Particles: reduce excessive hash accesses to a more passable level.

By caching the group id in the particle emitter, and groups in the
painter.

Test case: samegame, 1 player, click 1 set of 3 stones, quit.
QQuickParticleSystem::emittersChanged(), before patch:
 - 64M instr. inclusive
 - 145,880 calls to findNode (29M instr.)
after:
 - 21M instr. inclusive
 - 0 calls to findNode
 - 25 calls to QQuickParticlePainter::recalculateGroupIds (9800 instr.
   inclusive).

Change-Id: I4aba9d50100513c6b7cdd230e30b3aecaf84485a
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

10 files changed, 192 insertions, 92 deletions

diff --git a/src/particles/qquickcustomparticle.cpp b/src/particles/qquickcustomparticle.cpp
index 97cfdc7bf9..32b2fd847e 100644
--- a/src/particles/qquickcustomparticle.cpp
+++ b/src/particles/qquickcustomparticle.cpp
@@ -324,19 +324,18 @@ QQuickShaderEffectNode* QQuickCustomParticle::buildCustomNodes()
         return 0;
     }
 
-    if (m_groups.isEmpty())
+    if (groups().isEmpty())
         return 0;
 
     QQuickShaderEffectNode *rootNode = 0;
     QQuickShaderEffectMaterial *material = new QQuickShaderEffectMaterial;
     m_dirtyProgram = true;
 
-    foreach (const QString &str, m_groups){
-        int gIdx = m_system->groupIds[str];
-        int count = m_system->groupData[gIdx]->size();
+    for (auto groupId : groupIds()) {
+        int count = m_system->groupData[groupId]->size();
 
         QQuickShaderEffectNode* node = new QQuickShaderEffectNode();
-        m_nodes.insert(gIdx, node);
+        m_nodes.insert(groupId, node);
 
         node->setMaterial(material);
 
@@ -349,7 +348,7 @@ QQuickShaderEffectNode* QQuickCustomParticle::buildCustomNodes()
         node->setFlag(QSGNode::OwnsGeometry, true);
         PlainVertex *vertices = (PlainVertex *) g->vertexData();
         for (int p=0; p < count; ++p) {
-            commit(gIdx, p);
+            commit(groupId, p);
             vertices[0].tx = 0;
             vertices[0].ty = 0;
 
diff --git a/src/particles/qquickimageparticle.cpp b/src/particles/qquickimageparticle.cpp
index c88a0b002b..5f2d1db663 100644
--- a/src/particles/qquickimageparticle.cpp
+++ b/src/particles/qquickimageparticle.cpp
@@ -1020,10 +1020,13 @@ void QQuickImageParticle::setEntryEffect(EntryEffect arg)
 void QQuickImageParticle::resetColor()
 {
     m_explicitColor = false;
-    foreach (const QString &str, m_groups)
-        foreach (QQuickParticleData* d, m_system->groupData[m_system->groupIds[str]]->data)
-            if (d->colorOwner == this)
+    for (auto groupId : groupIds()) {
+        for (QQuickParticleData* d : qAsConst(m_system->groupData[groupId]->data)) {
+            if (d->colorOwner == this) {
                 d->colorOwner = 0;
+            }
+        }
+    }
     m_color = QColor();
     m_color_variation = 0.0f;
     m_redVariation = 0.0f;
@@ -1036,10 +1039,13 @@ void QQuickImageParticle::resetColor()
 void QQuickImageParticle::resetRotation()
 {
     m_explicitRotation = false;
-    foreach (const QString &str, m_groups)
-        foreach (QQuickParticleData* d, m_system->groupData[m_system->groupIds[str]]->data)
-            if (d->rotationOwner == this)
+    for (auto groupId : groupIds()) {
+        for (QQuickParticleData* d : qAsConst(m_system->groupData[groupId]->data)) {
+            if (d->rotationOwner == this) {
                 d->rotationOwner = 0;
+            }
+        }
+    }
     m_rotation = 0;
     m_rotationVariation = 0;
     m_rotationVelocity = 0;
@@ -1050,10 +1056,13 @@ void QQuickImageParticle::resetRotation()
 void QQuickImageParticle::resetDeformation()
 {
     m_explicitDeformation = false;
-    foreach (const QString &str, m_groups)
-        foreach (QQuickParticleData* d, m_system->groupData[m_system->groupIds[str]]->data)
-            if (d->deformationOwner == this)
+    for (auto groupId : groupIds()) {
+        for (QQuickParticleData* d : qAsConst(m_system->groupData[groupId]->data)) {
+            if (d->deformationOwner == this) {
                 d->deformationOwner = 0;
+            }
+        }
+    }
     if (m_xVector)
         delete m_xVector;
     if (m_yVector)
@@ -1255,9 +1264,9 @@ void QQuickImageParticle::finishBuildParticleNodes(QSGNode** node)
         perfLevel = Simple;
     }
 
