Mega Evolution support is mostly done.

asm/macros/battle_script.inc

@@ -1473,6 +1473,10 @@
 	.4byte \ptr
 	.endm
 	
+	.macro handlemegaevo battler
+	various \battler, VARIOUS_HANDLE_MEGA_EVO
+	.endm
+	
 @ helpful macros
 	.macro setstatchanger stat, stages, down
 	setbyte sSTATCHANGER \stat | \stages << 4 | \down << 7

data/battle_anim_scripts.s

@@ -672,6 +672,7 @@ gBattleAnims_VariousTable:: @ 82C9320
 	.4byte Anim_Table_x14
 	.4byte Status_Ingrain
 	.4byte Anim_WishHeal
+	.4byte General_MegaEvolution
 
 .align 2
 gBattleAnims_Special:: @ 82C937C
@@ -12054,6 +12055,9 @@ Anim_WishHeal:
 	waitforvisualfinish
 	createsprite gUnknown_08597274, 0x2, 1, 3, 10, 0, 0
 	end
+	
+General_MegaEvolution:
+	end
 
 AnimScript_82D85A3:
 	createvisualtask sub_8172E9C, 0x2

data/battle_scripts_1.s

@@ -4610,6 +4610,16 @@ BattleScript_FocusPunchSetUp::
 	printstring STRINGID_PKMNTIGHTENINGFOCUS
 	waitmessage 0x40
 	end2
+	
+BattleScript_MegaEvolution::
+	printstring STRINGID_MEGAEVOREACTING
+	waitmessage 0x40
+	playanimation BS_ATTACKER, B_ANIM_MEGA_EVOLUTION, NULL
+	waitanimation
+	handlemegaevo BS_ATTACKER
+	printstring STRINGID_MEGAEVOEVOLVED
+	waitmessage 0x40
+	end2
 
 BattleScript_MoveUsedIsAsleep::
 	printstring STRINGID_PKMNFASTASLEEP

include/battle.h

@@ -598,6 +598,11 @@ struct BattleStruct
     bool8 ateBoost[MAX_BATTLERS_COUNT];
     u32 debugAIFlags;
     bool8 notfirstTimeAIFlags;
+    u8 toMegaEvolve; // As flags using gBitTable.
+    u8 megaEvolvedPartyIds[2]; // As flags using gBitTable;
+    bool8 alreadyMegaEvolved[4]; // Array id is used for mon position.
+    u16 speciesToMegaEvolve[MAX_BATTLERS_COUNT];
+    u8 megaEvoBattlerId;
 };
 
 #define GET_MOVE_TYPE(move, typeArg)                        \

include/battle_controllers.h

@@ -84,6 +84,7 @@ enum
 
 // Special return values in gBattleBufferB from Battle Controller functions.
 #define RET_VALUE_LEVELED_UP   11
+#define RET_MEGA_EVOLUTION 0x80
 
 struct UnusedControllerStruct
 {

include/battle_scripts.h

@@ -271,5 +271,6 @@ extern const u8 BattleScript_SelectingNotAllowedMoveHealBlockInPalace[];
 extern const u8 BattleScript_ToxicSpikesFree[];
 extern const u8 BattleScript_StickyWebFree[];
 extern const u8 BattleScript_StealthRockFree[];
+extern const u8 BattleScript_MegaEvolution[];
 
 #endif // GUARD_BATTLE_SCRIPTS_H

include/battle_util.h

@@ -88,5 +88,6 @@ u16 CalcTypeEffectivenessMultiplier(u16 move, u8 moveType, u8 battlerAtk, u8 bat
 u16 CalcPartyMonTypeEffectivenessMultiplier(u16 move, u16 speciesDef, u8 abilityDef);
 u16 GetTypeModifier(u8 atkType, u8 defType);
 s32 GetStealthHazardDamage(u8 hazardType, u8 battlerId);
+bool32 CanMegaEvolve(u8 battlerId);
 
 #endif // GUARD_BATTLE_UTIL_H

include/constants/battle_anim.h

@@ -60,6 +60,7 @@
 #define B_ANIM_x14                      0x14
 #define B_ANIM_INGRAIN_HEAL             0x15
 #define B_ANIM_WISH_HEAL                0x16
+#define B_ANIM_MEGA_EVOLUTION           0x17
 
 // special animations table
 #define B_ANIM_LVL_UP                   0x0

include/constants/battle_script_commands.h

@@ -99,6 +99,7 @@
 #define VARIOUS_TRY_ELECTRIFY                   48
 #define VARIOUS_TRY_REFLECT_TYPE                49
 #define VARIOUS_TRY_SOAK                        50
+#define VARIOUS_HANDLE_MEGA_EVO					51
 
