[Ability] Implement Forecast (#3534)

* initial forecast implementation

* updates

* bug fixes and add tests

* bug fixes

* update docs

* fix issues post-merge

* add show ability

* add support for simulated abilities

* add simulated conditions and fix tests

* fix simulated conditions

src/data/ability.ts

@@ -13,7 +13,7 @@ import { ArenaTagSide, ArenaTrapTag } from "./arena-tag";
 import { Stat, getStatName } from "./pokemon-stat";
 import { BerryModifier, PokemonHeldItemModifier } from "../modifier/modifier";
 import { TerrainType } from "./terrain";
-import { SpeciesFormChangeManualTrigger } from "./pokemon-forms";
+import { SpeciesFormChangeManualTrigger, SpeciesFormChangeRevertWeatherFormTrigger, SpeciesFormChangeWeatherTrigger } from "./pokemon-forms";
 import i18next from "i18next";
 import { Localizable } from "#app/interfaces/locales.js";
 import { Command } from "../ui/command-ui-handler";
@@ -25,10 +25,11 @@ import { ArenaTagType } from "#enums/arena-tag-type";
 import { BattlerTagType } from "#enums/battler-tag-type";
 import { Moves } from "#enums/moves";
 import { Species } from "#enums/species";
-import { MovePhase } from "#app/phases/move-phase.js";
+import { MovePhase } from "#app/phases/move-phase";
-import { PokemonHealPhase } from "#app/phases/pokemon-heal-phase.js";
+import { PokemonHealPhase } from "#app/phases/pokemon-heal-phase";
-import { ShowAbilityPhase } from "#app/phases/show-ability-phase.js";
+import { ShowAbilityPhase } from "#app/phases/show-ability-phase";
-import { StatChangePhase } from "#app/phases/stat-change-phase.js";
+import { StatChangePhase } from "#app/phases/stat-change-phase";
+import BattleScene from "#app/battle-scene";
 
 export class Ability implements Localizable {
   public id: Abilities;
@@ -2361,6 +2362,73 @@ export class PostSummonTransformAbAttr extends PostSummonAbAttr {
   }
 }
 
+/**
+ * Reverts weather-based forms to their normal forms when the user is summoned.
+ * Used by Cloud Nine and Air Lock.
+ * @extends PostSummonAbAttr
+ */
+export class PostSummonWeatherSuppressedFormChangeAbAttr extends PostSummonAbAttr {
+  /**
+   * Triggers {@linkcode Arena.triggerWeatherBasedFormChangesToNormal | triggerWeatherBasedFormChangesToNormal}
+   * @param {Pokemon} pokemon the Pokemon with this ability
+   * @param passive n/a
+   * @param args n/a
+   * @returns whether a Pokemon was reverted to its normal form
+   */
+  applyPostSummon(pokemon: Pokemon, passive: boolean, simulated: boolean, args: any[]) {
+    const pokemonToTransform = getPokemonWithWeatherBasedForms(pokemon.scene);
+
+    if (pokemonToTransform.length < 1) {
+      return false;
+    }
+
+    if (!simulated) {
+      pokemon.scene.arena.triggerWeatherBasedFormChangesToNormal();
+    }
+
+    return true;
+  }
+}
+
+/**
+ * Triggers weather-based form change when summoned into an active weather.
+ * Used by Forecast.
+ * @extends PostSummonAbAttr
+ */
+export class PostSummonFormChangeByWeatherAbAttr extends PostSummonAbAttr {
+  private ability: Abilities;
+
+  constructor(ability: Abilities) {
+    super(false);
+
+    this.ability = ability;
+  }
+
+  /**
+   * Calls the {@linkcode BattleScene.triggerPokemonFormChange | triggerPokemonFormChange} for both
+   * {@linkcode SpeciesFormChange.SpeciesFormChangeWeatherTrigger | SpeciesFormChangeWeatherTrigger} and
+   * {@linkcode SpeciesFormChange.SpeciesFormChangeWeatherTrigger | SpeciesFormChangeRevertWeatherFormTrigger} if it
+   * is the specific Pokemon and ability
+   * @param {Pokemon} pokemon the Pokemon with this ability
+   * @param passive n/a
+   * @param args n/a
+   * @returns whether the form change was triggered
+   */
+  applyPostSummon(pokemon: Pokemon, passive: boolean, simulated: boolean, args: any[]): boolean {
+    if (pokemon.species.speciesId === Species.CASTFORM && this.ability === Abilities.FORECAST) {
+      if (simulated) {
+        return simulated;
+      }
+
+      pokemon.scene.triggerPokemonFormChange(pokemon, SpeciesFormChangeWeatherTrigger);
+      pokemon.scene.triggerPokemonFormChange(pokemon, SpeciesFormChangeRevertWeatherFormTrigger);
+      queueShowAbility(pokemon, passive);
+      return true;
+    }
+    return false;
+  }
+}
+
 export class PreSwitchOutAbAttr extends AbAttr {
   constructor() {
     super(true);
@@ -3014,6 +3082,49 @@ export class PostWeatherChangeAbAttr extends AbAttr {
   }
 }
 
