Don't use rand()%n. The function specified by the ANSI committee is a terrible linear congruential generator. In fact, it's so bad that the low order bit alternates. The algorithm implemented as RandomFloat() seems to be Park and Miller's "minimum standard" generator which is better (but not great). [See Knuth for more information.]

Any place where you would use rand()%n, use RandomInt(0, n) instead.

stepmania/src/BGAnimationLayer.cpp

@@ -326,7 +326,7 @@ void BGAnimationLayer::LoadFromAniLayerFile( const RString& sPath )
 
 	if( sHint.find("startonrandomframe") != RString::npos )
 		for( unsigned i=0; i<m_SubActors.size(); i++ )
-			m_SubActors[i]->SetState( rand()%m_SubActors[i]->GetNumStates() );
+			m_SubActors[i]->SetState( RandomInt(m_SubActors[i]->GetNumStates()) );
 
 	if( sHint.find("dontanimate") != RString::npos )
 		for( unsigned i=0; i<m_SubActors.size(); i++ )
@@ -519,7 +519,7 @@ void BGAnimationLayer::LoadFromNode( const RString& sDir, const XNode* pNode )
 	if( bStartOnRandomFrame )
 	{
 		for( unsigned i=0; i<m_SubActors.size(); i++ )
-			m_SubActors[i]->SetState( rand()%m_SubActors[i]->GetNumStates() );
+			m_SubActors[i]->SetState( RandomInt(m_SubActors[i]->GetNumStates()) );
 	}
 }
 

stepmania/src/Character.cpp

@@ -72,7 +72,7 @@ RString GetRandomFileInDir( RString sDir )
 	if( asFiles.empty() )
 		return RString();
 	else
-		return asFiles[rand()%asFiles.size()];
+		return asFiles[RandomInt(asFiles.size())];
 }
 
 

stepmania/src/CharacterManager.cpp

@@ -61,7 +61,7 @@ Character* CharacterManager::GetRandomCharacter()
 	vector<Character*> apCharacters;
 	GetCharacters( apCharacters );
 	if( apCharacters.size() )
-		return apCharacters[rand()%apCharacters.size()];
+		return apCharacters[RandomInt(apCharacters.size())];
 	else
 		return GetDefaultCharacter();
 }

stepmania/src/DancingCharacters.cpp

@@ -160,7 +160,7 @@ void DancingCharacters::LoadNextSong()
 			m_pCharacter[p]->PlayAnimation( "rest" );
 }
 
-int Neg1OrPos1() { return rand()%2 ? -1 : +1; }
+int Neg1OrPos1() { return RandomInt( 2 ) ? -1 : +1; }
 
 void DancingCharacters::Update( float fDelta )
 {
@@ -213,7 +213,7 @@ void DancingCharacters::Update( float fDelta )
 	// time for a new sweep?
 	if( GAMESTATE->m_fSongBeat > m_fThisCameraEndBeat )
 	{
-		if( (rand()%5) >= 2 )
+		if( RandomInt(5) >= 2 )
 		{
 			// sweeping camera
 			m_CameraDistance = CAMERA_SWEEP_DISTANCE + RandomInt(-1,1) * CAMERA_SWEEP_DISTANCE_VARIANCE;

stepmania/src/GameSoundManager.cpp

@@ -270,7 +270,7 @@ static void DoPlayOnceFromDir( RString sPath )
 	if( arraySoundFiles.empty() )
 		return;
 
-	int index = rand() % arraySoundFiles.size();
+	int index = RandomInt( arraySoundFiles.size( ));
 	SOUNDMAN->PlayOnce( sPath + arraySoundFiles[index] );
 }
 