 // atk80, dmg manipulation
 #define ATK80_DMG_CHANGE_SIGN                               0

include/constants/battle_string_ids.h

@@ -502,6 +502,8 @@
 #define	STRINGID_TERRAINBECOMESELECTRIC 499
 #define	STRINGID_TERRAINBECOMESPSYCHIC 500
 #define	STRINGID_TARGETELECTRIFIED 501
+#define	STRINGID_MEGAEVOREACTING 502
+#define	STRINGID_MEGAEVOEVOLVED 503
 
 #define BATTLESTRINGS_COUNT     516
 

include/constants/hold_effects.h

@@ -109,6 +109,7 @@
 
 // Gen6 hold effects
 #define HOLD_EFFECT_FAIRY_POWER    130
+#define HOLD_EFFECT_MEGA_STONE     131
 
 // Gen7 hold effects
 #define HOLD_EFFECT_PROTECTIVE_PADS 150

include/constants/items.h

@@ -239,7 +239,7 @@
 #define ITEM_METAL_POWDER 223
 #define ITEM_THICK_CLUB 224
 #define ITEM_STICK 225
-#define ITEM_0E2 226
+#define ITEM_MEGA_STONE_TESTING 226
 #define ITEM_0E3 227
 #define ITEM_0E4 228
 #define ITEM_0E5 229

include/pokemon.h

@@ -422,6 +422,8 @@ enum
 #define EVO_LEVEL_SHEDINJA   0x000e // Pokémon reaches the specified level (special value for Shedinja)
 #define EVO_BEAUTY           0x000f // Pokémon levels up with beauty ≥ specified value
 
+#define EVO_MEGA_EVOLUTION   0xffff // Not an actual evolution, used to temporarily mega evolve in battle.
+
 struct Evolution
 {
     u16 method;

src/battle_controller_opponent.c

@@ -1548,16 +1548,15 @@ static void OpponentHandleChooseMove(void)
 
         if (gBattleTypeFlags & (BATTLE_TYPE_TRAINER | BATTLE_TYPE_FIRST_BATTLE | BATTLE_TYPE_SAFARI | BATTLE_TYPE_ROAMER))
         {
-
             BattleAI_SetupAIData(0xF);
             chosenMoveId = BattleAI_ChooseMoveOrAction();
 
             switch (chosenMoveId)
             {
-            case 5:
+            case AI_CHOICE_WATCH:
                 BtlController_EmitTwoReturnValues(1, B_ACTION_SAFARI_WATCH_CAREFULLY, 0);
                 break;
-            case 4:
+            case AI_CHOICE_FLEE:
                 BtlController_EmitTwoReturnValues(1, B_ACTION_RUN, 0);
                 break;
             case 6:
@@ -1572,7 +1571,10 @@ static void OpponentHandleChooseMove(void)
                     if (gAbsentBattlerFlags & gBitTable[gBattlerTarget])
                         gBattlerTarget = GetBattlerAtPosition(B_POSITION_PLAYER_RIGHT);
                 }
-                BtlController_EmitTwoReturnValues(1, 10, (chosenMoveId) | (gBattlerTarget << 8));
+                if (CanMegaEvolve(gActiveBattler)) // If opponent can mega evolve, do it.
+                    BtlController_EmitTwoReturnValues(1, 10 | RET_MEGA_EVOLUTION, (chosenMoveId) | (gBattlerTarget << 8));
+                else
+                    BtlController_EmitTwoReturnValues(1, 10, (chosenMoveId) | (gBattlerTarget << 8));
                 break;
             }
             OpponentBufferExecCompleted();

