QHash/QMultiHash: fix operator==

The existing QHash::operator== does not work when the same
keys appear in different order between the two hashes being compared.
However, relying on iteration order on a QHash is (as usual) a bad
idea and one should never do it.

Task-number: QTBUG-60395
Change-Id: Ifb39a6779230e26bbd6fdba82ccc0247b9cdc6ed
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

2 files changed, 85 insertions, 12 deletions

diff --git a/src/corelib/tools/qhash.h b/src/corelib/tools/qhash.h
index c59f789cb2..b2c3cf574d 100644
--- a/src/corelib/tools/qhash.h
+++ b/src/corelib/tools/qhash.h
@@ -51,6 +51,8 @@
 #include <initializer_list>
 #endif
 
+#include <algorithm>
+
 #if defined(Q_CC_MSVC)
 #pragma warning( push )
 #pragma warning( disable : 4311 ) // disable pointer truncation warning
@@ -936,18 +938,24 @@ Q_OUTOFLINE_TEMPLATE bool QHash<Key, T>::operator==(const QHash &other) const
     const_iterator it = begin();
 
     while (it != end()) {
-        const Key &akey = it.key();
+        // Build two equal ranges for i.key(); one for *this and one for other.
+        // For *this we can avoid a lookup via equal_range, as we know the beginning of the range.
+        auto thisEqualRangeEnd = it;
+        while (thisEqualRangeEnd != end() && it.key() == thisEqualRangeEnd.key())
+            ++thisEqualRangeEnd;
 
-        const_iterator it2 = other.find(akey);
-        do {
-            if (it2 == other.end() || !(it2.key() == akey))
-                return false;
-            if (!(it.value() == it2.value()))
-                return false;
-            ++it;
-            ++it2;
-        } while (it != end() && it.key() == akey);
+        const auto otherEqualRange = other.equal_range(it.key());
+
+        if (std::distance(it, thisEqualRangeEnd) != std::distance(otherEqualRange.first, otherEqualRange.second))
+            return false;
+
+        // Keys in the ranges are equal by construction; this checks only the values.
+        if (!std::is_permutation(it, thisEqualRangeEnd, otherEqualRange.first))
+            return false;
+
+        it = thisEqualRangeEnd;
     }
+
     return true;
 }
 
diff --git a/tests/auto/corelib/tools/qhash/tst_qhash.cpp b/tests/auto/corelib/tools/qhash/tst_qhash.cpp
index 0c5f1a7afb..943b4316ff 100644
--- a/tests/auto/corelib/tools/qhash/tst_qhash.cpp
+++ b/tests/auto/corelib/tools/qhash/tst_qhash.cpp
@@ -51,7 +51,7 @@ private slots:
     void contains(); // copied from tst_QMap
     void qhash();
     void take(); // copied from tst_QMap
-    void operator_eq(); // copied from tst_QMap
+    void operator_eq(); // slightly modified from tst_QMap
     void rehash_isnt_quadratic();
     void dont_need_default_constructor();
     void qmultihash_specific();
@@ -771,7 +771,7 @@ void tst_QHash::take()
     QVERIFY(!map.contains(3));
 }
 
-//copied from tst_QMap
+// slightly modified from tst_QMap
 void tst_QHash::operator_eq()
 {
     {
@@ -848,6 +848,71 @@ void tst_QHash::operator_eq()
         QVERIFY(a != b);
         QVERIFY(!(a == b));
     }
+
+    // unlike multi-maps, multi-hashes should be equal iff their contents are equal,
+    // regardless of insertion or iteration order
+
+    {
+        QHash<int, int> a;
+        QHash<int, int> b;
+
+        a.insertMulti(0, 0);
+        a.insertMulti(0, 1);
+
+        b.insertMulti(0, 1);
+        b.insertMulti(0, 0);
+
+        QVERIFY(a == b);
+        QVERIFY(!(a != b));
+    }
+
+    {
+        QHash<int, int> a;
+        QHash<int, int> b;
+
+        enum { Count = 100 };
+
+        for (int key = 0; key < Count; ++key) {
+            for (int value = 0; value < Count; ++value)
+                a.insertMulti(key, value);
+        }
+
+        for (int key = Count - 1; key >= 0; --key) {
+            for (int value = 0; value < Count; ++value)
+                b.insertMulti(key, value);
+        }
+
+        QVERIFY(a == b);
+        QVERIFY(!(a != b));
+    }
+
+    {
+        QHash<int, int> a;
+        QHash<int, int> b;
+
+        enum {
+            Count = 100,
+            KeyStep = 17,   // coprime with Count
+            ValueStep = 23, // coprime with Count
+        };
+
+        for (int key = 0; key < Count; ++key) {
+            for (int value = 0; value < Count; ++value)
+                a.insertMulti(key, value);
+        }
+
+        // Generates two permutations of [0, Count) for the keys and values,
+        // so that b will be identical to a, just built in a very different order.
+
+        for (int k = 0; k < Count; ++k) {
+           const int key = (k * KeyStep) % Count;
+           for (int v = 0; v < Count; ++v)
+               b.insertMulti(key, (v * ValueStep) % Count);
+        }
+
+        QVERIFY(a == b);
+        QVERIFY(!(a != b));
+    }
 }
 
 void tst_QHash::compare()