Add MOD to match mod with powers of 2

include/global.h

@@ -85,6 +85,12 @@
 #define SAFE_DIV(a, b) ((a) / (b))
 #endif
 
+// The below macro does a%n, but (to match) will switch to a&(n-1) if n is a power of 2.
+// There are cases where GF does a&(n-1) where we would really like to have a%n, because
+// if n is changed to a value that isn't a power of 2 then a&(n-1) is unlikely to work as
+// intended, and a%n for powers of 2 isn't always optimized to use &.
+#define MOD(a, n)(((n) & ((n)-1)) ? ((a) % (n)) : ((a) & ((n)-1)))
+
 // Extracts the upper 16 bits of a 32-bit number
 #define HIHALF(n) (((n) & 0xFFFF0000) >> 16)
 

src/battle_controller_opponent.c

@@ -1588,7 +1588,7 @@ static void OpponentHandleChooseMove(void)
             u16 move;
             do
             {
-                chosenMoveId = Random() & 3;
+                chosenMoveId = MOD(Random(), MAX_MON_MOVES);
                 move = moveInfo->moves[chosenMoveId];
             } while (move == MOVE_NONE);
 

src/battle_pyramid.c

@@ -38,7 +38,7 @@
 #include "constants/moves.h"
 #include "constants/trainers.h"
 
-#define NUM_LAYOUT_OFFSETS 8 // Assumed to be a power of 2
+#define NUM_LAYOUT_OFFSETS 8
 
 extern const struct MapLayout *const gMapLayouts[];
 
@@ -1904,7 +1904,7 @@ static void GetPyramidFloorLayoutOffsets(u8 *layoutOffsets)
 
     for (i = 0; i < NUM_PYRAMID_FLOOR_SQUARES; i++)
     {
-        layoutOffsets[i] = sPyramidFloorTemplates[id].layoutOffsets[rand & (NUM_LAYOUT_OFFSETS - 1)];
+        layoutOffsets[i] = sPyramidFloorTemplates[id].layoutOffsets[MOD(rand, NUM_LAYOUT_OFFSETS)];
         rand >>= 3;
         if (i == 7)
         {

src/battle_script_commands.c

@@ -7350,7 +7350,7 @@ static void Cmd_tryconversiontypechange(void)
     {
         do
         {
-            while ((moveChecked = Random() & (MAX_MON_MOVES - 1)) >= validMoves);
+            while ((moveChecked = MOD(Random(), MAX_MON_MOVES)) >= validMoves);
 
             moveType = gBattleMoves[gBattleMons[gBattlerAttacker].moves[moveChecked]].type;
 
@@ -8189,7 +8189,7 @@ static void Cmd_trychoosesleeptalkmove(void)
 
         do
         {
-            movePosition = Random() & (MAX_MON_MOVES - 1);
+            movePosition = MOD(Random(), MAX_MON_MOVES);
         } while ((gBitTable[movePosition] & unusableMovesBits));
 
         gCalledMove = gBattleMons[gBattlerAttacker].moves[movePosition];

src/battle_util.c

@@ -2773,11 +2773,12 @@ u8 AbilityBattleEffects(u8 caseID, u8 battler, u8 ability, u8 special, u16 moveA
                 {
                     do
                     {
+                        // Pick either MOVE_EFFECT_SLEEP, MOVE_EFFECT_POISON or MOVE_EFFECT_BURN
                         gBattleCommunication[MOVE_EFFECT_BYTE] = Random() & 3;
                     } while (gBattleCommunication[MOVE_EFFECT_BYTE] == 0);
-
+                    // Replace MOVE_EFFECT_BURN with MOVE_EFFECT_PARALYSIS
                     if (gBattleCommunication[MOVE_EFFECT_BYTE] == MOVE_EFFECT_BURN)
-                        gBattleCommunication[MOVE_EFFECT_BYTE] += 2; // 5 MOVE_EFFECT_PARALYSIS
+                        gBattleCommunication[MOVE_EFFECT_BYTE] += (MOVE_EFFECT_PARALYSIS - MOVE_EFFECT_BURN);
 