stepmania/src/MusicWheel.cpp

@@ -1065,7 +1065,7 @@ bool MusicWheel::Select()	// return true if this selection ends the screen
 	if( m_WheelState == STATE_ROULETTE_SPINNING )
 	{
 		m_WheelState = STATE_ROULETTE_SLOWING_DOWN;
-		m_iSwitchesLeftInSpinDown = ROULETTE_SLOW_DOWN_SWITCHES/2+1 + rand()%(ROULETTE_SLOW_DOWN_SWITCHES/2);
+		m_iSwitchesLeftInSpinDown = ROULETTE_SLOW_DOWN_SWITCHES/2+1 + RandomInt( ROULETTE_SLOW_DOWN_SWITCHES/2 );
 		m_fTimeLeftInState = 0.1f;
 		return false;
 	}
@@ -1331,7 +1331,7 @@ Song *MusicWheel::GetPreferredSelectionForRandomOrPortal()
 		if( i == NUM_PROBES/2 )
 			vDifficultiesToRequire.clear();
 
-		int iSelection = rand() % wid.size();
+		int iSelection = RandomInt( wid.size() );
 		if( wid[iSelection].m_Type != TYPE_SONG )
 			continue;
 

stepmania/src/NoteDataUtil.cpp

@@ -1068,7 +1068,7 @@ static void SuperShuffleTaps( NoteData &inout, int iStartIndex, int iEndIndex )
 			set<int> vTriedTracks;
 			for( int i=0; i<4; i++ )	// probe max 4 times
 			{
-				int t2 = rand() % inout.GetNumTracks();
+				int t2 = RandomInt( inout.GetNumTracks() );
 				if( vTriedTracks.find(t2) != vTriedTracks.end() )	// already tried this track
 					continue;	// skip
 				vTriedTracks.insert( t2 );
@@ -1941,7 +1941,7 @@ void NoteDataUtil::AddTapAttacks( NoteData &nd, Song* pSong )
 			TapNote::attack,
 			TapNote::SubType_invalid,
 			TapNote::original, 
-			szAttacks[rand()%ARRAYSIZE(szAttacks)],
+			szAttacks[RandomInt(ARRAYSIZE(szAttacks))],
 			15.0f, 
 			false,
 			0 );

stepmania/src/PlayerOptions.cpp

@@ -460,9 +460,9 @@ void PlayerOptions::ChooseRandomModifiers()
 	float f;
 	f = RandomFloat(0,1);
 	if( f>0.66f )
-		m_fAccels[rand()%NUM_ACCELS] = 1;
+		m_fAccels[RandomInt(NUM_ACCELS)] = 1;
 	else if( f>0.33f )
-		m_fEffects[rand()%NUM_EFFECTS] = 1;
+		m_fEffects[RandomInt(NUM_EFFECTS)] = 1;
 	f = RandomFloat(0,1);
 	if( f>0.95f )
 		m_fAppearances[APPEARANCE_HIDDEN] = 1;

stepmania/src/RageUtil.h

@@ -176,6 +176,12 @@ inline int RandomInt( int nLow, int nHigh )
 	return int( RandomFloat() * (nHigh - nLow + 1) + nLow );
 }
 
+// Returns an integer between 0 and n-1 inclusive (replacement for rand() % n).
+inline int RandomInt( int n )
+{
+	return RandomInt( 0, n-1 );
+}
+
 /* Alternative: */
 class RandomGen
 {

stepmania/src/RandomSample.cpp

@@ -92,7 +92,7 @@ int RandomSample::GetNextToPlay()
 	int iIndexToPlay = 0;
 	for( int i=0; i<5; i++ )
 	{
-		iIndexToPlay = rand() % m_pSamples.size();
+		iIndexToPlay = RandomInt( m_pSamples.size() );
 		if( iIndexToPlay != m_iIndexLastPlayed )
 			break;
 	}

stepmania/src/ScoreKeeperRave.cpp

@@ -131,12 +131,12 @@ void ScoreKeeperRave::LaunchAttack( AttackLevel al )
 	RString sAttackToGive;
 
 	if (GAMESTATE->m_pCurCharacters[pn] != NULL)		
-		sAttackToGive = asAttacks[ rand()%NUM_ATTACKS_PER_LEVEL ];
+		sAttackToGive = asAttacks[ RandomInt(NUM_ATTACKS_PER_LEVEL) ];
 	else
 	{
 		/* If you add any note skins here, you need to make sure they're cached, too. */
 		RString DefaultAttacks[8] = { "1.5x", "2.0x", "0.5x", "reverse", "sudden", "boost", "brake", "wave" };
-		sAttackToGive = DefaultAttacks[ rand()%8 ];
+		sAttackToGive = DefaultAttacks[ RandomInt(8) ];
 	}
 
