Fix Forewarn edge cases and random selection (#9049)

include/random.h

@@ -167,6 +167,7 @@ enum RandomTag
     RNG_QUICK_DRAW,
     RNG_QUICK_CLAW,
     RNG_TRACE,
+    RNG_FOREWARN,
     RNG_FICKLE_BEAM,
     RNG_AI_ABILITY,
     RNG_AI_SWITCH_HASBADODDS,

src/battle_util.c

@@ -3335,12 +3335,43 @@ static void ForewarnChooseMove(u32 battler)
         }
     }
 
-    for (bestId = 0, i = 1; i < count; i++)
+    if (count == 0)
     {
-        if (data[i].power > data[bestId].power)
-            bestId = i;
-        else if (data[i].power == data[bestId].power && Random() & 1)
+        Free(data);
+        return;
+    }
+
+    u32 tieCount = 1;
+    u8 bestPower = data[0].power;
+
+    bestId = 0;
+    for (i = 1; i < count; i++)
+    {
+        if (data[i].power > bestPower)
+        {
+            bestPower = data[i].power;
             bestId = i;
+            tieCount = 1;
+        }
+        else if (data[i].power == bestPower)
+        {
+            tieCount++;
+        }
+    }
+
+    if (tieCount > 1)
+    {
+        u32 tieIndex = RandomUniform(RNG_FOREWARN, 0, tieCount - 1);
+        for (i = 0, bestId = 0; i < count; i++)
+        {
+            if (data[i].power != bestPower)
+                continue;
+            if (tieIndex-- == 0)
+            {
+                bestId = i;
+                break;
+            }
+        }
     }
 
     gEffectBattler = data[bestId].battler;
@@ -4155,7 +4186,7 @@ u32 AbilityBattleEffects(enum AbilityEffect caseID, u32 battler, enum Ability ab
             }
             return effect; // Note: It returns effect as to not record the ability if Frisk does not activate.
         case ABILITY_FOREWARN:
-            if (!gSpecialStatuses[battler].switchInAbilityDone)
+            if (!gSpecialStatuses[battler].switchInAbilityDone && !IsOpposingSideEmpty(battler))
             {
                 ForewarnChooseMove(battler);
                 gBattleCommunication[MULTISTRING_CHOOSER] = B_MSG_SWITCHIN_FOREWARN;

test/battle/ability/forewarn.c

@@ -1,4 +1,84 @@
 #include "global.h"
 #include "test/battle.h"
 
-TO_DO_BATTLE_TEST("TODO: Write Forewarn (Ability) test titles")
+DOUBLE_BATTLE_TEST("Forewarn warns about the highest power move among all opposing battlers")
+{
+    GIVEN {
+        PLAYER(SPECIES_MUSHARNA) { Ability(ABILITY_FOREWARN); }
+        PLAYER(SPECIES_WYNAUT);
+        OPPONENT(SPECIES_ZUBAT) { Moves(MOVE_CRUNCH, MOVE_CELEBRATE); }
+        OPPONENT(SPECIES_EXCADRILL) { Moves(MOVE_FISSURE, MOVE_CELEBRATE); }
+    } WHEN {
+        TURN {}
+    } SCENE {
+        ABILITY_POPUP(playerLeft, ABILITY_FOREWARN);
+        MESSAGE("Forewarn alerted Musharna to the opposing Excadrill's Fissure!");
+    }
+}
+
+SINGLE_BATTLE_TEST("Forewarn randomly chooses between same-power moves on one opponent")
+{
+    PASSES_RANDOMLY(1, 3, RNG_FOREWARN);
+    GIVEN {
+        ASSUME(GetMovePower(MOVE_TACKLE) == GetMovePower(MOVE_POUND));
+        ASSUME(GetMovePower(MOVE_TACKLE) == GetMovePower(MOVE_SCRATCH));
+        PLAYER(SPECIES_MUSHARNA) { Ability(ABILITY_FOREWARN); }
+        OPPONENT(SPECIES_ZUBAT) { Moves(MOVE_TACKLE, MOVE_POUND, MOVE_SCRATCH, MOVE_CELEBRATE); }
+    } WHEN {
+        TURN {}
+    } SCENE {
+        ABILITY_POPUP(player, ABILITY_FOREWARN);
+        MESSAGE("Forewarn alerted Musharna to the opposing Zubat's Tackle!");
+    }
+}
+
+DOUBLE_BATTLE_TEST("Forewarn randomly chooses between opponents with same-power moves")
+{
+    PASSES_RANDOMLY(1, 4, RNG_FOREWARN);
+    GIVEN {
+        ASSUME(GetMovePower(MOVE_TACKLE) == GetMovePower(MOVE_POUND));
+        ASSUME(GetMovePower(MOVE_TACKLE) == GetMovePower(MOVE_SCRATCH));
+        ASSUME(GetMovePower(MOVE_TACKLE) == GetMovePower(MOVE_QUICK_ATTACK));
+        PLAYER(SPECIES_MUSHARNA) { Ability(ABILITY_FOREWARN); }
+        PLAYER(SPECIES_WYNAUT);
+        OPPONENT(SPECIES_ZUBAT) { Moves(MOVE_TACKLE, MOVE_POUND, MOVE_PECK, MOVE_CELEBRATE); }
+        OPPONENT(SPECIES_EXCADRILL) { Moves(MOVE_SCRATCH, MOVE_QUICK_ATTACK, MOVE_ABSORB, MOVE_CELEBRATE); }
+    } WHEN {
+        TURN {}
+    } SCENE {
+        ABILITY_POPUP(playerLeft, ABILITY_FOREWARN);
+        MESSAGE("Forewarn alerted Musharna to the opposing Zubat's Tackle!");
+    }
+}
+
+DOUBLE_BATTLE_TEST("Forewarn does not trigger if a mon switches in while the opposing field is empty")
+{
+    GIVEN {
+        ASSUME(GetMoveEffect(MOVE_U_TURN) == EFFECT_HIT_ESCAPE);
+        ASSUME(GetMoveEffect(MOVE_HEALING_WISH) == EFFECT_HEALING_WISH);
+        PLAYER(SPECIES_WYNAUT);
+        PLAYER(SPECIES_WOBBUFFET);
+        PLAYER(SPECIES_MUSHARNA) { Ability(ABILITY_FOREWARN); }
+        OPPONENT(SPECIES_WYNAUT) { HP(1); }
+        OPPONENT(SPECIES_WOBBUFFET);
+        OPPONENT(SPECIES_TREECKO);
+        OPPONENT(SPECIES_TORCHIC);
+    } WHEN {
+        TURN {
+            MOVE(opponentRight, MOVE_HEALING_WISH);
+            MOVE(playerLeft, MOVE_U_TURN, target: opponentLeft);
+            SEND_OUT(playerLeft, 2);
+            SEND_OUT(opponentLeft, 2);
+            SEND_OUT(opponentRight, 3);
+        }
+    } SCENE {
+        ANIMATION(ANIM_TYPE_MOVE, MOVE_CELEBRATE, playerRight);
+        ANIMATION(ANIM_TYPE_MOVE, MOVE_HEALING_WISH, opponentRight);
+        ANIMATION(ANIM_TYPE_MOVE, MOVE_U_TURN, playerLeft);
+        HP_BAR(opponentLeft);
+        NOT ABILITY_POPUP(playerLeft, ABILITY_FOREWARN);
+        MESSAGE("2 sent out Treecko!");
+        MESSAGE("2 sent out Torchic!");
+        NOT ABILITY_POPUP(playerLeft, ABILITY_FOREWARN);
+    }
+}