+/**
+ * Triggers weather-based form change when weather changes.
+ * Used by Forecast.
+ * @extends PostWeatherChangeAbAttr
+ */
+export class PostWeatherChangeFormChangeAbAttr extends PostWeatherChangeAbAttr {
+  private ability: Abilities;
+
+  constructor(ability: Abilities) {
+    super(false);
+
+    this.ability = ability;
+  }
+
+  /**
+   * Calls {@linkcode Arena.triggerWeatherBasedFormChangesToNormal | triggerWeatherBasedFormChangesToNormal} when the
+   * weather changed to form-reverting weather, otherwise calls {@linkcode Arena.triggerWeatherBasedFormChanges | triggerWeatherBasedFormChanges}
+   * @param {Pokemon} pokemon the Pokemon that changed the weather
+   * @param passive n/a
+   * @param weather n/a
+   * @param args n/a
+   * @returns whether the form change was triggered
+   */
+  applyPostWeatherChange(pokemon: Pokemon, passive: boolean, simulated: boolean, weather: WeatherType, args: any[]): boolean {
+    if (pokemon.species.speciesId === Species.CASTFORM && this.ability === Abilities.FORECAST) {
+      if (simulated) {
+        return simulated;
+      }
+
+      const formRevertingWeathers: WeatherType[] = [ WeatherType.NONE, WeatherType.SANDSTORM, WeatherType.STRONG_WINDS, WeatherType.FOG ];
+      const weatherType = pokemon.scene.arena.weather?.weatherType;
+
+      if (weatherType && formRevertingWeathers.includes(weatherType)) {
+        pokemon.scene.arena.triggerWeatherBasedFormChangesToNormal();
+      } else {
+        pokemon.scene.arena.triggerWeatherBasedFormChanges();
+      }
+      return true;
+    }
+    return false;
+  }
+}
+
 export class PostWeatherChangeAddBattlerTagAttr extends PostWeatherChangeAbAttr {
   private tagType: BattlerTagType;
   private turnCount: integer;
@@ -3784,6 +3895,38 @@ export class PostFaintAbAttr extends AbAttr {
   }
 }
 
+/**
+ * Used for weather suppressing abilities to trigger weather-based form changes upon being fainted.
+ * Used by Cloud Nine and Air Lock.
+ * @extends PostFaintAbAttr
+ */
+export class PostFaintUnsuppressedWeatherFormChangeAbAttr extends PostFaintAbAttr {
+  /**
+   * Triggers {@linkcode Arena.triggerWeatherBasedFormChanges | triggerWeatherBasedFormChanges}
+   * when the user of the ability faints
+   * @param {Pokemon} pokemon the fainted Pokemon
+   * @param passive n/a
+   * @param attacker n/a
+   * @param move n/a
+   * @param hitResult n/a
+   * @param args n/a
+   * @returns whether the form change was triggered
+   */
+  applyPostFaint(pokemon: Pokemon, passive: boolean, simulated: boolean, attacker: Pokemon, move: Move, hitResult: HitResult, args: any[]): boolean {
+    const pokemonToTransform = getPokemonWithWeatherBasedForms(pokemon.scene);
+
+    if (pokemonToTransform.length < 1) {
+      return false;
+    }
+
+    if (!simulated) {
+      pokemon.scene.arena.triggerWeatherBasedFormChanges();
+    }
+
+    return true;
+  }
+}
+
 /**
  * Clears Desolate Land/Primordial Sea/Delta Stream upon the Pokemon fainting
  */
@@ -4559,6 +4702,16 @@ function setAbilityRevealed(pokemon: Pokemon): void {
   }
 }
 
+/**
+ * Returns the Pokemon with weather-based forms
+ * @param {BattleScene} scene - The current scene
+ */
+function getPokemonWithWeatherBasedForms(scene: BattleScene) {
+  return scene.getField(true).filter(p =>
+    p.hasAbility(Abilities.FORECAST) && p.species.speciesId === Species.CASTFORM
+  );
+}
+
 export const allAbilities = [ new Ability(Abilities.NONE, 3) ];
 
 export function initAbilities() {
@@ -4605,7 +4758,10 @@ export function initAbilities() {
       .ignorable(),
     new Ability(Abilities.CLOUD_NINE, 3)
       .attr(SuppressWeatherEffectAbAttr, true)
-      .attr(PostSummonUnnamedMessageAbAttr, i18next.t("abilityTriggers:weatherEffectDisappeared")),
+      .attr(PostSummonUnnamedMessageAbAttr, i18next.t("abilityTriggers:weatherEffectDisappeared"))
+      .attr(PostSummonWeatherSuppressedFormChangeAbAttr)
+      .attr(PostFaintUnsuppressedWeatherFormChangeAbAttr)
+      .bypassFaint(),
     new Ability(Abilities.COMPOUND_EYES, 3)
       .attr(BattleStatMultiplierAbAttr, BattleStat.ACC, 1.3),
     new Ability(Abilities.INSOMNIA, 3)
@@ -4750,7 +4906,8 @@ export function initAbilities() {
     new Ability(Abilities.FORECAST, 3)
       .attr(UncopiableAbilityAbAttr)
       .attr(NoFusionAbilityAbAttr)
-      .unimplemented(),
+      .attr(PostSummonFormChangeByWeatherAbAttr, Abilities.FORECAST)
+      .attr(PostWeatherChangeFormChangeAbAttr, Abilities.FORECAST),
     new Ability(Abilities.STICKY_HOLD, 3)
       .attr(BlockItemTheftAbAttr)
       .bypassFaint()
@@ -4800,7 +4957,10 @@ export function initAbilities() {
       .ignorable(),
     new Ability(Abilities.AIR_LOCK, 3)
       .attr(SuppressWeatherEffectAbAttr, true)
-      .attr(PostSummonUnnamedMessageAbAttr, i18next.t("abilityTriggers:weatherEffectDisappeared")),
+      .attr(PostSummonUnnamedMessageAbAttr, i18next.t("abilityTriggers:weatherEffectDisappeared"))
+      .attr(PostSummonWeatherSuppressedFormChangeAbAttr)
+      .attr(PostFaintUnsuppressedWeatherFormChangeAbAttr)
+      .bypassFaint(),
     new Ability(Abilities.TANGLED_FEET, 4)
       .conditionalAttr(pokemon => !!pokemon.getTag(BattlerTagType.CONFUSED), BattleStatMultiplierAbAttr, BattleStat.EVA, 2)
       .ignorable(),