   	PlayerNumber pnToAttack = OPPOSITE_PLAYER[pn];

stepmania/src/ScreenDebugOverlay.cpp

@@ -650,8 +650,8 @@ static HighScore MakeRandomHighScore( float fPercentDP )
 {
 	HighScore hs;
 	hs.SetName( "FAKE" );
-	hs.SetGrade( (Grade)SCALE( rand()%5, 0, 4, Grade_Tier01, Grade_Tier05 ) );
-	hs.SetScore( rand()%100*1000 );
+	hs.SetGrade( (Grade)SCALE( RandomInt(5), 0, 4, Grade_Tier01, Grade_Tier05 ) );
+	hs.SetScore( RandomInt(100*1000) );
 	hs.SetPercentDP( fPercentDP );
 	hs.SetSurviveSeconds( randomf(30.0f, 100.0f) );
 	PlayerOptions po;
@@ -660,11 +660,11 @@ static HighScore MakeRandomHighScore( float fPercentDP )
 	hs.SetDateTime( DateTime::GetNowDateTime() );
 	hs.SetPlayerGuid( Profile::MakeGuid() );
 	hs.SetMachineGuid( Profile::MakeGuid() );
-	hs.SetProductID( rand()%10 );
+	hs.SetProductID( RandomInt(10) );
 	FOREACH_TapNoteScore( tns )
-		hs.SetTapNoteScore( tns, rand() % 100 );
+		hs.SetTapNoteScore( tns, RandomInt(100) );
 	FOREACH_HoldNoteScore( hns )
-		hs.SetHoldNoteScore( hns, rand() % 100 );
+		hs.SetHoldNoteScore( hns, RandomInt(100) );
 	RadarValues rv;
 	FOREACH_RadarCategory( rc )
 		rv.m_Values.f[rc] = randomf( 0, 1 );

stepmania/src/ScreenDemonstration.cpp

@@ -48,7 +48,7 @@ void ScreenDemonstration::Init()
 		}
 
 		ASSERT( vStylePossible.size() > 0 );
-		const Style* pStyle = vStylePossible[ rand() % vStylePossible.size() ];
+		const Style* pStyle = vStylePossible[ RandomInt(vStylePossible.size()) ];
 		GAMESTATE->m_pCurStyle.Set( pStyle );
 	}
 

stepmania/src/ScreenEdit.cpp

@@ -604,7 +604,7 @@ static RString GetOneBakedRandomFile( Song *pSong, bool bTryGenre = true )
 		vsNamesOut,
 		bTryGenre );
 	if( !vsNamesOut.empty() )
-		return vsNamesOut[rand()%vsNamesOut.size()];
+		return vsNamesOut[RandomInt(vsNamesOut.size())];
 	else
 		return RString();
 }

stepmania/src/ScreenEnding.cpp

@@ -53,9 +53,9 @@ ScreenEnding::ScreenEnding() : ScreenAttract( false/*dont reset GAMESTATE*/ )
 			STATSMAN->m_CurStageStats.m_player[PLAYER_2].SetLifeRecordAt( 1-fP1, f );
 		}
 	
