Make EXP boosting items stack additively and not multiplicatively (#3094)

Fixes #2040

src/modifier/modifier.ts

@@ -1774,7 +1774,7 @@ export class PokemonExpBoosterModifier extends PokemonHeldItemModifier {
   }
 
   apply(args: any[]): boolean {
-    (args[1] as Utils.NumberHolder).value = Math.floor((args[1] as Utils.NumberHolder).value * (1 + (this.getStackCount() * this.boostMultiplier)));
+    (args[1] as Utils.NumberHolder).value += (this.getStackCount() * this.boostMultiplier);
 
     return true;
   }

src/phases.ts

@@ -3927,7 +3927,9 @@ export class VictoryPhase extends PokemonPhase {
           expMultiplier = Overrides.XP_MULTIPLIER_OVERRIDE;
         }
         const pokemonExp = new Utils.NumberHolder(expValue * expMultiplier);
-        this.scene.applyModifiers(PokemonExpBoosterModifier, true, partyMember, pokemonExp);
+        const modifierBonusExp = new Utils.NumberHolder(1);
+        this.scene.applyModifiers(PokemonExpBoosterModifier, true, partyMember, modifierBonusExp);
+        pokemonExp.value *= modifierBonusExp.value;
         partyMemberExp.push(Math.floor(pokemonExp.value));
       }
 

src/test/items/exp_booster.test.ts

@@ -0,0 +1,40 @@
+import { afterEach, beforeAll, beforeEach, describe, expect, it, vi } from "vitest";
+import Phase from "phaser";
+import GameManager from "#app/test/utils/gameManager";
+import overrides from "#app/overrides";
+import { Abilities } from "#app/enums/abilities.js";
+import { PokemonExpBoosterModifier } from "#app/modifier/modifier.js";
+import * as Utils from "#app/utils";
+
+describe("EXP Modifier Items", () => {
+  let phaserGame: Phaser.Game;
+  let game: GameManager;
+
+  beforeAll(() => {
+    phaserGame = new Phase.Game({
+      type: Phaser.HEADLESS,
+    });
+  });
+
+  afterEach(() => {
+    game.phaseInterceptor.restoreOg();
+  });
+
+  beforeEach(() => {
+    game = new GameManager(phaserGame);
+
+    vi.spyOn(overrides, "OPP_ABILITY_OVERRIDE", "get").mockReturnValue(Abilities.BALL_FETCH);
+    vi.spyOn(overrides, "ABILITY_OVERRIDE", "get").mockReturnValue(Abilities.BALL_FETCH);
+    vi.spyOn(overrides, "SINGLE_BATTLE_OVERRIDE", "get").mockReturnValue(true);
+  });
+
+  it("EXP booster items stack additively", async() => {
+    vi.spyOn(overrides, "STARTING_HELD_ITEMS_OVERRIDE", "get").mockReturnValue([{name: "LUCKY_EGG"}, {name: "GOLDEN_EGG"}]);
+    await game.startBattle();
+
+    const partyMember = game.scene.getPlayerPokemon();
+    const modifierBonusExp = new Utils.NumberHolder(1);
+    partyMember.scene.applyModifiers(PokemonExpBoosterModifier, true, partyMember, modifierBonusExp);
+    expect(modifierBonusExp.value).toBe(2.4);
+  }, 20000);
+});