src/data/move.ts

@@ -26,16 +26,17 @@ import { BattlerTagType } from "#enums/battler-tag-type";
 import { Biome } from "#enums/biome";
 import { Moves } from "#enums/moves";
 import { Species } from "#enums/species";
-import { MoveUsedEvent } from "#app/events/battle-scene.js";
+import { MoveUsedEvent } from "#app/events/battle-scene";
-import { PartyStatusCurePhase } from "#app/phases/party-status-cure-phase.js";
+import { PartyStatusCurePhase } from "#app/phases/party-status-cure-phase";
-import { BattleEndPhase } from "#app/phases/battle-end-phase.js";
+import { BattleEndPhase } from "#app/phases/battle-end-phase";
-import { MoveEndPhase } from "#app/phases/move-end-phase.js";
+import { MoveEndPhase } from "#app/phases/move-end-phase";
-import { MovePhase } from "#app/phases/move-phase.js";
+import { MovePhase } from "#app/phases/move-phase";
-import { NewBattlePhase } from "#app/phases/new-battle-phase.js";
+import { NewBattlePhase } from "#app/phases/new-battle-phase";
-import { PokemonHealPhase } from "#app/phases/pokemon-heal-phase.js";
+import { PokemonHealPhase } from "#app/phases/pokemon-heal-phase";
-import { StatChangePhase } from "#app/phases/stat-change-phase.js";
+import { StatChangePhase } from "#app/phases/stat-change-phase";
-import { SwitchPhase } from "#app/phases/switch-phase.js";
+import { SwitchPhase } from "#app/phases/switch-phase";
-import { SwitchSummonPhase } from "#app/phases/switch-summon-phase.js";
+import { SwitchSummonPhase } from "#app/phases/switch-summon-phase";
+import { SpeciesFormChangeRevertWeatherFormTrigger } from "./pokemon-forms";
 
 export enum MoveCategory {
   PHYSICAL,
@@ -5739,7 +5740,10 @@ export class AbilityChangeAttr extends MoveEffectAttr {
       return false;
     }
 
-    (this.selfTarget ? user : target).summonData.ability = this.ability;
+    const moveTarget = this.selfTarget ? user : target;
+
+    moveTarget.summonData.ability = this.ability;
+    user.scene.triggerPokemonFormChange(moveTarget, SpeciesFormChangeRevertWeatherFormTrigger);
 
     user.scene.queueMessage(i18next.t("moveTriggers:acquiredAbility", {pokemonName: getPokemonNameWithAffix((this.selfTarget ? user : target)), abilityName: allAbilities[this.ability].name}));
 
@@ -5825,6 +5829,10 @@ export class SwitchAbilitiesAttr extends MoveEffectAttr {
     target.summonData.ability = tempAbilityId;
 
     user.scene.queueMessage(i18next.t("moveTriggers:swappedAbilitiesWithTarget", {pokemonName: getPokemonNameWithAffix(user)}));
+    // Swaps Forecast from Castform
+    user.scene.arena.triggerWeatherBasedFormChangesToNormal();
+    // Swaps Forecast to Castform (edge case)
+    user.scene.arena.triggerWeatherBasedFormChanges();
 
     return true;
   }
@@ -5850,6 +5858,7 @@ export class SuppressAbilitiesAttr extends MoveEffectAttr {
     }
 
     target.summonData.abilitySuppressed = true;
+    target.scene.arena.triggerWeatherBasedFormChangesToNormal();
 
     target.scene.queueMessage(i18next.t("moveTriggers:suppressAbilities", {pokemonName: getPokemonNameWithAffix(target)}));
 

src/data/pokemon-forms.ts

@@ -10,6 +10,7 @@ import { Species } from "#enums/species";
 import { TimeOfDay } from "#enums/time-of-day";
 import { getPokemonNameWithAffix } from "#app/messages.js";
 import i18next from "i18next";
+import { WeatherType } from "./weather";
 
 export enum FormChangeItem {
   NONE,
@@ -356,6 +357,78 @@ export class SpeciesDefaultFormMatchTrigger extends SpeciesFormChangeTrigger {
   }
 }
 