-    foreach (const QString &str, m_groups){//For sharing higher levels, need to have highest used so it renders
-        int gIdx = m_system->groupIds[str];
-        foreach (QQuickParticlePainter* p, m_system->groupData[gIdx]->painters){
+    for (auto groupId : groupIds()) {
+        //For sharing higher levels, need to have highest used so it renders
+        for (QQuickParticlePainter* p : qAsConst(m_system->groupData[groupId]->painters)) {
             QQuickImageParticle* other = qobject_cast<QQuickImageParticle*>(p);
             if (other){
                 if (other->perfLevel > perfLevel) {
@@ -1382,16 +1391,15 @@ void QQuickImageParticle::finishBuildParticleNodes(QSGNode** node)
     }
 
     m_nodes.clear();
-    foreach (const QString &str, m_groups){
-        int gIdx = m_system->groupIds[str];
-        int count = m_system->groupData[gIdx]->size();
+    for (auto groupId : groupIds()) {
+        int count = m_system->groupData[groupId]->size();
         QSGGeometryNode* node = new QSGGeometryNode();
         node->setMaterial(m_material);
         node->markDirty(QSGNode::DirtyMaterial);
 
-        m_nodes.insert(gIdx, node);
-        m_idxStarts.insert(gIdx, m_lastIdxStart);
-        m_startsIdx.append(qMakePair<int,int>(m_lastIdxStart, gIdx));
+        m_nodes.insert(groupId, node);
+        m_idxStarts.insert(groupId, m_lastIdxStart);
+        m_startsIdx.append(qMakePair<int,int>(m_lastIdxStart, groupId));
         m_lastIdxStart += count;
 
         //Create Particle Geometry
@@ -1423,7 +1431,7 @@ void QQuickImageParticle::finishBuildParticleNodes(QSGNode** node)
             g->setDrawingMode(GL_TRIANGLES);
 
         for (int p=0; p < count; ++p)
-            commit(gIdx, p);//commit sets geometry for the node, has its own perfLevel switch
+            commit(groupId, p);//commit sets geometry for the node, has its own perfLevel switch
 
         if (perfLevel == Sprites)
             initTexCoords<SpriteVertex>((SpriteVertex*)g->vertexData(), vCount);
@@ -1538,10 +1546,9 @@ void QQuickImageParticle::prepareNextFrame(QSGNode **node)
 void QQuickImageParticle::spritesUpdate(qreal time)
 {
     // Sprite progression handled CPU side, so as to have per-frame control.
-    foreach (const QString &str, m_groups) {
-        int gIdx = m_system->groupIds[str];
-        foreach (QQuickParticleData* mainDatum, m_system->groupData[gIdx]->data) {
-            QSGGeometryNode *node = m_nodes[gIdx];
+    for (auto groupId : groupIds()) {
+        for (QQuickParticleData* mainDatum : qAsConst(m_system->groupData[groupId]->data)) {
+            QSGGeometryNode *node = m_nodes[groupId];
             if (!node)
                 continue;
             //TODO: Interpolate between two different animations if it's going to transition next frame
@@ -1549,7 +1556,7 @@ void QQuickImageParticle::spritesUpdate(qreal time)
             QQuickParticleData* datum = (mainDatum->animationOwner == this ? mainDatum : getShadowDatum(mainDatum));
             int spriteIdx = 0;
             for (int i = 0; i<m_startsIdx.count(); i++) {
-                if (m_startsIdx[i].second == gIdx){
+                if (m_startsIdx[i].second == groupId){
                     spriteIdx = m_startsIdx[i].first + datum->index;
                     break;
                 }
diff --git a/src/particles/qquickitemparticle.cpp b/src/particles/qquickitemparticle.cpp
index 4cd8ee9db8..ba6c6f915d 100644
--- a/src/particles/qquickitemparticle.cpp
+++ b/src/particles/qquickitemparticle.cpp
@@ -231,10 +231,10 @@ void QQuickItemParticle::reset()
     // delete all managed items which had their logical particles cleared
     // but leave it alone if the logical particle is maintained
     QSet<QQuickItem*> lost = QSet<QQuickItem*>::fromList(m_managed);
-    foreach (const QString group, m_groups){
-        int gIdx = m_system->groupIds[group];
-        foreach (QQuickParticleData* d, m_system->groupData[gIdx]->data)
-                lost.remove(d->delegate);
+    for (auto groupId : groupIds()) {
+        for (QQuickParticleData* d : qAsConst(m_system->groupData[groupId]->data)) {
+            lost.remove(d->delegate);
+        }
     }
     m_deletables.append(lost.toList());
     //TODO: This doesn't yet handle calling detach on taken particles in the system reset case
@@ -249,11 +249,12 @@ QSGNode* QQuickItemParticle::updatePaintNode(QSGNode* n, UpdatePaintNodeData* d)
         m_pleaseReset = false;
         //Refill loadables, delayed here so as to only happen once per frame max
         //### Constant resetting might lead to m_loadables never being populated when tick() occurs
-        foreach (const QString group, m_groups){
-            int gIdx = m_system->groupIds[group];
-            foreach (QQuickParticleData* d, m_system->groupData[gIdx]->data)
-                if (!d->delegate && d->t != -1  && d->stillAlive())
+        for (auto groupId : groupIds()) {
+            for (QQuickParticleData* d : qAsConst(m_system->groupData[groupId]->data)) {
+                if (!d->delegate && d->t != -1  && d->stillAlive()) {
                     m_loadables << d;
+                }
+            }
         }
     }
     prepareNextFrame();
@@ -276,31 +277,26 @@ void QQuickItemParticle::prepareNextFrame()
         return;
 
