Add qHash(QHash) and qHash(QMultiHash)

The hash function is carefully designed to give the same result
as the straight-forward implementation of qHash(unordered_map),
which we'll probably add at some point, namely: std::accumulate
over a container of std::pair.

This is one reason to use std:: and not QPair in the implemen-
tation of qHash(QHash). The other is that qHash(QPair) uses a
bad hash combiner, which may xor out the 'seed' from the result.

We can't fix that until Qt 6, but the qHash(std::pair) overload
uses the well-known boost::hash_combine algorithm (implemented
in Qt as QtPrivate::QHashCombine), so we can use that.

I also trust std::pair to work without problems with reference
template arguments, while QPair only very recently gained a very
basic auto-test for reference parameters.

[ChangeLog][QtCore] Added qHash() overloads for QHash, QMultiHash.

Change-Id: I90879d8a99cf1aadb6e84ecc0c3704f52f3691da
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qhash.cpp

@@ -2644,4 +2644,24 @@ uint qHash(long double key, uint seed) Q_DECL_NOTHROW
     \sa QHash::constFind()
 */
 
+/*!
+    \fn uint qHash(const QHash<Key, T> &key, uint seed = 0)
+    \since 5.8
+    \relates QHash
+
+    Returns the hash value for the \a key, using \a seed to seed the calculation.
+
+    Type \c T must be supported by qHash().
+*/
+
+/*!
+    \fn uint qHash(const QMultiHash<Key, T> &key, uint seed = 0)
+    \since 5.8
+    \relates QMultiHash
+
+    Returns the hash value for the \a key, using \a seed to seed the calculation.
+
+    Type \c T must be supported by qHash().
+*/
+
 QT_END_NAMESPACE

src/corelib/tools/qhash.h

@@ -1095,6 +1095,27 @@ Q_INLINE_TEMPLATE int QMultiHash<Key, T>::count(const Key &key, const T &value)
 Q_DECLARE_ASSOCIATIVE_ITERATOR(Hash)
 Q_DECLARE_MUTABLE_ASSOCIATIVE_ITERATOR(Hash)
 
+template <class Key, class T>
+uint qHash(const QHash<Key, T> &key, uint seed = 0)
+    Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(std::declval<Key&>())) && noexcept(qHash(std::declval<T&>())))
+{
+    QtPrivate::QHashCombineCommutative hash;
+    for (auto it = key.begin(), end = key.end(); it != end; ++it) {
+        const Key &k = it.key();
+        const T   &v = it.value();
+        seed = hash(seed, std::pair<const Key&, const T&>(k, v));
+    }
+    return seed;
+}
+
+template <class Key, class T>
+inline uint qHash(const QMultiHash<Key, T> &key, uint seed = 0)
+    Q_DECL_NOEXCEPT_EXPR(noexcept(qHash(std::declval<Key&>())) && noexcept(qHash(std::declval<T&>())))
+{
+    const QHash<Key, T> &key2 = key;
+    return qHash(key2, seed);
+}
+
 QT_END_NAMESPACE
 
 #if defined(Q_CC_MSVC)

tests/auto/corelib/tools/qhash/tst_qhash.cpp

@@ -49,6 +49,7 @@ private slots:
     void find(); // copied from tst_QMap
     void constFind(); // copied from tst_QMap
     void contains(); // copied from tst_QMap
+    void qhash();
     void take(); // copied from tst_QMap
     void operator_eq(); // copied from tst_QMap
     void rehash_isnt_quadratic();
@@ -695,6 +696,69 @@ void tst_QHash::contains()
     QVERIFY(!map1.contains(43));
 }
 
+namespace {
+class QGlobalQHashSeedResetter
+{
+    int oldSeed;
+public:
+    // not entirely correct (may lost changes made by another thread between the query
+    // of the old and the setting of the new seed), but qSetGlobalQHashSeed doesn't
+    // return the old value, so this is the best we can do:
+    explicit QGlobalQHashSeedResetter(int newSeed)
+        : oldSeed(qGlobalQHashSeed())
+    {
+        qSetGlobalQHashSeed(newSeed);
+    }
+    ~QGlobalQHashSeedResetter()
+    {
+        qSetGlobalQHashSeed(oldSeed);
+    }
+};
+
+template <typename Key, typename T>
+QHash<T, Key> inverted(const QHash<Key, T> &in)
+{
+    QHash<T, Key> result;
+    for (auto it = in.begin(), end = in.end(); it != end; ++it)
+        result[it.value()] = it.key();
+    return result;
+}
+
+template <typename AssociativeContainer>
+void make_test_data(AssociativeContainer &c)
+{
+    c["one"] = "1";
+    c["two"] = "2";
+}
+
+}
+
+void tst_QHash::qhash()
+{
+    const QGlobalQHashSeedResetter seed1(0);
+
+    QHash<QString, QString> hash1;
+    make_test_data(hash1);
+    const QHash<QString, QString> hsah1 = inverted(hash1);
+
+    const QGlobalQHashSeedResetter seed2(1);
+
+    QHash<QString, QString> hash2;
+    make_test_data(hash2);
+    const QHash<QString, QString> hsah2 = inverted(hash2);
+
+    QCOMPARE(hash1, hash2);
+    QCOMPARE(hsah1, hsah2);
+    QCOMPARE(qHash(hash1), qHash(hash2));
+    QCOMPARE(qHash(hsah1), qHash(hsah2));
+
+    // by construction this is almost impossible to cause false collisions:
+    QVERIFY(hash1 != hsah1);
+    QVERIFY(hash2 != hsah2);
+    QVERIFY(qHash(hash1) != qHash(hsah1));
+    QVERIFY(qHash(hash2) != qHash(hsah2));
+}
+
 //copied from tst_QMap
 void tst_QHash::take()
 {