+/**
+ * Class used for triggering form changes based on weather.
+ * Used by Castform.
+ * @extends SpeciesFormChangeTrigger
+ */
+export class SpeciesFormChangeWeatherTrigger extends SpeciesFormChangeTrigger {
+  /** The ability that  triggers the form change */
+  public ability: Abilities;
+  /** The list of weathers that trigger the form change */
+  public weathers: WeatherType[];
+
+  constructor(ability: Abilities, weathers: WeatherType[]) {
+    super();
+    this.ability = ability;
+    this.weathers = weathers;
+  }
+
+  /**
+   * Checks if the Pokemon has the required ability and is in the correct weather while
+   * the weather or ability is also not suppressed.
+   * @param {Pokemon} pokemon the pokemon that is trying to do the form change
+   * @returns `true` if the Pokemon can change forms, `false` otherwise
+   */
+  canChange(pokemon: Pokemon): boolean {
+    const currentWeather = pokemon.scene.arena.weather?.weatherType ?? WeatherType.NONE;
+    const isWeatherSuppressed = pokemon.scene.arena.weather?.isEffectSuppressed(pokemon.scene);
+    const isAbilitySuppressed = pokemon.summonData.abilitySuppressed;
+
+    return !isAbilitySuppressed && !isWeatherSuppressed && (pokemon.hasAbility(this.ability) && this.weathers.includes(currentWeather));
+  }
+}
+
+/**
+ * Class used for reverting to the original form when the weather runs out
+ * or when the user loses the ability/is suppressed.
+ * Used by Castform.
+ * @extends SpeciesFormChangeTrigger
+ */
+export class SpeciesFormChangeRevertWeatherFormTrigger extends SpeciesFormChangeTrigger {
+  /** The ability that triggers the form change*/
+  public ability: Abilities;
+  /** The list of weathers that will also trigger a form change to original form */
+  public weathers: WeatherType[];
+
+  constructor(ability: Abilities, weathers: WeatherType[]) {
+    super();
+    this.ability = ability;
+    this.weathers = weathers;
+  }
+
+  /**
+   * Checks if the Pokemon has the required ability and the weather is one that will revert
+   * the Pokemon to its original form or the weather or ability is suppressed
+   * @param {Pokemon} pokemon the pokemon that is trying to do the form change
+   * @returns `true` if the Pokemon will revert to its original form, `false` otherwise
+   */
+  canChange(pokemon: Pokemon): boolean {
+    if (pokemon.hasAbility(this.ability, false, true)) {
+      const currentWeather = pokemon.scene.arena.weather?.weatherType ?? WeatherType.NONE;
+      const isWeatherSuppressed = pokemon.scene.arena.weather?.isEffectSuppressed(pokemon.scene);
+      const isAbilitySuppressed = pokemon.summonData.abilitySuppressed;
+      const summonDataAbility = pokemon.summonData.ability;
+      const isAbilityChanged = summonDataAbility !== this.ability && summonDataAbility !== Abilities.NONE;
+
+      if (this.weathers.includes(currentWeather) || isWeatherSuppressed || isAbilitySuppressed || isAbilityChanged) {
+        return true;
+      }
+    }
+    return false;
+  }
+}
+
 export function getSpeciesFormChangeMessage(pokemon: Pokemon, formChange: SpeciesFormChange, preName: string): string {
   const isMega = formChange.formKey.indexOf(SpeciesFormKey.MEGA) > -1;
   const isGmax = formChange.formKey.indexOf(SpeciesFormKey.GIGANTAMAX) > -1;
@@ -839,7 +912,24 @@ export const pokemonFormChanges: PokemonFormChanges = {
     new SpeciesFormChange(Species.CRAMORANT, "gorging", "", new SpeciesFormChangeManualTrigger, true),
     new SpeciesFormChange(Species.CRAMORANT, "gulping", "", new SpeciesFormChangeActiveTrigger(false), true),
     new SpeciesFormChange(Species.CRAMORANT, "gorging", "", new SpeciesFormChangeActiveTrigger(false), true),
-  ]
+  ],
+  [Species.CASTFORM]: [
+    new SpeciesFormChange(Species.CASTFORM, "", "sunny", new SpeciesFormChangeWeatherTrigger(Abilities.FORECAST, [WeatherType.SUNNY, WeatherType.HARSH_SUN]), true),
+    new SpeciesFormChange(Species.CASTFORM, "rainy", "sunny", new SpeciesFormChangeWeatherTrigger(Abilities.FORECAST, [WeatherType.SUNNY, WeatherType.HARSH_SUN]), true),
+    new SpeciesFormChange(Species.CASTFORM, "snowy", "sunny", new SpeciesFormChangeWeatherTrigger(Abilities.FORECAST, [WeatherType.SUNNY, WeatherType.HARSH_SUN]), true),
+    new SpeciesFormChange(Species.CASTFORM, "", "rainy", new SpeciesFormChangeWeatherTrigger(Abilities.FORECAST, [WeatherType.RAIN, WeatherType.HEAVY_RAIN]), true),
+    new SpeciesFormChange(Species.CASTFORM, "sunny", "rainy", new SpeciesFormChangeWeatherTrigger(Abilities.FORECAST, [WeatherType.RAIN, WeatherType.HEAVY_RAIN]), true),
+    new SpeciesFormChange(Species.CASTFORM, "snowy", "rainy", new SpeciesFormChangeWeatherTrigger(Abilities.FORECAST, [WeatherType.RAIN, WeatherType.HEAVY_RAIN]), true),
+    new SpeciesFormChange(Species.CASTFORM, "", "snowy", new SpeciesFormChangeWeatherTrigger(Abilities.FORECAST, [WeatherType.HAIL, WeatherType.SNOW]), true),
+    new SpeciesFormChange(Species.CASTFORM, "sunny", "snowy", new SpeciesFormChangeWeatherTrigger(Abilities.FORECAST, [WeatherType.HAIL, WeatherType.SNOW]), true),
+    new SpeciesFormChange(Species.CASTFORM, "rainy", "snowy", new SpeciesFormChangeWeatherTrigger(Abilities.FORECAST, [WeatherType.HAIL, WeatherType.SNOW]), true),
+    new SpeciesFormChange(Species.CASTFORM, "sunny", "", new SpeciesFormChangeRevertWeatherFormTrigger(Abilities.FORECAST, [WeatherType.NONE, WeatherType.SANDSTORM, WeatherType.STRONG_WINDS, WeatherType.FOG]), true),
+    new SpeciesFormChange(Species.CASTFORM, "rainy", "", new SpeciesFormChangeRevertWeatherFormTrigger(Abilities.FORECAST, [WeatherType.NONE, WeatherType.SANDSTORM, WeatherType.STRONG_WINDS, WeatherType.FOG]), true),
+    new SpeciesFormChange(Species.CASTFORM, "snowy", "", new SpeciesFormChangeRevertWeatherFormTrigger(Abilities.FORECAST, [WeatherType.NONE, WeatherType.SANDSTORM, WeatherType.STRONG_WINDS, WeatherType.FOG]), true),
+    new SpeciesFormChange(Species.CASTFORM, "sunny", "", new SpeciesFormChangeActiveTrigger(), true),
+    new SpeciesFormChange(Species.CASTFORM, "rainy", "", new SpeciesFormChangeActiveTrigger(), true),
+    new SpeciesFormChange(Species.CASTFORM, "snowy", "", new SpeciesFormChangeActiveTrigger(), true),
+  ],
 };
 
