Fix Gen 4 interaction between U-turn/Substitute/Intimidate

data/mods/gen4/abilities.js

@@ -133,6 +133,38 @@ let BattleAbilities = {
 		inherit: true,
 		rating: 2.5,
 	},
+	"intimidate": {
+		inherit: true,
+		onStart(pokemon) {
+			let activated = false;
+			for (const target of pokemon.side.foe.active) {
+				if (target && this.isAdjacent(target, pokemon) &&
+					!(target.volatiles['substitute'] ||
+						target.volatiles['substitutebroken'] && target.volatiles['substitutebroken'].move === 'uturn')) {
+					activated = true;
+					break;
+				}
+			}
+
+			if (!activated) {
+				this.hint('In Gen 4, Intimidate does not activate if every target has a Substitute (or the Substitute was just broken by U-turn).', false, pokemon.side.id);
+				return;
+			}
+			this.add('-ability', pokemon, 'Intimidate', 'boost');
+
+			for (const target of pokemon.side.foe.active) {
+				if (!target || !this.isAdjacent(target, pokemon)) continue;
+
+				if (target.volatiles['substitute']) {
+					this.add('-immune', target);
+				} else if (target.volatiles['substitutebroken'] && target.volatiles['substitutebroken'].move === 'uturn') {
+					this.hint('In Gen 4, if U-turn breaks Substitute the incoming Intimidate does nothing.');
+				} else {
+					this.boost({atk: -1}, target, pokemon);
+				}
+			}
+		},
+	},
 	"leafguard": {
 		inherit: true,
 		desc: "If Sunny Day is active, this Pokemon cannot gain a major status condition, but can use Rest normally.",

data/mods/gen4/moves.js

@@ -1473,6 +1473,55 @@ let BattleMovedex = {
 		inherit: true,
 		desc: "If the target lost HP, the user takes recoil damage equal to 1/4 the HP lost by the target, rounded down, but not less than 1 HP.",
 	},
+	substitute: {
+		inherit: true,
+		effect: {
+			onStart(target) {
+				this.add('-start', target, 'Substitute');
+				this.effectData.hp = Math.floor(target.maxhp / 4);
+				delete target.volatiles['partiallytrapped'];
+			},
+			onTryPrimaryHitPriority: -1,
+			onTryPrimaryHit(target, source, move) {
+				if (target === source || move.flags['authentic']) {
+					return;
+				}
+				let damage = this.getDamage(source, target, move);
+				if (!damage && damage !== 0) {
+					this.add('-fail', source);
+					this.attrLastMove('[still]');
+					return null;
+				}
+				damage = this.runEvent('SubDamage', target, source, move, damage);
+				if (!damage) {
+					return damage;
+				}
+				if (damage > target.volatiles['substitute'].hp) {
+					damage = /** @type {number} */ (target.volatiles['substitute'].hp);
+				}
+				target.volatiles['substitute'].hp -= damage;
+				source.lastDamage = damage;
+				if (target.volatiles['substitute'].hp <= 0) {
+					target.removeVolatile('substitute');
+					target.addVolatile('substitutebroken');
+					if (target.volatiles['substitutebroken']) target.volatiles['substitutebroken'].move = move.id;
+				} else {
+					this.add('-activate', target, 'Substitute', '[damage]');
+				}
+				if (move.recoil && damage) {
+					this.damage(this.calcRecoilDamage(damage, move), source, target, 'recoil');
+				}
+				if (move.drain) {
+					this.heal(Math.ceil(damage * move.drain[0] / move.drain[1]), source, target, 'drain');
+				}
+				this.runEvent('AfterSubDamage', target, source, move, damage);
+				return 0; // hit
+			},
+			onEnd(target) {
+				this.add('-end', target, 'Substitute');
+			},
+		},
+	},
 	suckerpunch: {
 		inherit: true,
 		desc: "Fails if the target did not select a physical or special attack for use this turn, or if the target moves before the user.",

data/mods/gen4/statuses.js

@@ -50,6 +50,9 @@ let BattleStatuses = {
 			return false;
 		},
 	},
+	substitutebroken: {
+		noCopy: true,
+	},
 	trapped: {
 		inherit: true,
 		noCopy: false,

sim/battle.ts

@@ -2755,6 +2755,11 @@ export class Battle extends Dex.ModdedDex {
 				action.pokemon.abilityOrder = this.abilityOrder++;
 				this.singleEvent('Start', action.pokemon.getItem(), action.pokemon.itemData, action.pokemon);
 			}
+			if (this.gen === 4) {
+				for (const foeActive of action.pokemon.side.foe.active) {
+					foeActive.removeVolatile('substitutebroken');
+				}
+			}
 			delete action.pokemon.draggedIn;
 			break;
 		case 'runPrimal':

test/simulator/abilities/intimidate.js

@@ -31,6 +31,31 @@ describe('Intimidate', function () {
 		assert.statStage(battle.p1.active[0], 'atk', 0);
 	});
 
+	it('should not activate if U-turn breaks the Substitute in Gen 4', function () {
+		battle = common.gen(4).createBattle({gameType: 'doubles'});
+		battle.join('p1', 'Guest 1', 1, [
+			{species: "Gengar", level: 1, item: 'leftovers', ability: 'levitate', moves: ['substitute']},
+			{species: "Suicune", level: 1, item: 'leftovers', ability: 'pressure', moves: ['substitute']},
+		]);
+		battle.join('p2', 'Guest 2', 1, [
+			{species: "Gliscor", item: 'laggingtail', ability: 'sandveil', moves: ['uturn']},
+			{species: "Scizor", item: 'laggingtail', ability: 'technician', moves: ['batonpass']},
+			{species: "Gyarados", item: 'leftovers', ability: 'intimidate', moves: ['splash']},
+			{species: "Salamence", item: 'leftovers', ability: 'intimidate', moves: ['splash']},
+		]);
+		battle.makeChoices('move substitute, move substitute', 'move uturn, move batonpass');
+		battle.makeChoices('pass, pass', 'switch 3, pass');
+
+		const activate = '|-ability|p2a: Gyarados|Intimidate|boost';
+		assert.strictEqual(battle.log.filter(m => m === activate).length, 0);
+		assert.statStage(battle.p1.active[0], 'atk', 0);
+		assert.statStage(battle.p1.active[1], 'atk', 0);
+
+		battle.makeChoices('pass, pass', 'pass, switch 4');
+		assert.statStage(battle.p1.active[0], 'atk', -1);
+		assert.statStage(battle.p1.active[1], 'atk', 0);
+	});
+
 	it('should affect adjacent foes only', function () {
 		battle = common.createBattle({gameType: 'triples'});
 		const p1 = battle.join('p1', 'Guest 1', 1, [