     //TODO: Size, better fade?
-    foreach (const QString &str, m_groups){
-        const int gIdx = m_system->groupIds[str];
-        const QVector<QQuickParticleData*> dataVector = m_system->groupData[gIdx]->data;
-        const int count = dataVector.size();
-
-        for (int i=0; i<count; i++){
-            QQuickParticleData* data = dataVector.at(i);
+    for (auto groupId : groupIds()) {
+        for (QQuickParticleData* data : qAsConst(m_system->groupData[groupId]->data)) {
             QQuickItem* item = data->delegate;
             if (!item)
                 continue;
-            qreal t = ((timeStamp/1000.0) - data->t) / data->lifeSpan;
+            float t = ((timeStamp / 1000.0f) - data->t) / data->lifeSpan;
             if (m_stasis.contains(item)) {
                 data->t += dt;//Stasis effect
                 continue;
             }
-            if (t >= 1.0){//Usually happens from load
+            if (t >= 1.0f){//Usually happens from load
                 m_deletables << item;
                 data->delegate = 0;
             }else{//Fade
                 data->delegate->setVisible(true);
                 if (m_fade){
-                    qreal o = 1.;
-                    if (t<0.2)
-                        o = t*5;
-                    if (t>0.8)
+                    float o = 1.f;
+                    if (t <0.2f)
+                        o = t * 5;
+                    if (t > 0.8f)
                         o = (1-t)*5;
                     item->setOpacity(o);
                 }
diff --git a/src/particles/qquickparticleemitter.cpp b/src/particles/qquickparticleemitter.cpp
index 5e5c750e0e..7cce4d481a 100644
--- a/src/particles/qquickparticleemitter.cpp
+++ b/src/particles/qquickparticleemitter.cpp
@@ -237,6 +237,8 @@ QQuickParticleEmitter::QQuickParticleEmitter(QQuickItem *parent) :
   , m_reset_last(true)
   , m_last_timestamp(-1)
   , m_last_emission(0)
+  , m_groupIdNeedRecalculation(false)
+  , m_groupId(QQuickParticleGroupData::DefaultGroupID)
 
 {
     //TODO: Reset velocity/acc back to null vector? Or allow null pointer?
@@ -257,6 +259,16 @@ bool QQuickParticleEmitter::isEmitConnected()
     IS_SIGNAL_CONNECTED(this, QQuickParticleEmitter, emitParticles, (QQmlV4Handle));
 }
 