 export function initPokemonForms() {

src/field/arena.ts

@@ -20,7 +20,10 @@ import { Moves } from "#enums/moves";
 import { Species } from "#enums/species";
 import { TimeOfDay } from "#enums/time-of-day";
 import { TrainerType } from "#enums/trainer-type";
-import { CommonAnimPhase } from "#app/phases/common-anim-phase.js";
+import { Abilities } from "#app/enums/abilities";
+import { SpeciesFormChangeRevertWeatherFormTrigger, SpeciesFormChangeWeatherTrigger } from "#app/data/pokemon-forms";
+import { CommonAnimPhase } from "#app/phases/common-anim-phase";
+import { ShowAbilityPhase } from "#app/phases/show-ability-phase";
 
 export class Arena {
   public scene: BattleScene;
@@ -331,6 +334,30 @@ export class Arena {
     return true;
   }
 
+  /**
+   * Function to trigger all weather based form changes
+   */
+  triggerWeatherBasedFormChanges(): void {
+    this.scene.getField(true).forEach( p => {
+      if (p.hasAbility(Abilities.FORECAST) && p.species.speciesId === Species.CASTFORM) {
+        new ShowAbilityPhase(this.scene, p.getBattlerIndex());
+        this.scene.triggerPokemonFormChange(p, SpeciesFormChangeWeatherTrigger);
+      }
+    });
+  }
+
+  /**
+   * Function to trigger all weather based form changes back into their normal forms
+   */
+  triggerWeatherBasedFormChangesToNormal(): void {
+    this.scene.getField(true).forEach( p => {
+      if (p.hasAbility(Abilities.FORECAST, false, true) && p.species.speciesId === Species.CASTFORM) {
+        new ShowAbilityPhase(this.scene, p.getBattlerIndex());
+        return this.scene.triggerPokemonFormChange(p, SpeciesFormChangeRevertWeatherFormTrigger);
+      }
+    });
+  }
+
   trySetTerrain(terrain: TerrainType, hasPokemonSource: boolean, ignoreAnim: boolean = false): boolean {
     if (this.terrain?.terrainType === (terrain || undefined)) {
       return false;

src/field/pokemon.ts

@@ -1140,7 +1140,7 @@ export default abstract class Pokemon extends Phaser.GameObjects.Container {
    * effects which can affect whether an ability will be present or in effect, and both passive and
    * non-passive. This is the primary way to check whether a pokemon has a particular ability.
    * @param {Abilities} ability The ability to check for
-   * @param {boolean} canApply If false, it doesn't check whether the abiltiy is currently active
+   * @param {boolean} canApply If false, it doesn't check whether the ability is currently active
    * @param {boolean} ignoreOverride If true, it ignores ability changing effects
    * @returns {boolean} Whether the ability is present and active
    */

src/phases/switch-summon-phase.ts

@@ -160,6 +160,8 @@ export class SwitchSummonPhase extends SummonPhase {
     this.lastPokemon?.resetSummonData();
 
     this.scene.triggerPokemonFormChange(pokemon, SpeciesFormChangeActiveTrigger, true);
+    // Reverts to weather-based forms when weather suppressors (Cloud Nine/Air Lock) are switched out
+    this.scene.arena.triggerWeatherBasedFormChanges();
   }
 
   queuePostSummon(): void {

src/phases/turn-end-phase.ts

@@ -60,6 +60,7 @@ export class TurnEndPhase extends FieldPhase {
 
     if (this.scene.arena.weather && !this.scene.arena.weather.lapse()) {
       this.scene.arena.trySetWeather(WeatherType.NONE, false);
+      this.scene.arena.triggerWeatherBasedFormChangesToNormal();
     }
 
