[Bug] Fix eggs having exploitable RNG (#3913)

* [Bug] Fix eggs having exploitable RNG

* Fix Wind Rider test having random chance to fail

* Revert egg's ID back to its own unseeded generation

* Remove change from wind rider test

---------

Co-authored-by: NightKev <34855794+DayKev@users.noreply.github.com>
Co-authored-by: Mumble <171087428+frutescens@users.noreply.github.com>

src/battle-scene.ts

@@ -974,6 +974,7 @@ export default class BattleScene extends SceneBase {
 
     this.setSeed(Overrides.SEED_OVERRIDE || Utils.randomString(24));
     console.log("Seed:", this.seed);
+    this.resetSeed(); // Properly resets RNG after saving and quitting a session
 
     this.disableMenu = false;
 

src/data/egg.ts

@@ -139,6 +139,7 @@ export class Egg {
   ////
 
   constructor(eggOptions?: IEggOptions) {
+    const generateEggProperties = (eggOptions?: IEggOptions) => {
       //if (eggOptions.tier && eggOptions.species) throw Error("Error egg can't have species and tier as option. only choose one of them.")
 
       this._sourceType = eggOptions?.sourceType!; // TODO: is this bang correct?
@@ -180,6 +181,16 @@ export class Egg {
         this.increasePullStatistic(eggOptions.scene!); // TODO: is this bang correct?
         this.addEggToGameData(eggOptions.scene!); // TODO: is this bang correct?
       }
+    };
+
+    if (eggOptions?.scene) {
+      const seedOverride = Utils.randomString(24);
+      eggOptions?.scene.executeWithSeedOffset(() => {
+        generateEggProperties(eggOptions);
+      }, 0, seedOverride);
+    } else { // For legacy eggs without scene
+      generateEggProperties(eggOptions);
+    }
   }
 
   ////
@@ -200,6 +211,9 @@ export class Egg {
 
   // Generates a PlayerPokemon from an egg
   public generatePlayerPokemon(scene: BattleScene): PlayerPokemon {
+    let ret: PlayerPokemon;
+
+    const generatePlayerPokemonHelper = (scene: BattleScene) => {
       // Legacy egg wants to hatch. Generate missing properties
       if (!this._species) {
         this._isShiny = this.rollShiny();
@@ -222,7 +236,7 @@ export class Egg {
       }
 
       // This function has way to many optional parameters
-    const ret: PlayerPokemon = scene.addPlayerPokemon(pokemonSpecies, 1, abilityIndex, undefined, undefined, false);
+      ret = scene.addPlayerPokemon(pokemonSpecies, 1, abilityIndex, undefined, undefined, false);
       ret.shiny = this._isShiny;
       ret.variant = this._variantTier;
 
@@ -231,6 +245,12 @@ export class Egg {
       for (let s = 0; s < ret.ivs.length; s++) {
         ret.ivs[s] = Math.max(ret.ivs[s], secondaryIvs[s]);
       }
+    };
+
+    ret = ret!;  // Tell TS compiler it's defined now
+    scene.executeWithSeedOffset(() => {
+      generatePlayerPokemonHelper(scene);
+    }, this._id, EGG_SEED.toString());
 
     return ret;
   }

src/test/battle-scene.test.ts

@@ -1,5 +1,5 @@
 import { LoadingScene } from "#app/loading-scene";
-import { afterEach, beforeAll, beforeEach, describe, expect, it } from "vitest";
+import { afterEach, beforeAll, beforeEach, describe, expect, it, vi } from "vitest";
 import GameManager from "./utils/gameManager";
 
 describe("BattleScene", () => {
@@ -24,4 +24,12 @@ describe("BattleScene", () => {
     // `BattleScene.create()` is called during the `new GameManager()` call
     expect(game.scene.scene.remove).toHaveBeenCalledWith(LoadingScene.KEY);
   });
+
+  it("should also reset RNG on reset", () => {
+    vi.spyOn(game.scene, "resetSeed");
+
+    game.scene.reset();
+
+    expect(game.scene.resetSeed).toHaveBeenCalled();
+  });
 });

src/test/eggs/egg.test.ts

@@ -12,6 +12,7 @@ import { afterEach, beforeAll, beforeEach, describe, expect, it, vi } from "vite
 describe("Egg Generation Tests", () => {
   let phaserGame: Phaser.Game;
   let game: GameManager;
+  const EGG_HATCH_COUNT: integer = 1000;
 
   beforeAll(() => {
     phaserGame = new Phaser.Game({
@@ -47,14 +48,21 @@ describe("Egg Generation Tests", () => {
 
     expect(result).toBe(expectedSpecies);
   });
-  it("should hatch an Arceus. Set from legendary gacha", async () => {
+  it("should hatch an Arceus around half the time. Set from legendary gacha", async () => {
     const scene = game.scene;
     const timestamp = new Date(2024, 6, 10, 15, 0, 0, 0).getTime();
     const expectedSpecies = Species.ARCEUS;
+    let gachaSpeciesCount = 0;
 