src/battle_main.c

@@ -166,6 +166,7 @@ static void SetActionsAndBattlersTurnOrder(void);
 static void sub_803CDF8(void);
 static bool8 AllAtActionConfirmed(void);
 static void CheckFocusPunch_ClearVarsBeforeTurnStarts(void);
+static void CheckMegaEvolutionBeforeTurn(void);
 static void FreeResetData_ReturnToOvOrDoEvolutions(void);
 static void ReturnFromBattleToOverworld(void);
 static void TryEvolvePokemon(void);
@@ -4375,9 +4376,12 @@ static void HandleTurnActionSelectionState(void)
         case STATE_WAIT_ACTION_CASE_CHOSEN:
             if (!(gBattleControllerExecFlags & ((gBitTable[gActiveBattler]) | (0xF0000000) | (gBitTable[gActiveBattler] << 4) | (gBitTable[gActiveBattler] << 8) | (gBitTable[gActiveBattler] << 0xC))))
             {
+                bool32 shouldMegaEvolve;
                 switch (gChosenActionByBattler[gActiveBattler])
                 {
                 case B_ACTION_USE_MOVE:
+                    shouldMegaEvolve = gBattleBufferB[gActiveBattler][1] & RET_MEGA_EVOLUTION;
+                    gBattleBufferB[gActiveBattler][1] &= ~(RET_MEGA_EVOLUTION);
                     switch (gBattleBufferB[gActiveBattler][1])
                     {
                     case 3:
@@ -4419,6 +4423,8 @@ static void HandleTurnActionSelectionState(void)
                             *(gBattleStruct->chosenMovePositions + gActiveBattler) = gBattleBufferB[gActiveBattler][2];
                             gChosenMoveByBattler[gActiveBattler] = gBattleMons[gActiveBattler].moves[*(gBattleStruct->chosenMovePositions + gActiveBattler)];
                             *(gBattleStruct->moveTarget + gActiveBattler) = gBattleBufferB[gActiveBattler][3];
+                            if (shouldMegaEvolve)
+                                gBattleStruct->toMegaEvolve |= gBitTable[gActiveBattler];
                             gBattleCommunication[gActiveBattler]++;
                         }
                         break;
@@ -4844,8 +4850,8 @@ static void SetActionsAndBattlersTurnOrder(void)
                     turnOrderId++;
                 }
             }
-            gBattleMainFunc = CheckFocusPunch_ClearVarsBeforeTurnStarts;
+            gBattleMainFunc = CheckMegaEvolutionBeforeTurn;
-            gBattleStruct->focusPunchBattlerId = 0;
+            gBattleStruct->megaEvoBattlerId = 0;
             return;
         }
         else
@@ -4886,8 +4892,8 @@ static void SetActionsAndBattlersTurnOrder(void)
             }
         }
     }
-    gBattleMainFunc = CheckFocusPunch_ClearVarsBeforeTurnStarts;
+    gBattleMainFunc = CheckMegaEvolutionBeforeTurn;
-    gBattleStruct->focusPunchBattlerId = 0;
+    gBattleStruct->megaEvoBattlerId = 0;
 }
 
 static void TurnValuesCleanUp(bool8 var0)
@@ -4939,6 +4945,29 @@ static void SpecialStatusesClear(void)
     }
 }
 
+static void CheckMegaEvolutionBeforeTurn(void)
+{
+    if (!(gHitMarker & HITMARKER_RUN))
+    {
+        while (gBattleStruct->megaEvoBattlerId < gBattlersCount)
+        {
+            gActiveBattler = gBattlerAttacker = gBattleStruct->megaEvoBattlerId;
+            gBattleStruct->megaEvoBattlerId++;
+            if (gBattleStruct->toMegaEvolve & gBitTable[gActiveBattler]
+                && !(gProtectStructs[gActiveBattler].noValidMoves))
+            {
+                gBattleStruct->toMegaEvolve &= ~(gBitTable[gActiveBattler]);
+                gLastUsedItem = gBattleMons[gActiveBattler].item;
+                BattleScriptExecute(BattleScript_MegaEvolution);
+                return;
+            }
+        }
+    }
+
+    gBattleMainFunc = CheckFocusPunch_ClearVarsBeforeTurnStarts;
+    gBattleStruct->focusPunchBattlerId = 0;
+}
+
 static void CheckFocusPunch_ClearVarsBeforeTurnStarts(void)
 {
     u32 i;

