QHeaderView::moveSection performance boost

The patch also speeds up swapSections and hideSection a lot.

It work by eliminating the Span. (That is forcing each 'Span'
to have exactly one element) That saves a lot of loops since
we can often lookup info fast - and/or change it fast.

Since it is often a complexity change, it is difficult to
put %-increase on. (The most used linear function is
recalcSectionStartPos() - and it has a very low constant)

However comparing with the new benchmark (2500 rows)
swapSection, showHideSection and moveSection are about
20-40 factors faster. (Yes, it is a lot faster!)

In the benchmark moveSection is about 300 factors faster.

Beside being a far better model it is also far more simple.

This fix partly solves:
Task-number: QTBUG-19092

Change-Id: I8deeb9315276d15c68e8a27d5dcb8e0c0badf367
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

1 files changed, 16 insertions, 19 deletions

diff --git a/src/widgets/itemviews/qheaderview_p.h b/src/widgets/itemviews/qheaderview_p.h
index e26c4a6475..9d7d97f582 100644
--- a/src/widgets/itemviews/qheaderview_p.h
+++ b/src/widgets/itemviews/qheaderview_p.h
@@ -97,7 +97,8 @@ public:
           lastSectionSize(0),
           sectionIndicatorOffset(0),
           sectionIndicator(0),
-          globalResizeMode(QHeaderView::Interactive)
+          globalResizeMode(QHeaderView::Interactive),
+          sectionStartposRecalc(true)
     {}
 
 
@@ -281,22 +282,24 @@ public:
     QLabel *sectionIndicator;
     QHeaderView::ResizeMode globalResizeMode;
     QList<QPersistentModelIndex> persistentHiddenSections;
-
+    mutable bool sectionStartposRecalc;
     // header section spans
 
     struct SectionSpan {
         int size;
-        int count;
+        mutable int calculated_startpos;
         QHeaderView::ResizeMode resizeMode;
-        inline SectionSpan() : size(0), count(0), resizeMode(QHeaderView::Interactive) {}
-        inline SectionSpan(int length, int sections, QHeaderView::ResizeMode mode)
-            : size(length), count(sections), resizeMode(mode) {}
-        inline int sectionSize() const { return (count > 0 ? size / count : 0); }
+        inline SectionSpan() : size(0), resizeMode(QHeaderView::Interactive) {}
+        inline SectionSpan(int length, QHeaderView::ResizeMode mode)
+            : size(length), calculated_startpos(-1), resizeMode(mode) {}
+        inline int sectionSize() const { return size; }
+        inline int calculatedEndPos() const { return calculated_startpos + size; }
 #ifndef QT_NO_DATASTREAM
+        int tmpDataStreamSectionCount;
         inline void write(QDataStream &out) const
-        { out << size; out << count; out << (int)resizeMode; }
+        { out << size; out << 1; out << (int)resizeMode; }
         inline void read(QDataStream &in)
-        { in >> size; in >> count; int m; in >> m; resizeMode = (QHeaderView::ResizeMode)m; }
+        { in >> size; in >> tmpDataStreamSectionCount; int m; in >> m; resizeMode = (QHeaderView::ResizeMode)m; }
 #endif
     };
 
@@ -306,12 +309,10 @@ public:
     void removeSectionsFromSpans(int start, int end);
     void resizeSectionSpan(int visualIndex, int oldSize, int newSize);
     void setDefaultSectionSize(int size);
+    void recalcSectionStartPos() const; // not really const
 
     inline int headerSectionCount() const { // for debugging
-        int count = 0;
-        for (int i = 0; i < sectionSpans.count(); ++i)
-            count += sectionSpans.at(i).count;
-        return count;
+        return sectionSpans.count();
     }
 
     inline int headerLength() const { // for debugging
@@ -329,12 +330,8 @@ public:
     }
 
     inline int sectionSpanIndex(int visual) const {
-        int section_start = 0;
-        for (int i = 0; i < sectionSpans.count(); ++i) {
-            int section_end = section_start + sectionSpans.at(i).count - 1;
-            if (visual >= section_start && visual <= section_end)
-                return i;
-            section_start = section_end + 1;
+        if (visual < sectionSpans.count() && visual >= 0) {
+            return visual;
         }
         return -1;
     }