Ensure stable sort in QListWidget

QlistWidgets with sorting enabled do not sort stable. A re-sort is
triggered when any Qt::ItemDataRole is changed and not only when
Qt::DisplayRole is changed. Due to an unstable optimization, the changed
element gets inserted at the beginning of their respective "equivalence
group".

This patch disables the optimization and ensures stable sorting
with std::stable_sort. Sorting is only performed, if the subset of
changed items in the range [begin, end] is not already sorted in the
whole list. For this purpose, it is assumed that the list has already been sorted before begin and after end. This assumption minimizes the subset to check.

Limits / side effect:
The patch focuses on the most common use case, which is a single item being changed. Replacing the optimization by std:stable_sort can potentially slow down the sorting performance of large data sets.

Task-number: QTBUG-113123
Pick-to: 6.5 6.6
Change-Id: Ib2bd08f21422eb7d6aeff7cdd6a91be7114ebcba
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>

1 files changed, 60 insertions, 0 deletions

diff --git a/tests/auto/widgets/itemviews/qlistwidget/tst_qlistwidget.cpp b/tests/auto/widgets/itemviews/qlistwidget/tst_qlistwidget.cpp
index 07973d7225..f017e0e06b 100644
--- a/tests/auto/widgets/itemviews/qlistwidget/tst_qlistwidget.cpp
+++ b/tests/auto/widgets/itemviews/qlistwidget/tst_qlistwidget.cpp
@@ -75,6 +75,8 @@ private slots:
     void sortItems();
     void sortHiddenItems();
     void sortHiddenItems_data();
+    void sortCheckStability_data();
+    void sortCheckStability();
     void closeEditor();
     void setData_data();
     void setData();
@@ -1131,6 +1133,64 @@ void tst_QListWidget::sortHiddenItems()
     delete tw;
 }
 
+void tst_QListWidget::sortCheckStability_data() {
+    QTest::addColumn<Qt::SortOrder>("order");
+    QTest::addColumn<QVariantList>("initialList");
+    QTest::addColumn<QVariantList>("expectedList");
+
+    QTest::newRow("ascending strings")
+            << Qt::AscendingOrder
+            << QVariantList{ QString("a"), QString("b"), QString("b"), QString("a")}
+            << QVariantList{ QString("a"), QString("a"), QString("b"), QString("b")};
+
+    QTest::newRow("descending strings")
+            << Qt::DescendingOrder
+            << QVariantList{ QString("a"), QString("b"), QString("b"), QString("a")}
+            << QVariantList{ QString("b"), QString("b"), QString("a"), QString("a")};
+
+    QTest::newRow("ascending numbers")
+            << Qt::AscendingOrder
+            << QVariantList{ 1, 2, 2, 1}
+            << QVariantList{ 1, 1, 2, 2};
+
+    QTest::newRow("descending numbers")
+            << Qt::DescendingOrder
+            << QVariantList{ 1, 2, 2, 1}
+            << QVariantList{ 2, 2, 1, 1};
+}
+
+void tst_QListWidget::sortCheckStability() {
+    QFETCH(Qt::SortOrder, order);
+    QFETCH(const QVariantList, initialList);
+    QFETCH(const QVariantList, expectedList);
+
+    for (const QVariant &data : initialList) {
+        QListWidgetItem *item = new QListWidgetItem(testWidget);
+        item->setData(Qt::DisplayRole, data);
+    }
+
+    QAbstractItemModel *model = testWidget->model();
+    QList<QPersistentModelIndex> persistent;
+    for (int j = 0; j < model->rowCount(QModelIndex()); ++j)
+        persistent << model->index(j, 0, QModelIndex());
+
+    testWidget->sortItems(order);
+
+    QCOMPARE(testWidget->count(), expectedList.size());
+    for (int i = 0; i < testWidget->count(); ++i)
+        QCOMPARE(testWidget->item(i)->text(), expectedList.at(i).toString());
+
+    QVector<QListWidgetItem*> itemOrder(testWidget->count());
+    for (int i = 0; i < testWidget->count(); ++i)
+        itemOrder[i] = testWidget->item(i);
+
+    qobject_cast<QListModel*>(testWidget->model())->ensureSorted(0, order, 1, 1);
+    testWidget->sortItems(order);
+
+    for (int i = 0; i < testWidget->count(); ++i)
+        QCOMPARE(itemOrder[i],testWidget->item(i));
+}
+
 class TestListWidget : public QListWidget
 {
     Q_OBJECT