig, GameID, TeamCountConfig } from "../Schemas";
 import { NukeType } from "../StatsSchemas";
-import { assertNever, sigmoid, simpleHash, within } from "../Util";
+import { assertNever, sigmoid, simpleHash, toInt, within } from "../Util";
 import { Config, GameEnv, NukeMagnitude, ServerConfig, Theme } from "./Config";
 import { Env } from "./Env";
 import { PastelTheme } from "./PastelTheme";
@@ -273,22 +273,28 @@ export class DefaultConfig implements Config {
     // expected number of trains = numPlayerFactories  / trainSpawnRate(numPlayerFactories)
     return (numPlayerFactories + 10) * 18;
   }
-  trainGold(rel: "self" | "team" | "ally" | "other"): Gold {
-    const multiplier = this.goldMultiplier();
-    let baseGold: bigint;
+  trainGold(
+    rel: "self" | "team" | "ally" | "other",
+    citiesVisited: number,
+  ): Gold {
+    // No penalty for the first 3 cities.
+    citiesVisited = Math.max(0, citiesVisited - 2);
+    let baseGold: number;
     switch (rel) {
       case "ally":
-        baseGold = 35_000n;
+        baseGold = 35_000;
         break;
       case "team":
       case "other":
-        baseGold = 25_000n;
+        baseGold = 25_000;
         break;
       case "self":
-        baseGold = 10_000n;
+        baseGold = 10_000;
         break;
     }
-    return BigInt(Math.floor(Number(baseGold) * multiplier));
+    const distPenalty = citiesVisited * 5_000;
+    const gold = Math.max(5000, baseGold - distPenalty);
+    return toInt(gold * this.goldMultiplier());
   }
 
   trainStationMinRange(): number {

src/core/execution/TrainExecution.ts

@@ -24,6 +24,7 @@ export class TrainExecution implements Execution {
   private stations: TrainStation[] = [];
   private currentRailroad: OrientedRailroad | null = null;
   private speed: number = 2;
+  private _tradeStopsVisited: number = 0;
 
   constructor(
     private railNetwork: RailNetwork,
@@ -37,6 +38,10 @@ export class TrainExecution implements Execution {
     return this.player;
   }
 
+  public tradeStopsVisited(): number {
+    return this._tradeStopsVisited;
+  }
+
   init(mg: Game, ticks: number): void {
     this.mg = mg;
     const stations = this.railNetwork.findStationsPath(
@@ -261,6 +266,10 @@ export class TrainExecution implements Execution {
       throw new Error("Not initialized");
     }
     this.stations[1].onTrainStop(this);
+    const stationType = this.stations[1].unit.type();
+    if (stationType === UnitType.City || stationType === UnitType.Port) {
+      this._tradeStopsVisited++;
+    }
     return;
   }
 

src/core/execution/nation/NationStructureBehavior.ts

@@ -697,7 +697,10 @@ export class NationStructureBehavior {
       unitToCluster.set(station.unit, station.getCluster());
     }
 
-    const maxTradeGold = Math.max(Number(game.config().trainGold("ally")), 1);
+    const maxTradeGold = Math.max(
+      Number(game.config().trainGold("ally", 0)),
+      1,
+    );
     const result: Array<{
       tile: TileRef;
       cluster: Cluster | null;
@@ -705,7 +708,8 @@ export class NationStructureBehavior {
     }> = [];
 
     // Own structures — weighted by "self" trade gold.
-    const selfWeight = Number(game.config().trainGold("self")) / maxTradeGold;
+    const selfWeight =
+      Number(game.config().trainGold("self", 0)) / maxTradeGold;
     for (const unit of player.units(
       UnitType.City,
       UnitType.Port,
@@ -730,7 +734,7 @@ export class NationStructureBehavior {
         : player.isAlliedWith(neighbor)
           ? "ally"
           : "other";
-      const weight = Number(game.config().trainGold(relType)) / maxTradeGold;
+      const weight = Number(game.config().trainGold(relType, 0)) / maxTradeGold;
       for (const unit of neighbor.units(
         UnitType.City,
         UnitType.Port,

src/core/game/TrainStation.ts

@@ -20,7 +20,12 @@ class TradeStationStopHandler implements TrainStopHandler {
   ): void {
     const stationOwner = station.unit.owner();
     const trainOwner = trainExecution.owner();
-    const gold = mg.config().trainGold(rel(trainOwner, stationOwner));
+    const gold = mg
+      .config()
+      .trainGold(
+        rel(trainOwner, stationOwner),
+        trainExecution.tradeStopsVisited(),
+      );
     // Share revenue with the station owner if it's not the current player
     if (trainOwner !== stationOwner) {
       stationOwner.addGold(gold, station.tile());