     if (this.scene.arena.terrain && !this.scene.arena.terrain.lapse()) {

src/test/abilities/forecast.test.ts

@@ -0,0 +1,347 @@
+import { Abilities } from "#app/enums/abilities.js";
+import GameManager from "#test/utils/gameManager";
+import { Moves } from "#enums/moves";
+import { Species } from "#enums/species";
+import Phaser from "phaser";
+import { afterEach, beforeAll, beforeEach, describe, expect, it, vi } from "vitest";
+import { SPLASH_ONLY } from "#test/utils/testUtils";
+import { WeatherType } from "#app/enums/weather-type";
+import { BattlerIndex } from "#app/battle";
+import { QuietFormChangePhase } from "#app/phases/quiet-form-change-phase";
+import { DamagePhase } from "#app/phases/damage-phase";
+import { MovePhase } from "#app/phases/move-phase";
+import { PostSummonPhase } from "#app/phases/post-summon-phase";
+import { TurnEndPhase } from "#app/phases/turn-end-phase";
+import { VictoryPhase } from "#app/phases/victory-phase";
+import { allAbilities } from "#app/data/ability";
+
+describe("Abilities - Forecast", () => {
+  let phaserGame: Phaser.Game;
+  let game: GameManager;
+  const NORMAL_FORM = 0;
+  const SUNNY_FORM = 1;
+  const RAINY_FORM = 2;
+  const SNOWY_FORM = 3;
+
+  /**
+   * Tests form changes based on weather changes
+   * @param {GameManager} game The game manager instance
+   * @param {WeatherType} weather The active weather to set
+   * @param form The expected form based on the active weather
+   * @param initialForm The initial form pre form change
+   */
+  const testWeatherFormChange = async (game: GameManager, weather: WeatherType, form: number, initialForm?: number) => {
+    game.override.weather(weather).starterForms({[Species.CASTFORM]: initialForm});
+    await game.startBattle([Species.CASTFORM]);
+
+    game.move.select(Moves.SPLASH);
+
+    expect(game.scene.getPlayerPokemon()?.formIndex).toBe(form);
+  };
+
+  /**
+   * Tests reverting to normal form when Cloud Nine/Air Lock is active on the field
+   * @param {GameManager} game The game manager instance
+   * @param {Abilities} ability The ability that is active on the field
+   */
+  const testRevertFormAgainstAbility = async (game: GameManager, ability: Abilities) => {
+    game.override.starterForms({ [Species.CASTFORM]: SUNNY_FORM }).enemyAbility(ability);
+    await game.startBattle([Species.CASTFORM]);
+
+    game.move.select(Moves.SPLASH);
+
+    expect(game.scene.getPlayerPokemon()?.formIndex).toBe(NORMAL_FORM);
+  };
+
+  /**
+   * Tests transforming back to match the weather when Cloud Nine/Air Lock user is fainted
+   * @param {GameManager} game The game manager instance
+   * @param {Abilities} ability The ability that will go out of battle (faint)
+   */
+  const testTransformAfterAbilityFaint = async (game: GameManager, ability: Abilities) => {
+    game.override.enemyAbility(ability).weather(WeatherType.SNOW).enemySpecies(Species.SHUCKLE);
+    await game.startBattle([Species.CASTFORM]);
+    const castform = game.scene.getPlayerPokemon();
+
+    expect(castform?.formIndex).toBe(NORMAL_FORM);
+
+    game.move.select(Moves.TACKLE);
+    await game.setTurnOrder([ BattlerIndex.PLAYER, BattlerIndex.ENEMY ]);
+    await game.phaseInterceptor.to(DamagePhase);
+
+    await game.doKillOpponents();
+    await game.phaseInterceptor.to(VictoryPhase);
+
+    expect(castform?.formIndex).toBe(SNOWY_FORM);
+  };
+
+  /**
+   * Tests transforming back to match the weather when Cloud Nine/Air Lock user is switched out
+   * @param {GameManager} game The game manager instance
+   * @param {Abilities} ability The ability that will go out of battle (switched out)
+   */
+  const testTransformAfterAbilitySwitchOut = async (game: GameManager, ability: Abilities) => {
+    game.override
+      .weather(WeatherType.SNOW)
+      .enemySpecies(Species.CASTFORM)
+      .enemyAbility(Abilities.FORECAST)
+      .ability(ability);
+    await game.startBattle([Species.PICHU, Species.PIKACHU]);
+
+    const castform = game.scene.getEnemyPokemon();
+
+    // We mock the return value of the second Pokemon to be other than Air Lock/Cloud Nine
+    vi.spyOn(game.scene.getParty()[1]!, "getAbility").mockReturnValue(allAbilities[Abilities.BALL_FETCH]);
+    expect(game.scene.getParty()[1]?.hasAbility(Abilities.BALL_FETCH));
+
+    expect(castform?.formIndex).toBe(NORMAL_FORM);
+
+    game.doSwitchPokemon(1);
+    await game.phaseInterceptor.to(MovePhase);
+    expect(castform?.formIndex).toBe(SNOWY_FORM);