src/battle_message.c

@@ -631,8 +631,6 @@ static const u8 sText_NotDoneYet[] = _("This move effect is not done yet!\p");
 static const u8 sText_PkmnBlewAwayToxicSpikes[] = _("{B_ATK_NAME_WITH_PREFIX} blew away\nTOXIC SPIKES!");
 static const u8 sText_PkmnBlewAwayStickyWeb[] = _("{B_ATK_NAME_WITH_PREFIX} blew away\nSTICKY WEB!");
 static const u8 sText_PkmnBlewAwayStealthRock[] = _("{B_ATK_NAME_WITH_PREFIX} blew away\nSTEALTH ROCK!");
-
-// To do.
 static const u8 sText_StickyWebUsed[] = _("A sticky web spreads out on the\nground around your team!");
 static const u8 sText_QuashSuccess[] = _("The opposing {B_ATK_NAME_WITH_PREFIX}’s move was postponed!");
 static const u8 sText_IonDelugeOn[] = _("A deluge of ions showers\nthe battlefield!");
@@ -642,12 +640,11 @@ static const u8 sText_TerrainBecomesGrassy[] = _("Grass grew to cover\nthe battl
 static const u8 sText_TerrainBecomesElectric[] = _("An electric current runs across\nthe battlefield!");
 static const u8 sText_TerrainBecomesPsychic[] = _("The battlefield got weird!");
 static const u8 sText_TargetElectrified[] = _("The opposing {B_ATK_NAME_WITH_PREFIX}’s moves\nhave been electrified!");
-
-// New selection strings, they must end with "\p".
-// Use {B_LAST_ITEM} and {B_CURRENT_MOVE}.
 static const u8 sText_AssaultVestDoesntAllow[] = _("The effects of the {B_LAST_ITEM} prevent status\nmoves from being used!\p");
 static const u8 sText_GravityPreventsUsage[] = _("{B_SCR_ACTIVE_NAME_WITH_PREFIX} can’t use {B_CURRENT_MOVE}\nbecause of gravity!\p");
 static const u8 sText_HealBlockPreventsUsage[] = _("The opposing {B_ATK_NAME_WITH_PREFIX} was\nprevented from healing!\p");
+static const u8 sText_MegaEvoReacting[] = _("{B_ATK_NAME_WITH_PREFIX}’s {B_LAST_ITEM} is reacting\nto {B_ATK_TRAINER_NAME}’s Mega Ring!");
+static const u8 sText_MegaEvoEvolved[] = _("{B_ATK_NAME_WITH_PREFIX} has Mega\nEvolved into Mega {B_BUFF1}!");
 
 const u8 *const gBattleStringsTable[BATTLESTRINGS_COUNT] =
 {
@@ -1142,6 +1139,8 @@ const u8 *const gBattleStringsTable[BATTLESTRINGS_COUNT] =
     sText_TerrainBecomesElectric,
     sText_TerrainBecomesPsychic,
     sText_TargetElectrified,
+    sText_MegaEvoReacting,
+    sText_MegaEvoEvolved,
 };
 
 const u16 gTerrainStringIds[] =

src/battle_script_commands.c

@@ -6119,6 +6119,7 @@ static void HandleTerrainMove(u32 moveEffect)
 
 static void atk76_various(void)
 {
+    struct Pokemon *mon;
     u8 side;
     s32 i, j;
     u8 data[10];
@@ -6588,6 +6589,28 @@ static void atk76_various(void)
             gBattlescriptCurrInstr += 7;
         }
         return;
+    case VARIOUS_HANDLE_MEGA_EVO:
+        if (GetBattlerSide(gActiveBattler) == B_SIDE_OPPONENT)
+            mon = &gEnemyParty[gBattlerPartyIndexes[gActiveBattler]];
+        else
+            mon = &gPlayerParty[gBattlerPartyIndexes[gActiveBattler]];
+        gBattleMons[gActiveBattler].species = gBattleStruct->speciesToMegaEvolve[gActiveBattler];
+        SetMonData(mon, MON_DATA_SPECIES, &gBattleMons[gActiveBattler].species);
+        PREPARE_SPECIES_BUFFER(gBattleTextBuff1, gBattleMons[gActiveBattler].species);
+
+        CalculateMonStats(mon);
+        gBattleMons[gActiveBattler].hp = GetMonData(mon, MON_DATA_HP);
+        gBattleMons[gActiveBattler].maxHP = GetMonData(mon, MON_DATA_MAX_HP);
+        gBattleMons[gActiveBattler].attack = GetMonData(mon, MON_DATA_ATK);
+        gBattleMons[gActiveBattler].defense = GetMonData(mon, MON_DATA_DEF);
+        gBattleMons[gActiveBattler].speed = GetMonData(mon, MON_DATA_SPEED);
+        gBattleMons[gActiveBattler].spAttack = GetMonData(mon, MON_DATA_SPATK);
+        gBattleMons[gActiveBattler].spDefense = GetMonData(mon, MON_DATA_SPDEF);
+        UpdateHealthboxAttribute(gHealthboxSpriteIds[gActiveBattler], mon, HEALTHBOX_ALL);
+
+        gBattleStruct->alreadyMegaEvolved[GetBattlerPosition(gActiveBattler)] = TRUE;
+        gBattleStruct->megaEvolvedPartyIds[GetBattlerSide(gActiveBattler)] |= gBitTable[gBattlerPartyIndexes[gActiveBattler]];
+        break;
     }
 
