[Bug] Fix rare egg move and species rates for Manaphy eggs (#4125)

src/data/egg.ts

@@ -222,7 +222,7 @@ export class Egg {
 
       let pokemonSpecies = getPokemonSpecies(this._species);
       // Special condition to have Phione eggs also have a chance of generating Manaphy
-      if (this._species === Species.PHIONE) {
+      if (this._species === Species.PHIONE && this._sourceType === EggSourceType.SAME_SPECIES_EGG) {
         pokemonSpecies = getPokemonSpecies(Utils.randSeedInt(MANAPHY_EGG_MANAPHY_RATE) ? Species.PHIONE : Species.MANAPHY);
       }
 
@@ -326,7 +326,8 @@ export class Egg {
       break;
     }
 
-    return Utils.randSeedInt(baseChance * Math.pow(2, 3 - this.tier)) ? Utils.randSeedInt(3) : 3;
+    const tierMultiplier = this.isManaphyEgg() ? 2 : Math.pow(2, 3 - this.tier);
+    return Utils.randSeedInt(baseChance * tierMultiplier) ? Utils.randSeedInt(3) : 3;
   }
 
   private getEggTierDefaultHatchWaves(eggTier?: EggTier): number {
@@ -361,7 +362,12 @@ export class Egg {
      * the species that was the legendary focus at the time
      */
     if (this.isManaphyEgg()) {
-      const rand = Utils.randSeedInt(MANAPHY_EGG_MANAPHY_RATE);
+      /**
+       * Adding a technicality to make unit tests easier: By making this check pass
+       * when Utils.randSeedInt(8) = 1, and by making the generatePlayerPokemon() species
+       * check pass when Utils.randSeedInt(8) = 0, we can tell them apart during tests.
+       */
+      const rand = (Utils.randSeedInt(MANAPHY_EGG_MANAPHY_RATE) !== 1);
       return rand ? Species.PHIONE : Species.MANAPHY;
     } else if (this.tier === EggTier.MASTER
       && this._sourceType === EggSourceType.GACHA_LEGENDARY) {

src/test/eggs/manaphy-egg.test.ts

@@ -0,0 +1,118 @@
+import { Egg } from "#app/data/egg";
+import { EggSourceType } from "#app/enums/egg-source-types";
+import { EggTier } from "#app/enums/egg-type";
+import { Species } from "#enums/species";
+import GameManager from "#test/utils/gameManager";
+import Phaser from "phaser";
+import { afterEach, beforeAll, beforeEach, describe, expect, it, vi } from "vitest";
+
+describe("Manaphy Eggs", () => {
+  let phaserGame: Phaser.Game;
+  let game: GameManager;
+  const EGG_HATCH_COUNT: integer = 48;
+  let rngSweepProgress: number = 0;
+
+  beforeAll(() => {
+    phaserGame = new Phaser.Game({
+      type: Phaser.HEADLESS,
+    });
+    game = new GameManager(phaserGame);
+  });
+
+  afterEach(() => {
+    game.phaseInterceptor.restoreOg();
+    vi.restoreAllMocks();
+  });
+
+  beforeEach(async () => {
+    await game.importData("src/test/utils/saves/everything.prsv");
+
+    /**
+     * In our tests, we will perform an "RNG sweep" by letting rngSweepProgress
+     * increase uniformly from 0 to 1 in order to get a uniform sample of the
+     * possible RNG outcomes. This will let us quickly and consistently find
+     * the probability of each RNG outcome.
+     */
+    vi.spyOn(Phaser.Math.RND, "realInRange").mockImplementation((min: number, max: number) => {
+      return rngSweepProgress * (max - min) + min;
+    });
+  });
+
+  it("should have correct Manaphy rates and Rare Egg Move rates, from the egg gacha", () => {
+    const scene = game.scene;
+
+    let manaphyCount = 0;
+    let phioneCount = 0;
+    let rareEggMoveCount = 0;
+    for (let i = 0; i < EGG_HATCH_COUNT; i++) {
+      rngSweepProgress = (2 * i + 1) / (2 * EGG_HATCH_COUNT);
+
+      const newEgg = new Egg({ scene, tier: EggTier.COMMON, sourceType: EggSourceType.GACHA_SHINY, id: 204 });
+      const newHatch = newEgg.generatePlayerPokemon(scene);
+      if (newHatch.species.speciesId === Species.MANAPHY) {
+        manaphyCount++;
+      } else if (newHatch.species.speciesId === Species.PHIONE) {
+        phioneCount++;
+      }
+      if (newEgg.eggMoveIndex === 3) {
+        rareEggMoveCount++;
+      }
+    }
+
+    expect(manaphyCount + phioneCount).toBe(EGG_HATCH_COUNT);
+    expect(manaphyCount).toBe(1/8 * EGG_HATCH_COUNT);
+    expect(rareEggMoveCount).toBe(1/12 * EGG_HATCH_COUNT);
+  });
+
+  it("should have correct Manaphy rates and Rare Egg Move rates, from Phione species eggs", () => {
+    const scene = game.scene;
+
+    let manaphyCount = 0;
+    let phioneCount = 0;
+    let rareEggMoveCount = 0;
+    for (let i = 0; i < EGG_HATCH_COUNT; i++) {
+      rngSweepProgress = (2 * i + 1) / (2 * EGG_HATCH_COUNT);
+
+      const newEgg = new Egg({ scene, species: Species.PHIONE, sourceType: EggSourceType.SAME_SPECIES_EGG });
+      const newHatch = newEgg.generatePlayerPokemon(scene);
+      if (newHatch.species.speciesId === Species.MANAPHY) {
+        manaphyCount++;
+      } else if (newHatch.species.speciesId === Species.PHIONE) {
+        phioneCount++;
+      }
+      if (newEgg.eggMoveIndex === 3) {
+        rareEggMoveCount++;
+      }
+    }
+
+    expect(manaphyCount + phioneCount).toBe(EGG_HATCH_COUNT);
+    expect(manaphyCount).toBe(1/8 * EGG_HATCH_COUNT);
+    expect(rareEggMoveCount).toBe(1/6 * EGG_HATCH_COUNT);
+  });
+
+  it("should have correct Manaphy rates and Rare Egg Move rates, from Manaphy species eggs", () => {
+    const scene = game.scene;
+
+    let manaphyCount = 0;
+    let phioneCount = 0;
+    let rareEggMoveCount = 0;
+    for (let i = 0; i < EGG_HATCH_COUNT; i++) {
+      rngSweepProgress = (2 * i + 1) / (2 * EGG_HATCH_COUNT);
+
+      const newEgg = new Egg({ scene, species: Species.MANAPHY, sourceType: EggSourceType.SAME_SPECIES_EGG });
+      const newHatch = newEgg.generatePlayerPokemon(scene);
+      if (newHatch.species.speciesId === Species.MANAPHY) {
+        manaphyCount++;
+      } else if (newHatch.species.speciesId === Species.PHIONE) {
+        phioneCount++;
+      }
+      if (newEgg.eggMoveIndex === 3) {
+        rareEggMoveCount++;
+      }
+    }
+
+    expect(phioneCount).toBe(0);
+    expect(manaphyCount).toBe(EGG_HATCH_COUNT);
+    expect(rareEggMoveCount).toBe(1/6 * EGG_HATCH_COUNT);
+  });
+});