+  };
+
+  beforeAll(() => {
+    phaserGame = new Phaser.Game({
+      type: Phaser.HEADLESS,
+    });
+  });
+
+  afterEach(() => {
+    game.phaseInterceptor.restoreOg();
+  });
+
+  beforeEach(() => {
+    game = new GameManager(phaserGame);
+    game.override.moveset([ Moves.SPLASH, Moves.RAIN_DANCE, Moves.SUNNY_DAY, Moves.TACKLE ])
+      .enemyMoveset(SPLASH_ONLY)
+      .enemyAbility(Abilities.BALL_FETCH);
+  });
+
+  it("changes to Sunny Form during Harsh Sunlight", async () => {
+    await testWeatherFormChange(game, WeatherType.SUNNY, SUNNY_FORM);
+  });
+
+  it("changes to Sunny Form during Extreme Harsh Sunlight", async () => {
+    await testWeatherFormChange(game, WeatherType.HARSH_SUN, SUNNY_FORM);
+  });
+
+  it("changes to Rainy Form during Rain", async () => {
+    await testWeatherFormChange(game, WeatherType.RAIN, RAINY_FORM);
+  });
+
+  it("changes to Rainy Form during Heavy Rain", async () => {
+    await testWeatherFormChange(game, WeatherType.HEAVY_RAIN, RAINY_FORM);
+  });
+
+  it("changes to Snowy Form during Hail", async () => {
+    await testWeatherFormChange(game, WeatherType.HAIL, SNOWY_FORM);
+  });
+
+  it("changes to Snowy Form during Snow", async () => {
+    await testWeatherFormChange(game, WeatherType.SNOW, SNOWY_FORM);
+  });
+
+  it("reverts to Normal Form during Sandstorm", async () => {
+    await testWeatherFormChange(game, WeatherType.SANDSTORM, NORMAL_FORM, SUNNY_FORM);
+  });
+
+  it("reverts to Normal Form during Fog", async () => {
+    await testWeatherFormChange(game, WeatherType.FOG, NORMAL_FORM, SUNNY_FORM);
+  });
+
+  it("reverts to Normal Form during Strong Winds", async () => {
+    await testWeatherFormChange(game, WeatherType.STRONG_WINDS, NORMAL_FORM, SUNNY_FORM);
+  });
+
+  it("reverts to Normal Form during Clear weather", async () => {
+    await testWeatherFormChange(game, WeatherType.NONE, NORMAL_FORM, SUNNY_FORM);
+  });
+
+  it("reverts to Normal Form if a Pokémon on the field has Cloud Nine", async () => {
+    await testRevertFormAgainstAbility(game, Abilities.CLOUD_NINE);
+  });
+
+  it("reverts to Normal Form if a Pokémon on the field has Air Lock", async () => {
+    await testRevertFormAgainstAbility(game, Abilities.AIR_LOCK);
+  });
+
+  it("has no effect on Pokémon other than Castform", async () => {
+    game.override.enemyAbility(Abilities.FORECAST).enemySpecies(Species.SHUCKLE);
+    await game.startBattle([Species.CASTFORM]);
+
+    game.move.select(Moves.RAIN_DANCE);
+    await game.phaseInterceptor.to(TurnEndPhase);
+
+    expect(game.scene.getPlayerPokemon()?.formIndex).toBe(RAINY_FORM);
+    expect(game.scene.getEnemyPokemon()?.formIndex).not.toBe(RAINY_FORM);
+  });
+
+  it("cannot be copied", async () => {
+    game.override.enemyAbility(Abilities.TRACE);
+    await game.startBattle([Species.CASTFORM]);
+
+    game.move.select(Moves.SPLASH);
+
+    expect(game.scene.getEnemyPokemon()?.hasAbility(Abilities.FORECAST)).toBe(false);
+  });
+
+  it("(Skill Swap) reverts to Normal Form when Castform loses Forecast, changes form to match the weather when it regains it", async () => {
+    game.override.moveset([Moves.SKILL_SWAP]).weather(WeatherType.RAIN);
+    await game.startBattle([Species.CASTFORM]);
+    const castform = game.scene.getPlayerPokemon();
+
+    expect(castform?.formIndex).toBe(RAINY_FORM);
+
+    // First turn - loses Forecast
+    game.move.select(Moves.SKILL_SWAP);
+    await game.phaseInterceptor.to(TurnEndPhase);
+
+    expect(castform?.formIndex).toBe(NORMAL_FORM);
+
+    // Second turn - regains Forecast
+    game.move.select(Moves.SKILL_SWAP);
+    await game.phaseInterceptor.to(TurnEndPhase);
+
+    expect(castform?.formIndex).toBe(RAINY_FORM);
+  });
+
+  it("(Worry Seed) reverts to Normal Form when Castform loses Forecast, changes form to match the weather when it regains it", async () => {
+    game.override.enemyMoveset(Array(4).fill(Moves.WORRY_SEED)).weather(WeatherType.RAIN);
+    await game.startBattle([Species.CASTFORM, Species.PIKACHU]);
+    const castform = game.scene.getPlayerPokemon();
+
+    expect(castform?.formIndex).toBe(RAINY_FORM);
+
+    // First turn - loses Forecast
+    game.move.select(Moves.SPLASH);
+    await game.phaseInterceptor.to(TurnEndPhase);
+
+    expect(castform?.formIndex).toBe(NORMAL_FORM);
+
+    await game.toNextTurn();
+
+    // Second turn - switch out Castform, regains Forecast
+    game.doSwitchPokemon(1);
+    await game.toNextTurn();
+