     gBattlescriptCurrInstr += 3;

src/battle_util.c

@@ -5572,3 +5572,60 @@ s32 GetStealthHazardDamage(u8 hazardType, u8 battlerId)
 
     return dmg;
 }
+
+static bool32 IsPartnerMonFromSameTrainer(u8 battlerId)
+{
+    if (GetBattlerSide(battlerId) == B_SIDE_OPPONENT && gBattleTypeFlags & BATTLE_TYPE_TWO_OPPONENTS)
+        return FALSE;
+    else if (GetBattlerSide(battlerId) == B_SIDE_PLAYER && gBattleTypeFlags & BATTLE_TYPE_INGAME_PARTNER)
+        return FALSE;
+    else if (gBattleTypeFlags & BATTLE_TYPE_MULTI)
+        return FALSE;
+    else
+        return TRUE;
+}
+
+bool32 CanMegaEvolve(u8 battlerId)
+{
+    u32 i;
+    u16 species, itemId;
+    u8 battlerPosition = GetBattlerPosition(battlerId);
+    u8 partnerPosition = GetBattlerPosition(BATTLE_PARTNER(battlerId));
+
+    // Check if trainer already mega evolved a pokemon.
+    if (gBattleStruct->alreadyMegaEvolved[battlerPosition])
+        return FALSE;
+    if (gBattleTypeFlags & BATTLE_TYPE_DOUBLE)
+    {
+        if (IsPartnerMonFromSameTrainer(battlerId) && gBattleStruct->alreadyMegaEvolved[partnerPosition])
+            return FALSE;
+    }
+    else
+    {
+        if (gBattleStruct->alreadyMegaEvolved[partnerPosition])
+            return FALSE;
+    }
+
+    // Check if the pokemon holds an appropriate item,
+    if (GetBattlerHoldEffect(battlerId, FALSE) != HOLD_EFFECT_MEGA_STONE)
+        return FALSE;
+
+    // Check if there is an entry in the evolution table.
+    species = gBattleMons[battlerId].species;
+    itemId = gBattleMons[battlerId].item;
+    for (i = 0; i < EVOS_PER_MON; i++)
+    {
+        if (gEvolutionTable[species][i].method == EVO_MEGA_EVOLUTION
+            && gEvolutionTable[species][i].param == itemId)
+        {
+            gBattleStruct->speciesToMegaEvolve[battlerId] = gEvolutionTable[species][i].targetSpecies;
+            break;
+        }
+    }
+
+    if (i == EVOS_PER_MON)
+        return FALSE;
+
+    // All checks passed, the mon CAN mega evolve.
+    return TRUE;
+}

src/data/items.h

@@ -3649,10 +3649,10 @@ const struct Item gItems[] =
         .secondaryId = 0,
     },
     {
-        .name = _("????????"),
+        .name = _("Mega Stone"),
-        .itemId = ITEM_NONE,
+        .itemId = ITEM_MEGA_STONE_TESTING,
         .price = 0,
-        .holdEffect = HOLD_EFFECT_NONE,
+        .holdEffect = HOLD_EFFECT_MEGA_STONE,
         .holdEffectParam = 0,
         .description = gDummyItemDescription,
         .importance = 0,

src/data/pokemon/evolution.h

@@ -173,7 +173,8 @@ const struct Evolution gEvolutionTable[NUM_SPECIES][EVOS_PER_MON] =
     [SPECIES_VIGOROTH]   = {{EVO_LEVEL, 36, SPECIES_SLAKING}},
     [SPECIES_GULPIN]     = {{EVO_LEVEL, 26, SPECIES_SWALOT}},
     [SPECIES_WHISMUR]    = {{EVO_LEVEL, 20, SPECIES_LOUDRED}},
-    [SPECIES_LOUDRED]    = {{EVO_LEVEL, 40, SPECIES_EXPLOUD}},
+    [SPECIES_LOUDRED]    = {{EVO_LEVEL, 40, SPECIES_EXPLOUD},
+                            {EVO_MEGA_EVOLUTION, ITEM_MEGA_STONE_TESTING, SPECIES_HO_OH}},
     [SPECIES_CLAMPERL]   = {{EVO_TRADE_ITEM, ITEM_DEEP_SEA_TOOTH, SPECIES_HUNTAIL},
                             {EVO_TRADE_ITEM, ITEM_DEEP_SEA_SCALE, SPECIES_GOREBYSS}},
     [SPECIES_SHUPPET]    = {{EVO_LEVEL, 37, SPECIES_BANETTE}},