+void QQuickParticleEmitter::reclaculateGroupId() const
+{
+    if (!m_system) {
+        m_groupId = QQuickParticleGroupData::InvalidID;
+        return;
+    }
+    m_groupId = m_system->groupIds.value(group(), QQuickParticleGroupData::InvalidID);
+    m_groupIdNeedRecalculation = m_groupId == QQuickParticleGroupData::InvalidID;
+}
+
 void QQuickParticleEmitter::componentComplete()
 {
     if (!m_system && qobject_cast<QQuickParticleSystem*>(parentItem()))
diff --git a/src/particles/qquickparticleemitter_p.h b/src/particles/qquickparticleemitter_p.h
index bb77d132cc..9b114ad46b 100644
--- a/src/particles/qquickparticleemitter_p.h
+++ b/src/particles/qquickparticleemitter_p.h
@@ -119,6 +119,13 @@ public:
         return m_group;
     }
 
+    QQuickParticleGroupData::ID groupId() const
+    {
+        if (m_groupIdNeedRecalculation)
+            reclaculateGroupId();
+        return m_groupId;
+    }
+
     int particleDurationVariation() const
     {
         return m_particleDurationVariation;
@@ -185,6 +192,7 @@ public Q_SLOTS:
     {
         if (m_system != arg) {
             m_system = arg;
+            m_groupIdNeedRecalculation = true;
             if (m_system)
                 m_system->registerParticleEmitter(this);
             Q_EMIT systemChanged(arg);
@@ -195,6 +203,7 @@ public Q_SLOTS:
     {
         if (m_group != arg) {
             m_group = arg;
+            m_groupIdNeedRecalculation = true;
             Q_EMIT groupChanged(arg);
         }
     }
@@ -319,7 +328,6 @@ protected:
        int m_particleDurationVariation;
        bool m_enabled;
        QQuickParticleSystem* m_system;
-       QString m_group;
        QQuickParticleExtruder* m_extruder;
        QQuickParticleExtruder* m_defaultExtruder;
        QQuickParticleExtruder* effectiveExtruder();
@@ -350,7 +358,14 @@ protected:
        QPointF m_last_last_last_emitter;
 
        bool isEmitConnected();
-private:
+
+private: // methods
+       void reclaculateGroupId() const;
+
+private: // data
+       QString m_group;
+       mutable bool m_groupIdNeedRecalculation;
+       mutable QQuickParticleGroupData::ID m_groupId;
        QQuickDirection m_nullVector;
 
 };
diff --git a/src/particles/qquickparticlepainter.cpp b/src/particles/qquickparticlepainter.cpp
index 134689713d..556d137fd4 100644
--- a/src/particles/qquickparticlepainter.cpp
+++ b/src/particles/qquickparticlepainter.cpp
@@ -64,9 +64,13 @@ QT_BEGIN_NAMESPACE
 
     If empty, it will paint the default particle group ("").
 */
-QQuickParticlePainter::QQuickParticlePainter(QQuickItem *parent) :
-    QQuickItem(parent),
-    m_system(0), m_count(0), m_pleaseReset(true), m_window(0)
+QQuickParticlePainter::QQuickParticlePainter(QQuickItem *parent)
+    : QQuickItem(parent)
+    , m_system(0)
+    , m_count(0)
+    , m_pleaseReset(true)
+    , m_window(0)
+    , m_groupIdsNeedRecalculation(false)
 {
 }
 
@@ -89,11 +93,32 @@ void QQuickParticlePainter::componentComplete()
     QQuickItem::componentComplete();
 }
 
+void QQuickParticlePainter::recalculateGroupIds() const
+{
+    if (!m_system) {
+        m_groupIds.clear();
+        return;
+    }
+
+    m_groupIdsNeedRecalculation = false;
+    m_groupIds.clear();
+
+    for (const QString &str : groups()) {
+        QQuickParticleGroupData::ID groupId = m_system->groupIds.value(str, QQuickParticleGroupData::InvalidID);
+        if (groupId == QQuickParticleGroupData::InvalidID) {
+            // invalid data, not finished setting up, or whatever. Fallback: do not cache.
+            m_groupIdsNeedRecalculation = true;
+        } else {
+            m_groupIds.append(groupId);
+        }
+    }
+}
 
 void QQuickParticlePainter::setSystem(QQuickParticleSystem *arg)
 {
     if (m_system != arg) {
         m_system = arg;
+        m_groupIdsNeedRecalculation = true;
         if (m_system){
             m_system->registerParticlePainter(this);
             reset();
@@ -102,6 +127,16 @@ void QQuickParticlePainter::setSystem(QQuickParticleSystem *arg)
     }
 }
 
