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Speed up the removal of items from a QGraphicsScene

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When using a linear index, all items in a scene are stored in a QList.
While adding new items is a constant operation, removal requires a
traversal through the entire list. This is especially problematic
when the scene contains millions of items and many of them are removed,
which requires a linear search for each item, resulting in a very slow
operation. Moreover, this behavior is actually inconsistent with the
current documentation which states for the linear index:
"Adding, moving and removing items, however, is done in constant time."

With this change, the list is sorted once an item needs to be removed.
The item to be removed is then found using binary search. To reduce
the overhead of sorting the list is not sorted from scratch. First the
newly inserted items are sorted and then the two parts of the list
are merged.

[ChangeLog][QtWidgets][QGraphicsScene] Speed up the removal of items
when using the linear index.

Change-Id: I28708622605d7fb0fac656df1f9b2f2fa3136759
Reviewed-by: Andreas Aardal Hanssen <andreas@hanssen.name>
This commit is contained in:
Dimitar Asenov 2014-09-02 14:15:27 +02:00
parent 5b2f77c598
commit 633647334c
1 changed files with 22 additions and 3 deletions
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25
src/widgets/graphicsview/qgraphicsscenelinearindex_p.h
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@ -70,7 +70,7 @@ class Q_AUTOTEST_EXPORT QGraphicsSceneLinearIndex : public QGraphicsSceneIndex
Q_OBJECT
public:
QGraphicsSceneLinearIndex(QGraphicsScene *scene = 0) : QGraphicsSceneIndex(scene)
QGraphicsSceneLinearIndex(QGraphicsScene *scene = 0) : QGraphicsSceneIndex(scene), m_numSortedElements(0)
{ }
QList<QGraphicsItem *> items(Qt::SortOrder order = Qt::DescendingOrder) const
@ -85,16 +85,35 @@ public:
protected :
virtual void clear()
{ m_items.clear(); }
{
m_items.clear();
m_numSortedElements = 0;
}
virtual void addItem(QGraphicsItem *item)
{ m_items << item; }
virtual void removeItem(QGraphicsItem *item)
{ m_items.removeOne(item); }
{
// Sort m_items if needed
if (m_numSortedElements < m_items.size())
{
std::sort(m_items.begin() + m_numSortedElements, m_items.end() );
std::inplace_merge(m_items.begin(), m_items.begin() + m_numSortedElements, m_items.end());
m_numSortedElements = m_items.size();
}
QList<QGraphicsItem*>::iterator element = std::lower_bound(m_items.begin(), m_items.end(), item);
if (element != m_items.end() && *element == item)
{
m_items.erase(element);
--m_numSortedElements;
}
}
private:
QList<QGraphicsItem*> m_items;
int m_numSortedElements;
};
#endif // QT_NO_GRAPHICSVIEW