+    // Third turn - switch in Castform
+    game.doSwitchPokemon(1);
+    await game.phaseInterceptor.to(MovePhase);
+
+    expect(castform?.formIndex).toBe(RAINY_FORM);
+  });
+
+  it("reverts to Normal Form when active weather ends", async () => {
+    await game.startBattle([Species.CASTFORM]);
+    const castform = game.scene.getPlayerPokemon();
+
+    game.move.select(Moves.SUNNY_DAY);
+    await game.phaseInterceptor.to(TurnEndPhase);
+
+    while (game.scene.arena.weather && game.scene.arena.weather.turnsLeft > 0) {
+      game.move.select(Moves.SPLASH);
+      expect(castform?.formIndex).toBe(SUNNY_FORM);
+      await game.toNextTurn();
+    }
+
+    expect(castform?.formIndex).toBe(NORMAL_FORM);
+  });
+
+  it("reverts to Normal Form when Forecast is suppressed, changes form to match the weather when it regains it", async () => {
+    game.override.enemyMoveset(Array(4).fill(Moves.GASTRO_ACID)).weather(WeatherType.RAIN);
+    await game.startBattle([Species.CASTFORM, Species.PIKACHU]);
+    const castform = game.scene.getPlayerPokemon();
+
+    expect(castform?.formIndex).toBe(RAINY_FORM);
+
+    // First turn - Forecast is suppressed
+    game.move.select(Moves.SPLASH);
+    await game.setTurnOrder([ BattlerIndex.ENEMY, BattlerIndex.PLAYER ]);
+    await game.move.forceHit();
+
+    await game.phaseInterceptor.to(TurnEndPhase);
+
+    expect(castform?.summonData.abilitySuppressed).toBe(true);
+    expect(castform?.formIndex).toBe(NORMAL_FORM);
+
+    await game.toNextTurn();
+
+    // Second turn - switch out Castform, regains Forecast
+    game.doSwitchPokemon(1);
+    await game.toNextTurn();
+
+    // Third turn - switch in Castform
+    game.doSwitchPokemon(1);
+    await game.phaseInterceptor.to(MovePhase);
+
+    expect(castform?.summonData.abilitySuppressed).toBe(false);
+    expect(castform?.formIndex).toBe(RAINY_FORM);
+  });
+
+  it("if a Pokémon transforms into Castform, the Pokémon will remain in the same form as the target Castform, regardless of the weather", async () => {
+    game.override.enemyMoveset(Array(4).fill(Moves.TRANSFORM));
+    await game.startBattle([Species.CASTFORM]);
+
+    game.move.select(Moves.SUNNY_DAY);
+    await game.setTurnOrder([ BattlerIndex.ENEMY, BattlerIndex.PLAYER ]);
+
+    await game.phaseInterceptor.to(TurnEndPhase);
+
+    expect(game.scene.getPlayerPokemon()?.formIndex).toBe(SUNNY_FORM);
+    expect(game.scene.getEnemyPokemon()?.formIndex).toBe(NORMAL_FORM);
+  });
+
+  it("does not change Castform's form until after Stealth Rock deals damage", async () => {
+    game.override.weather(WeatherType.RAIN).enemyMoveset(Array(4).fill(Moves.STEALTH_ROCK));
+    await game.startBattle([Species.PIKACHU, Species.CASTFORM]);
+
+    // First turn - set up stealth rock
+    game.move.select(Moves.SPLASH);
+    await game.toNextTurn();
+
+    // Second turn - switch in Castform, regains Forecast
+    game.doSwitchPokemon(1);
+    await game.phaseInterceptor.to(PostSummonPhase);
+
+    const castform = game.scene.getPlayerPokemon();
+
+    // Damage phase should come first
+    await game.phaseInterceptor.to(DamagePhase);
+    expect(castform?.hp).toBeLessThan(castform?.getMaxHp() ?? 0);
+
+    // Then change form
+    await game.phaseInterceptor.to(QuietFormChangePhase);
+    expect(castform?.formIndex).toBe(RAINY_FORM);
+  });
+
+  it("transforms to weather-based form when Pokemon with Air Lock is fainted", async () => {
+    await testTransformAfterAbilityFaint(game, Abilities.AIR_LOCK);
+  });
+
+  it("transforms to weather-based form when Pokemon with Cloud Nine is fainted", async () => {
+    await testTransformAfterAbilityFaint(game, Abilities.CLOUD_NINE);
+  });
+
+  it("transforms to weather-based form when Pokemon with Air Lock is switched out", async () => {
+    await testTransformAfterAbilitySwitchOut(game, Abilities.AIR_LOCK);
+  });
+
+  it("transforms to weather-based form when Pokemon with Cloud Nine is switched out", async () => {
+    await testTransformAfterAbilitySwitchOut(game, Abilities.CLOUD_NINE);
+  });
+
+  it("should be in Normal Form after the user is switched out", async () => {
+    game.override.weather(WeatherType.RAIN);
+
+    await game.startBattle([Species.CASTFORM, Species.MAGIKARP]);
+    const castform = game.scene.getPlayerPokemon()!;
+
+    expect(castform.formIndex).toBe(RAINY_FORM);
+
+    game.doSwitchPokemon(1);
+    await game.toNextTurn();
+
+    expect(castform.formIndex).toBe(NORMAL_FORM);
+  });
+});