+void QQuickParticlePainter::setGroups(const QStringList &arg)
+{
+    if (m_groups != arg) {
+        m_groups = arg;
+        m_groupIdsNeedRecalculation = true;
+        //Note: The system watches this as it has to recalc things when groups change. It will request a reset if necessary
+        Q_EMIT groupsChanged(arg);
+    }
+}
+
 void QQuickParticlePainter::load(QQuickParticleData* d)
 {
     initialize(d->group, d->index);
@@ -133,11 +168,6 @@ void QQuickParticlePainter::setCount(int c)//### TODO: some resizeing so that pa
     reset();
 }
 
-int QQuickParticlePainter::count()
-{
-    return m_count;
-}
-
 void QQuickParticlePainter::calcSystemOffset(bool resetPending)
 {
     if (!m_system || !parentItem())
diff --git a/src/particles/qquickparticlepainter_p.h b/src/particles/qquickparticlepainter_p.h
index 719dfdb3d8..064ce27fe8 100644
--- a/src/particles/qquickparticlepainter_p.h
+++ b/src/particles/qquickparticlepainter_p.h
@@ -64,25 +64,40 @@ class QQuickParticlePainter : public QQuickItem
     Q_PROPERTY(QQuickParticleSystem* system READ system WRITE setSystem NOTIFY systemChanged)
     Q_PROPERTY(QStringList groups READ groups WRITE setGroups NOTIFY groupsChanged)
 
+public: // data
+    typedef QQuickParticleVarLengthArray<QQuickParticleGroupData::ID, 4> GroupIDs;
+
 public:
     explicit QQuickParticlePainter(QQuickItem *parent = 0);
     //Data Interface to system
     void load(QQuickParticleData*);
     void reload(QQuickParticleData*);
     void setCount(int c);
-    int count();
+
+    int count() const
+    {
+        return m_count;
+    }
+
     void performPendingCommits();//Called from updatePaintNode
     QQuickParticleSystem* system() const
     {
         return m_system;
     }
 
-
     QStringList groups() const
     {
         return m_groups;
     }
 
+    const GroupIDs &groupIds() const
+    {
+        if (m_groupIdsNeedRecalculation) {
+            recalculateGroupIds();
+        }
+        return m_groupIds;
+    }
+
     void itemChange(ItemChange, const ItemChangeData &);
 
 Q_SIGNALS:
@@ -94,14 +109,7 @@ Q_SIGNALS:
 public Q_SLOTS:
     void setSystem(QQuickParticleSystem* arg);
 
-    void setGroups(const QStringList &arg)
-    {
-        if (m_groups != arg) {
-            m_groups = arg;
-            //Note: The system watches this as it has to recalc things when groups change. It will request a reset if necessary
-            Q_EMIT groupsChanged(arg);
-        }
-    }
+    void setGroups(const QStringList &arg);
 
     void calcSystemOffset(bool resetPending = false);
 
@@ -130,13 +138,18 @@ protected:
     friend class QQuickParticleSystem;
     int m_count;
     bool m_pleaseReset;//Used by subclasses, but it's a nice optimization to know when stuff isn't going to matter.
-    QStringList m_groups;
     QPointF m_systemOffset;
 
     QQuickWindow *m_window;
 
-private:
+private: // methods
+    void recalculateGroupIds() const;
+
+private: // data
+    QStringList m_groups;
     QSet<QPair<int,int> > m_pendingCommits;
+    mutable GroupIDs m_groupIds;
+    mutable bool m_groupIdsNeedRecalculation;
 };
 
 QT_END_NAMESPACE
diff --git a/src/particles/qquickparticlesystem.cpp b/src/particles/qquickparticlesystem.cpp
index e91ab40ffa..8ccdec4792 100644
--- a/src/particles/qquickparticlesystem.cpp
+++ b/src/particles/qquickparticlesystem.cpp
@@ -902,22 +902,21 @@ void QQuickParticleSystem::loadPainter(QObject *p)
 