+    for (let i = 0; i < EGG_HATCH_COUNT; i++) {
       const result = new Egg({ scene, timestamp, sourceType: EggSourceType.GACHA_LEGENDARY, tier: EggTier.MASTER }).generatePlayerPokemon(scene).species.speciesId;
+      if (result === expectedSpecies) {
+        gachaSpeciesCount++;
+      }
+    }
 
-    expect(result).toBe(expectedSpecies);
+    expect(gachaSpeciesCount).toBeGreaterThan(0.4 * EGG_HATCH_COUNT);
+    expect(gachaSpeciesCount).toBeLessThan(0.6 * EGG_HATCH_COUNT);
   });
   it("should hatch an Arceus. Set from species", () => {
     const scene = game.scene;
@@ -156,7 +164,7 @@ describe("Egg Generation Tests", () => {
     const scene = game.scene;
 
     const eggMoveIndex = new Egg({ scene }).eggMoveIndex;
-    const result = eggMoveIndex && eggMoveIndex >= 0 && eggMoveIndex <= 3;
+    const result = !Utils.isNullOrUndefined(eggMoveIndex) && eggMoveIndex >= 0 && eggMoveIndex <= 3;
 
     expect(result).toBe(true);
   });
@@ -309,4 +317,63 @@ describe("Egg Generation Tests", () => {
 
     expect(egg.variantTier).toBe(VariantTier.EPIC);
   });
+
+  it("should generate egg moves, species, shinyness, and ability unpredictably for the egg gacha", () => {
+    const scene = game.scene;
+    scene.setSeed("ABCDEFGHIJKLMNOPQRSTUVWXYZ");
+    scene.resetSeed();
+
+    const firstEgg = new Egg({scene, sourceType: EggSourceType.GACHA_SHINY, tier: EggTier.COMMON});
+    const firstHatch = firstEgg.generatePlayerPokemon(scene);
+    let diffEggMove = false;
+    let diffSpecies = false;
+    let diffShiny = false;
+    let diffAbility = false;
+    for (let i = 0; i < EGG_HATCH_COUNT; i++) {
+      scene.gameData.unlockPity[EggTier.COMMON] = 0;
+      scene.setSeed("ABCDEFGHIJKLMNOPQRSTUVWXYZ");
+      scene.resetSeed(); // Make sure that eggs are unpredictable even if using same seed
+
+      const newEgg = new Egg({scene, sourceType: EggSourceType.GACHA_SHINY, tier: EggTier.COMMON});
+      const newHatch = newEgg.generatePlayerPokemon(scene);
+      diffEggMove = diffEggMove || (newEgg.eggMoveIndex !== firstEgg.eggMoveIndex);
+      diffSpecies = diffSpecies || (newHatch.species.speciesId !== firstHatch.species.speciesId);
+      diffShiny = diffShiny || (newHatch.shiny !== firstHatch.shiny);
+      diffAbility = diffAbility || (newHatch.abilityIndex !== firstHatch.abilityIndex);
+    }
+
+    expect(diffEggMove).toBe(true);
+    expect(diffSpecies).toBe(true);
+    expect(diffShiny).toBe(true);
+    expect(diffAbility).toBe(true);
+  });
+
+  it("should generate egg moves, shinyness, and ability unpredictably for species eggs", () => {
+    const scene = game.scene;
+    scene.setSeed("ABCDEFGHIJKLMNOPQRSTUVWXYZ");
+    scene.resetSeed();
+
+    const firstEgg = new Egg({scene, species: Species.BULBASAUR});
+    const firstHatch = firstEgg.generatePlayerPokemon(scene);
+    let diffEggMove = false;
+    let diffSpecies = false;
+    let diffShiny = false;
+    let diffAbility = false;
+    for (let i = 0; i < EGG_HATCH_COUNT; i++) {
+      scene.setSeed("ABCDEFGHIJKLMNOPQRSTUVWXYZ");
+      scene.resetSeed(); // Make sure that eggs are unpredictable even if using same seed
+
+      const newEgg = new Egg({scene, species: Species.BULBASAUR});
+      const newHatch = newEgg.generatePlayerPokemon(scene);
+      diffEggMove = diffEggMove || (newEgg.eggMoveIndex !== firstEgg.eggMoveIndex);
+      diffSpecies = diffSpecies || (newHatch.species.speciesId !== firstHatch.species.speciesId);
+      diffShiny = diffShiny || (newHatch.shiny !== firstHatch.shiny);
+      diffAbility = diffAbility || (newHatch.abilityIndex !== firstHatch.abilityIndex);
+    }
+
+    expect(diffEggMove).toBe(true);
+    expect(diffSpecies).toBe(false);
+    expect(diffShiny).toBe(true);
+    expect(diffAbility).toBe(true);
+  });
 });