-		STATSMAN->m_CurStageStats.m_player[PLAYER_1].iActualDancePoints = rand()%3;
+		STATSMAN->m_CurStageStats.m_player[PLAYER_1].iActualDancePoints = RandomInt( 3 );
 		STATSMAN->m_CurStageStats.m_player[PLAYER_1].iPossibleDancePoints = 2;
-		STATSMAN->m_CurStageStats.m_player[PLAYER_2].iActualDancePoints = rand()%2;
+		STATSMAN->m_CurStageStats.m_player[PLAYER_2].iActualDancePoints = RandomInt( 2 );
 		STATSMAN->m_CurStageStats.m_player[PLAYER_2].iPossibleDancePoints = 1;
 		STATSMAN->m_CurStageStats.m_player[PLAYER_1].iCurCombo = 0;
 		STATSMAN->m_CurStageStats.m_player[PLAYER_1].UpdateComboList( 0, false );
@@ -66,12 +66,12 @@ ScreenEnding::ScreenEnding() : ScreenAttract( false/*dont reset GAMESTATE*/ )
 		STATSMAN->m_CurStageStats.m_player[PLAYER_1].iCurCombo = 250;
 		STATSMAN->m_CurStageStats.m_player[PLAYER_1].UpdateComboList( 100, false );
 
-		STATSMAN->m_CurStageStats.m_player[PLAYER_1].iTapNoteScores[TNS_W1] = rand()%2;
-		STATSMAN->m_CurStageStats.m_player[PLAYER_1].iTapNoteScores[TNS_W2] = rand()%2;
-		STATSMAN->m_CurStageStats.m_player[PLAYER_1].iTapNoteScores[TNS_W3] = rand()%2;
-		STATSMAN->m_CurStageStats.m_player[PLAYER_2].iTapNoteScores[TNS_W1] = rand()%2;
-		STATSMAN->m_CurStageStats.m_player[PLAYER_2].iTapNoteScores[TNS_W2] = rand()%2;
-		STATSMAN->m_CurStageStats.m_player[PLAYER_2].iTapNoteScores[TNS_W3] = rand()%2;
+		STATSMAN->m_CurStageStats.m_player[PLAYER_1].iTapNoteScores[TNS_W1] = RandomInt( 2 );
+		STATSMAN->m_CurStageStats.m_player[PLAYER_1].iTapNoteScores[TNS_W2] = RandomInt( 2 );
+		STATSMAN->m_CurStageStats.m_player[PLAYER_1].iTapNoteScores[TNS_W3] = RandomInt( 2 );
+		STATSMAN->m_CurStageStats.m_player[PLAYER_2].iTapNoteScores[TNS_W1] = RandomInt( 2 );
+		STATSMAN->m_CurStageStats.m_player[PLAYER_2].iTapNoteScores[TNS_W2] = RandomInt( 2 );
+		STATSMAN->m_CurStageStats.m_player[PLAYER_2].iTapNoteScores[TNS_W3] = RandomInt( 2 );
 
 		STATSMAN->m_vPlayedStageStats.clear();
 	}

stepmania/src/ScreenEvaluation.cpp

@@ -106,7 +106,7 @@ ScreenEvaluation::ScreenEvaluation()
 		
 		FOREACH_PlayerNumber( p )
 		{
-			if( rand() % 2 )
+			if( RandomInt(2) )
 				GAMESTATE->m_pPlayerState[p]->m_PlayerOptions.m_bTransforms[PlayerOptions::TRANSFORM_ECHO] = true;	// show "disqualified"
 