     QQuickParticlePainter* painter = qobject_cast<QQuickParticlePainter*>(p);
     Q_ASSERT(painter);//XXX
-    foreach (QQuickParticleGroupData* sg, groupData)
-        sg->painters.remove(painter);
+    for (QQuickParticleGroupData* sg : groupData) {
+        sg->painters.removeOne(painter);
+    }
+
     int particleCount = 0;
     if (painter->groups().isEmpty()) {//Uses default particle
-        QStringList def;
-        def << QString();
+        static QStringList def = QStringList() << QString();
         painter->setGroups(def);
         particleCount += groupData[0]->size();
         groupData[0]->painters << painter;
     } else {
-        foreach (const QString &group, painter->groups()) {
-            if (!group.isEmpty() && !groupIds.contains(group)) {//new group
-                new QQuickParticleGroupData(group, this);
-            }
-            particleCount += groupData[groupIds[group]]->size();
-            groupData[groupIds[group]]->painters << painter;
+        for (auto groupId : painter->groupIds()) {
+            QQuickParticleGroupData *gd = groupData[groupId];
+            particleCount += gd->size();
+            gd->painters << painter;
         }
     }
     painter->setCount(particleCount);
@@ -940,22 +939,23 @@ void QQuickParticleSystem::emittersChanged()
     }
 
     // Populate groups and set sizes.
-    for (int i = 0; i < m_emitters.count(); ++i) {
+    for (int i = 0; i < m_emitters.count(); ) {
         QQuickParticleEmitter *e = m_emitters.at(i);
         if (!e) {
             m_emitters.removeAt(i);
-            i--;
             continue;
         }
 
-        if (!e->group().isEmpty() &&
-            !groupIds.contains(e->group())) {
-            new QQuickParticleGroupData(e->group(), this);
+        int groupId = e->groupId();
+        if (groupId == QQuickParticleGroupData::InvalidID) {
+            groupId = (new QQuickParticleGroupData(e->group(), this))->index;
             previousSizes << 0;
             newSizes << 0;
         }
-        newSizes[groupIds[e->group()]] += e->particleCount();
+        newSizes[groupId] += e->particleCount();
         //###: Cull emptied groups?
+
+        ++i;
     }
 
     //TODO: Garbage collection?
diff --git a/src/particles/qquickparticlesystem_p.h b/src/particles/qquickparticlesystem_p.h
index 9ddbb2ff38..2745e73053 100644
--- a/src/particles/qquickparticlesystem_p.h
+++ b/src/particles/qquickparticlesystem_p.h
@@ -65,6 +65,30 @@
 
 QT_BEGIN_NAMESPACE
 
+template<class T, int Prealloc>
+class QQuickParticleVarLengthArray: public QVarLengthArray<T, Prealloc>
+{
+public:
+    void insert(const T &element)
+    {
+        if (!this->contains(element)) {
+            this->append(element);
+        }
+    }
+
+    bool removeOne(const T &element)
+    {
+        for (int i = 0; i < this->size(); ++i) {
+            if (this->at(i) == element) {
+                this->remove(i);
+                return true;
+            }
+        }
+
+        return false;
+    }
+};
+
 class QQuickParticleSystem;
 class QQuickParticleAffector;
 class QQuickParticleEmitter;
@@ -110,6 +134,10 @@ private:
 };
 
 class Q_QUICKPARTICLES_PRIVATE_EXPORT QQuickParticleGroupData {
+public: // types
+    typedef int ID;
+    enum { InvalidID = -1, DefaultGroupID = 0 };
+
 public:
     QQuickParticleGroupData(const QString &name, QQuickParticleSystem* sys);
     ~QQuickParticleGroupData();
@@ -121,8 +149,8 @@ public:
 
     void setSize(int newSize);
 
-    const int index;
-    QSet<QQuickParticlePainter*> painters;//TODO: What if they are dynamically removed?
+    const ID index;
+    QQuickParticleVarLengthArray<QQuickParticlePainter*, 4> painters;//TODO: What if they are dynamically removed?
 
     //TODO: Refactor particle data list out into a separate class
     QVector<QQuickParticleData*> data;
diff --git a/src/particles/qquicktrailemitter.cpp b/src/particles/qquicktrailemitter.cpp
index 89a68a0356..904421c296 100644
--- a/src/particles/qquicktrailemitter.cpp
+++ b/src/particles/qquicktrailemitter.cpp
@@ -179,7 +179,7 @@ void QQuickTrailEmitter::emitWindow(int timeStamp)
     qreal sizeAtEnd = m_particleEndSize >= 0 ? m_particleEndSize : m_particleSize;
 
     int gId = m_system->groupIds[m_follow];
-    int gId2 = m_system->groupIds[m_group];
+    int gId2 = groupId();
     for (int i=0; i<m_system->groupData[gId]->data.count(); i++) {
         QQuickParticleData *d = m_system->groupData[gId]->data[i];
         if (!d->stillAlive()){