Qt: cache root index in FileSystemProxyModel::filterAcceptsRow

The proxy model's filterAcceptsRow is called once per (row, parent)
pair the source QFileSystemModel surfaces, which can run into the
tens of thousands during a directory load, sort, or filter pass on
a populous tree. The original implementation resolved the source
model's root path to a QModelIndex via sm->index(sm->rootPath()) on
every single call - cheap individually, but it adds up.

The root path itself only changes when the user navigates, so cache
both the path and its resolved index on the proxy. On each call,
compare the cached path against the current one (a QString equality
check, much cheaper than re-walking the model); refresh on mismatch.

No behaviour change.

Author: LibretroAdmin

ui/drivers/ui_qt.cpp

@@ -1386,12 +1386,26 @@ void LogTextEdit::appendMessage(const QString& text)
 }
 
 /* Only accept indexes from current path.
- * https://www.qtcentre.org/threads/50700-QFileSystemModel-and-QSortFilterProxyModel-don-t-work-well-together */
+ * https://www.qtcentre.org/threads/50700-QFileSystemModel-and-QSortFilterProxyModel-don-t-work-well-together
+ *
+ * The root index is cached because this method is invoked once per
+ * (row, parent) pair the proxy considers - which can be tens of
+ * thousands per directory load on a populous tree - while the
+ * QFileSystemModel root path changes only when the user navigates.
+ * Resolving the root path to an index via sm->index(...) walks the
+ * model on every call without the cache. */
 bool FileSystemProxyModel::filterAcceptsRow(int sourceRow, const QModelIndex &sourceParent) const
 {
-   QFileSystemModel  *sm = qobject_cast<QFileSystemModel*>(sourceModel());
-   QModelIndex rootIndex = sm->index(sm->rootPath());
-   if (sourceParent == rootIndex)
+   QFileSystemModel *sm     = qobject_cast<QFileSystemModel*>(sourceModel());
+   const QString currentRoot = sm->rootPath();
+
+   if (currentRoot != m_cachedRootPath)
+   {
+      m_cachedRootPath  = currentRoot;
+      m_cachedRootIndex = sm->index(currentRoot);
+   }
+
+   if (sourceParent == m_cachedRootIndex)
       return QSortFilterProxyModel::filterAcceptsRow(sourceRow, sourceParent);
    return true;
 }

ui/drivers/ui_qt.h

@@ -386,6 +386,16 @@ class FileSystemProxyModel : public QSortFilterProxyModel
 protected:
    virtual bool filterAcceptsRow(int sourceRow, const QModelIndex &sourceParent) const;
    void sort(int column, Qt::SortOrder order = Qt::AscendingOrder);
+private:
+   /* Cache for filterAcceptsRow's "is this row a child of the
+    * source model's current root?" check. The QFileSystemModel
+    * root path rarely changes, but filterAcceptsRow can fire
+    * thousands of times per directory load / sort / filter, and
+    * resolving rootPath() to a QModelIndex via sm->index(...)
+    * is not free. Compare the cached path string against the
+    * current one on each call; refresh both members on mismatch. */
+   mutable QString m_cachedRootPath;
+   mutable QModelIndex m_cachedRootIndex;
 };
 
 class LoadCoreTableWidget : public QTableWidget