 			GAMESTATE->m_bSideIsJoined[p] = true;
@@ -131,9 +131,9 @@ ScreenEvaluation::ScreenEvaluation()
 		FOREACH_PlayerNumber( p )
 		{
 			float fSeconds = GAMESTATE->m_pCurSong->GetStepsSeconds();
-			STATSMAN->m_CurStageStats.m_player[p].iActualDancePoints = rand()%3;
+			STATSMAN->m_CurStageStats.m_player[p].iActualDancePoints = RandomInt( 3 );
 			STATSMAN->m_CurStageStats.m_player[p].iPossibleDancePoints = 2;
-			if( rand()%2 )
+			if( RandomInt(2) )
 				STATSMAN->m_CurStageStats.m_player[p].iCurCombo = 11000;
 			else
 				STATSMAN->m_CurStageStats.m_player[p].iCurCombo = 0;
@@ -154,13 +154,13 @@ ScreenEvaluation::ScreenEvaluation()
 			STATSMAN->m_CurStageStats.m_player[p].UpdateComboList( 0.50f * fSeconds, false );
 			STATSMAN->m_CurStageStats.m_player[p].iCurCombo = 100;
 			STATSMAN->m_CurStageStats.m_player[p].UpdateComboList( 1.00f * fSeconds, false );
-			if( rand()%5 == 0 )
+			if( RandomInt(5) == 0 )
 			{
 				STATSMAN->m_CurStageStats.m_player[p].bFailedEarlier = true;
 			}
-			STATSMAN->m_CurStageStats.m_player[p].iTapNoteScores[TNS_W1] = rand()%3;
-			STATSMAN->m_CurStageStats.m_player[p].iTapNoteScores[TNS_W2] = rand()%3;
-			STATSMAN->m_CurStageStats.m_player[p].iTapNoteScores[TNS_W3] = rand()%3;
+			STATSMAN->m_CurStageStats.m_player[p].iTapNoteScores[TNS_W1] = RandomInt( 3 );
+			STATSMAN->m_CurStageStats.m_player[p].iTapNoteScores[TNS_W2] = RandomInt( 3 );
+			STATSMAN->m_CurStageStats.m_player[p].iTapNoteScores[TNS_W3] = RandomInt( 3 );
 			STATSMAN->m_CurStageStats.m_player[p].iPossibleGradePoints = 4*ScoreKeeperNormal::TapNoteScoreToGradePoints(TNS_W1, false);
 			STATSMAN->m_CurStageStats.m_player[p].fLifeRemainingSeconds = randomf( 90, 580 );
 		}

stepmania/src/ScreenJukebox.cpp

@@ -76,7 +76,7 @@ void ScreenJukebox::SetSong()
 		if( vSongs.size() == 0 )
 			return;
 
-		Song* pSong = vSongs[rand()%vSongs.size()];
+		Song* pSong = vSongs[RandomInt(vSongs.size())];
 
 		if( !pSong->HasMusic() )
 			continue;	// skip
@@ -85,7 +85,7 @@ void ScreenJukebox::SetSong()
 		if( !pSong->ShowInDemonstrationAndRanking() )
 			continue;	// skip
 
-		Difficulty dc = vDifficultiesToShow[ rand()%vDifficultiesToShow.size() ];
+		Difficulty dc = vDifficultiesToShow[ RandomInt(vDifficultiesToShow.size()) ];
 		Steps* pSteps = SongUtil::GetStepsByDifficulty( pSong, GAMESTATE->GetCurrentStyle()->m_StepsType, dc );
 
 		if( pSteps == NULL )
@@ -148,7 +148,7 @@ void ScreenJukebox::SetSong()
 
 			if( !apOptions.empty() )
 			{
-				int iIndex = rand()%apOptions.size();
+				int iIndex = RandomInt( apOptions.size() );
 				m_pCourseEntry = apOptions[iIndex];
 				Course *pCourse = apPossibleCourses[iIndex]; 
 			

stepmania/src/arch/InputHandler/InputHandler_MonkeyKeyboard.cpp

@@ -50,7 +50,7 @@ static const DeviceButton g_keys[] =
 
 static DeviceButton GetRandomKeyboardKey()
 {
-	int index = rand()%ARRAYSIZE(g_keys);
+	int index = RandomInt( ARRAYSIZE(g_keys) );
 	return g_keys[index];
 }
 

stepmania/src/arch/MemoryCard/MemoryCardDriverThreaded_Windows.cpp

@@ -42,7 +42,7 @@ bool MemoryCardDriverThreaded_Windows::TestWrite( UsbStorageDevice* pDevice )
 	 * the chance of corruption if the user removes the device immediately, without doing anything. */
 	for( int i = 0; i < 10; ++i )
 	{
-		HANDLE hFile = CreateFile( ssprintf( "%stmp%i", pDevice->sOsMountDir.c_str(), rand() % 100000),
+		HANDLE hFile = CreateFile( ssprintf( "%stmp%i", pDevice->sOsMountDir.c_str(), RandomInt(100000)),
 			GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE,
 			NULL, CREATE_NEW, FILE_ATTRIBUTE_TEMPORARY | FILE_FLAG_DELETE_ON_CLOSE, NULL );