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QFileSystemWatcher: fix quadratic loop by porting away from QMutableListIterator

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QMutableListIterator::remove() is linear, so called in a loop, the
loop potentially becomes quadratic.

Fix by porting to std::remove_if. In this case, since the old code
unconditionally detached, anyway, we use remove_copy_if to build a new
list. That's still more efficient than the old code, even if nothing
is removed. It also prepares the code for when Java-style iterators
will be deprecated.

Since the same code appears in two different functions, do an Extract
Method into a file-static function.

Lastly, restore NRVO for most compilers by returning the same object
from all return statements of the function.

Change-Id: I6909c6483d8f7acfd1bf381828f020038b04e431
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>
This commit is contained in:
Marc Mutz 2019-05-19 10:42:04 +02:00
parent 1065777a2a
commit 94fbea2f30
1 changed files with 15 additions and 18 deletions
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33
src/corelib/io/qfilesystemwatcher.cpp
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@ -311,6 +311,17 @@ bool QFileSystemWatcher::addPath(const QString &path)
return paths.isEmpty();
}
static QStringList empty_paths_pruned(const QStringList &paths)
{
QStringList p;
p.reserve(paths.size());
const auto isEmpty = [](const QString &s) { return s.isEmpty(); };
std::remove_copy_if(paths.begin(), paths.end(),
std::back_inserter(p),
isEmpty);
return p;
}
/*!
Adds each path in \a paths to the file system watcher. Paths are
not added if they not exist, or if they are already being
@ -338,18 +349,11 @@ QStringList QFileSystemWatcher::addPaths(const QStringList &paths)
{
Q_D(QFileSystemWatcher);
QStringList p = paths;
QMutableListIterator<QString> it(p);
while (it.hasNext()) {
const QString &path = it.next();
if (path.isEmpty())
it.remove();
}
QStringList p = empty_paths_pruned(paths);
if (p.isEmpty()) {
qWarning("QFileSystemWatcher::addPaths: list is empty");
return QStringList();
return p;
}
const auto selectEngine = [this, d]() -> QFileSystemWatcherEngine* {
@ -421,18 +425,11 @@ QStringList QFileSystemWatcher::removePaths(const QStringList &paths)
{
Q_D(QFileSystemWatcher);
QStringList p = paths;
QMutableListIterator<QString> it(p);
while (it.hasNext()) {
const QString &path = it.next();
if (path.isEmpty())
it.remove();
}
QStringList p = empty_paths_pruned(paths);
if (p.isEmpty()) {
qWarning("QFileSystemWatcher::removePaths: list is empty");
return QStringList();
return p;
}
if (d->native)