                     gBattleCommunication[MOVE_EFFECT_BYTE] += MOVE_EFFECT_AFFECTS_USER;
                     BattleScriptPushCursor();
@@ -3956,7 +3957,7 @@ u8 IsMonDisobedient(void)
         {
             // Randomly select, then print a disobedient string
             // B_MSG_LOAFING, B_MSG_WONT_OBEY, B_MSG_TURNED_AWAY, or B_MSG_PRETEND_NOT_NOTICE
-            gBattleCommunication[MULTISTRING_CHOOSER] = Random() & (NUM_LOAF_STRINGS - 1);
+            gBattleCommunication[MULTISTRING_CHOOSER] = MOD(Random(), NUM_LOAF_STRINGS);
             gBattlescriptCurrInstr = BattleScript_MoveUsedLoafingAround;
             return 1;
         }
@@ -3964,7 +3965,7 @@ u8 IsMonDisobedient(void)
         {
             do
             {
-                gCurrMovePos = gChosenMovePos = Random() & (MAX_MON_MOVES - 1);
+                gCurrMovePos = gChosenMovePos = MOD(Random(), MAX_MON_MOVES);
             } while (gBitTable[gCurrMovePos] & calc);
 
             gCalledMove = gBattleMons[gBattlerAttacker].moves[gCurrMovePos];
@@ -4007,7 +4008,7 @@ u8 IsMonDisobedient(void)
         {
             // Randomly select, then print a disobedient string
             // B_MSG_LOAFING, B_MSG_WONT_OBEY, B_MSG_TURNED_AWAY, or B_MSG_PRETEND_NOT_NOTICE
-            gBattleCommunication[MULTISTRING_CHOOSER] = Random() & (NUM_LOAF_STRINGS - 1);
+            gBattleCommunication[MULTISTRING_CHOOSER] = MOD(Random(), NUM_LOAF_STRINGS);
             gBattlescriptCurrInstr = BattleScript_MoveUsedLoafingAround;
             return 1;
         }

src/contest_ai.c

@@ -326,7 +326,7 @@ u8 ContestAI_GetActionToUse(void)
     {
         // Randomly choose a move index. If it's the move
         // with the highest (or tied highest) score, return
-        u8 moveIdx = Random() & (MAX_MON_MOVES - 1); // % MAX_MON_MOVES doesn't match
+        u8 moveIdx = MOD(Random(), MAX_MON_MOVES);
         u8 score = eContestAI.moveScores[moveIdx];
         int i;
         for (i = 0; i < MAX_MON_MOVES; i++)

src/intro.c

@@ -1163,7 +1163,7 @@ void CB2_InitCopyrightScreenAfterTitleScreen(void)
 static void Task_Scene1_Load(u8 taskId)
 {
     SetVBlankCallback(NULL);
-    sIntroCharacterGender = Random() & 1;
+    sIntroCharacterGender = MOD(Random(), GENDER_COUNT);
     IntroResetGpuRegs();
     SetGpuReg(REG_OFFSET_BG3VOFS, 0);
     SetGpuReg(REG_OFFSET_BG2VOFS, 80);

src/naming_screen.c

@@ -1067,7 +1067,7 @@ static void SpriteCB_InputArrow(struct Sprite *sprite)
     if (sprite->sDelay == 0 || --sprite->sDelay == 0)
     {
         sprite->sDelay = 8;
-        sprite->sXPosId = (sprite->sXPosId + 1) & (ARRAY_COUNT(x) - 1);
+        sprite->sXPosId = MOD(sprite->sXPosId + 1, ARRAY_COUNT(x));
     }
     sprite->x2 = x[sprite->sXPosId];
 }
@@ -1097,7 +1097,7 @@ static void SpriteCB_Underscore(struct Sprite *sprite)
         sprite->sDelay++;
         if (sprite->sDelay > 8)
         {
-            sprite->sYPosId = (sprite->sYPosId + 1) & (ARRAY_COUNT(y) - 1);
+            sprite->sYPosId = MOD(sprite->sYPosId + 1, ARRAY_COUNT(y));
             sprite->sDelay = 0;
         }
     }