QMap 5.0 - keep track of leftmost node (BIC)

This suggestion keeps track of the most left node.
The point is that constBegin() becomes a lot faster.

That speeds up iteration a bit, and makes it O(1) to get the
first element. The penalty in insert and remove is very small.
On large trees it seems to be less than 1%.

It should be noticed that constBegin() is a very common hint
on my planned change to 5.1, and this opperation will without
this patch cost 2 x log N. One when the user calls the hint
with begin - and one where it is compared with begin.

Other std::maps has a very fast begin(). E.g
http://www.cplusplus.com/reference/stl/map/begin/
(begin with constant time)

Change-Id: I221f6755aa8bd16a5189771c5bc8ae56c8ee0fb4
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

1 files changed, 6 insertions, 2 deletions

diff --git a/src/corelib/tools/qmap.h b/src/corelib/tools/qmap.h
index 0e726e8394..e0b267ce20 100644
--- a/src/corelib/tools/qmap.h
+++ b/src/corelib/tools/qmap.h
@@ -173,11 +173,13 @@ struct Q_CORE_EXPORT QMapDataBase
     QtPrivate::RefCount ref;
     int size;
     QMapNodeBase header;
+    QMapNodeBase *mostLeftNode;
 
     void rotateLeft(QMapNodeBase *x);
     void rotateRight(QMapNodeBase *x);
     void rebalance(QMapNodeBase *x);
     void freeNodeAndRebalance(QMapNodeBase *z);
+    void recalcMostLeftNode();
 
     QMapNodeBase *createNode(int size, int alignment, QMapNodeBase *parent, bool left);
     void freeTree(QMapNodeBase *root, int alignment);
@@ -197,8 +199,8 @@ struct QMapData : public QMapDataBase
 
     const Node *end() const { return static_cast<const Node *>(&header); }
     Node *end() { return static_cast<Node *>(&header); }
-    const Node *begin() const { if (root()) return root()->minimumNode(); return end(); }
-    Node *begin() { if (root()) return root()->minimumNode(); return end(); }
+    const Node *begin() const { if (root()) return static_cast<const Node*>(mostLeftNode); return end(); }
+    Node *begin() { if (root()) return static_cast<Node*>(mostLeftNode); return end(); }
 
     void deleteNode(Node *z);
     Node *findNode(const Key &akey) const;
@@ -555,6 +557,7 @@ inline QMap<Key, T>::QMap(const QMap<Key, T> &other)
         if (other.d->header.left) {
             d->header.left = static_cast<Node *>(other.d->header.left)->copy(d);
             d->header.left->setParent(&d->header);
+            d->recalcMostLeftNode();
         }
     }
 }
@@ -780,6 +783,7 @@ Q_OUTOFLINE_TEMPLATE void QMap<Key, T>::detach_helper()
     if (!d->ref.deref())
         d->destroy();
     d = x;
+    d->recalcMostLeftNode();
 }
 
 template <class Key, class T>