Speed up QTextEngine::setBoundary

Before, setBoundary was O(N), now it is O(log N) assuming the boundaries
are added in order. (splitItem might still be linear because of the call
to QVector::insert)

This speeds up a lot QTextEngine::addRequiredBoundaries when there is a
lot of addFormats. (For example if there is a very long line with syntax
highlighting)

Task-number: QTBUG-8389
Change-Id: I10c9fca72343f46db24b1d4f519f7747188d4009
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>

src/gui/text/qtextengine.cpp

@@ -2535,20 +2535,28 @@ QString QTextEngine::elidedText(Qt::TextElideMode mode, const QFixed &width, int
     return layoutData->string.mid(from, to - from);
 }
 
+namespace {
+struct QScriptItemComparator {
+    bool operator()(const QScriptItem &a, const QScriptItem &b) { return a.position < b.position; }
+    bool operator()(int p, const QScriptItem &b) { return p < b.position; }
+    //bool operator()(const QScriptItem &a, int p) { return a.position < p; }
+};
+}
+
 void QTextEngine::setBoundary(int strPos) const
 {
     if (strPos <= 0 || strPos >= layoutData->string.length())
         return;
 
-    int itemToSplit = 0;
-    while (itemToSplit < layoutData->items.size() && layoutData->items.at(itemToSplit).position <= strPos)
-        itemToSplit++;
-    itemToSplit--;
-    if (layoutData->items.at(itemToSplit).position == strPos) {
+    const QScriptItem* it = qUpperBound(layoutData->items.constBegin(), layoutData->items.constEnd(),
+                                        strPos, QScriptItemComparator());
+    Q_ASSERT(it > layoutData->items.constBegin());
+    --it;
+    if (it->position == strPos) {
         // already a split at the requested position
         return;
     }
-    splitItem(itemToSplit, strPos - layoutData->items.at(itemToSplit).position);
+    splitItem(it - layoutData->items.constBegin(), strPos - it->position);
 }
 
 void QTextEngine::splitItem(int item, int pos) const