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>

2 files changed, 23 insertions, 3 deletions

diff --git a/src/corelib/tools/qmap.cpp b/src/corelib/tools/qmap.cpp
index dea87c6b63..7c33d60750 100644
--- a/src/corelib/tools/qmap.cpp
+++ b/src/corelib/tools/qmap.cpp
@@ -50,7 +50,7 @@
 
 QT_BEGIN_NAMESPACE
 
-const QMapDataBase QMapDataBase::shared_null = { Q_REFCOUNT_INITIALIZE_STATIC, 0, { 0, 0, 0 } };
+const QMapDataBase QMapDataBase::shared_null = { Q_REFCOUNT_INITIALIZE_STATIC, 0, { 0, 0, 0 }, 0 };
 
 const QMapNodeBase *QMapNodeBase::nextNode() const
 {
@@ -177,6 +177,12 @@ void QMapDataBase::freeNodeAndRebalance(QMapNodeBase *z)
     QMapNodeBase *x_parent;
     if (y->left == 0) {
         x = y->right;
+        if (y == mostLeftNode) {
+            if (x)
+                mostLeftNode = x; // It cannot have (left) children due the red black invariant.
+            else
+                mostLeftNode = y->parent();
+        }
     } else {
         if (y->right == 0) {
             x = y->left;
@@ -290,6 +296,13 @@ void QMapDataBase::freeNodeAndRebalance(QMapNodeBase *z)
     --size;
 }
 
+void QMapDataBase::recalcMostLeftNode()
+{
+    mostLeftNode = &header;
+    while (mostLeftNode->left)
+        mostLeftNode = mostLeftNode->left;
+}
+
 static inline int qMapAlignmentThreshold()
 {
     // malloc on 32-bit platforms should return pointers that are 8-byte
@@ -324,6 +337,8 @@ QMapNodeBase *QMapDataBase::createNode(int alloc, int alignment, QMapNodeBase *p
     if (parent) {
         if (left) {
             parent->left = node;
+            if (parent == mostLeftNode)
+                mostLeftNode = node;
         } else {
             parent->right = node;
         }
@@ -352,6 +367,7 @@ QMapDataBase *QMapDataBase::createData()
     d->header.p = 0;
     d->header.left = 0;
     d->header.right = 0;
+    d->mostLeftNode = &(d->header);
 
     return d;
 }
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>