QtConcurrent: Workaround GCC bug 58800 in median calculation

1) Revert 880b614 for libstdc++ <= 4.7.3 || (4.8.0 >= ... <= 4.8.2)
2) Fix off-by-one error in reverted code for Median::_bufferSize <= 2.

Task-number: QTBUG-35058
Change-Id: I9d226c2806c1cf06c3d5b9c9f371262d2d69bf2b
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>
Reviewed-by: Marc Mutz <marc.mutz@kdab.com>

src/concurrent/qtconcurrentmedian.h

@@ -102,10 +102,19 @@ public:
     {
         if (dirty) {
             dirty = false;
+
+// This is a workaround for http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58800
+// Avoid using std::nth_element for stdlibc++ <= 4.7.3 || (>= 4.8.0 && <= 4.8.2)
+#if defined(__GLIBCXX__) && (__GLIBCXX__ <= 20130411 || (__GLIBCXX__ >= 20130322 && __GLIBCXX__ <= 20131016))
+            QVector<T> sorted = values;
+            std::sort(sorted.begin(), sorted.end());
+            currentMedian = sorted.at(bufferSize / 2);
+#else
             QVector<T> copy = values;
             typename QVector<T>::iterator begin = copy.begin(), mid = copy.begin() + bufferSize/2, end = copy.end();
             std::nth_element(begin, mid, end);
             currentMedian = *mid;
+#endif
         }
         return currentMedian;
     }

tests/auto/concurrent/concurrent.pro

@@ -3,6 +3,7 @@ SUBDIRS=\
    qtconcurrentfilter \
    qtconcurrentiteratekernel \
    qtconcurrentmap \
+   qtconcurrentmedian \
    qtconcurrentrun \
    qtconcurrentthreadengine
 

tests/auto/concurrent/qtconcurrentmedian/qtconcurrentmedian.pro

@@ -0,0 +1,5 @@
+CONFIG += testcase parallel_test
+TARGET = tst_qtconcurrentmedian
+QT = core testlib concurrent
+SOURCES = tst_qtconcurrentmedian.cpp
+DEFINES += QT_STRICT_ITERATORS

tests/auto/concurrent/qtconcurrentmedian/tst_qtconcurrentmedian.cpp

@@ -0,0 +1,87 @@
+/****************************************************************************
+**
+** Copyright (C) 2013 Digia Plc and/or its subsidiary(-ies).
+** Contact: http://www.qt-project.org/legal
+**
+** This file is part of the test suite of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and Digia.  For licensing terms and
+** conditions see http://qt.digia.com/licensing.  For further information
+** use the contact form at http://qt.digia.com/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 2.1 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU Lesser General Public License version 2.1 requirements
+** will be met: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html.
+**
+** In addition, as a special exception, Digia gives you certain additional
+** rights.  These rights are described in the Digia Qt LGPL Exception
+** version 1.1, included in the file LGPL_EXCEPTION.txt in this package.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 3.0 as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL included in the
+** packaging of this file.  Please review the following information to
+** ensure the GNU General Public License version 3.0 requirements will be
+** met: http://www.gnu.org/copyleft/gpl.html.
+**
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+#include <qtconcurrentmedian.h>
+
+#include <QtTest/QtTest>
+
+class tst_QtConcurrentMedian: public QObject
+{
+    Q_OBJECT
+private slots:
+    void median_data();
+    void median();
+};
+
+void tst_QtConcurrentMedian::median_data()
+{
+    QTest::addColumn<QList<int> >("values");
+    QTest::addColumn<int>("expectedMedian");
+
+    QTest::newRow("size=1")
+        << (QList<int>() << 1)
+        << 1;
+
+    QTest::newRow("size=2")
+        << (QList<int>() << 3 << 2)
+        << 3;
+
+    QTest::newRow("size=3")
+        << (QList<int>() << 3 << 1 << 2)
+        << 2;
+
+    QTest::newRow("gcc bug 58800 (nth_element)")
+        << (QList<int>() << 207089 << 202585 << 180067 << 157549 << 211592 << 216096 << 207089)
+        << 207089;
+}
+
+void tst_QtConcurrentMedian::median()
+{
+    QFETCH(QList<int> , values);
+    QFETCH(int, expectedMedian);
+
+    QtConcurrent::Median<int> m(values.size());
+    foreach (int value, values)
+        m.addValue(value);
+    QCOMPARE(m.median(), expectedMedian);
+}
+
+QTEST_MAIN(tst_QtConcurrentMedian)
+#include "tst_qtconcurrentmedian.moc"