Optimize QJsonObject::operator==

The old code iterated through one QJsonObject and looked up the key
in the other, comparing the values. It had linearithmic complexity,
and created one QString 'key' per element.

Since the entries in a QJsonObject are lexicographically ordered,
we can, however, just walk through the two objects in lock-step and
compare corresponding entries (at the same index) with each other.

Doing so saves O(N) QString creations and QJsonObject::value()
calls, and makes operator== linear in the number of elements.

Change-Id: Ib46ee0c1008b7f114454e282b6bd2bfcdbe59e2a
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/json/qjson_p.h

@@ -670,6 +670,8 @@ public:
     bool operator >=(const Entry &other) const;
 };
 
+inline bool operator!=(const Entry &lhs, const Entry &rhs) { return !(lhs == rhs); }
+
 inline bool Entry::operator >=(const QString &key) const
 {
     if (value.latinKey)

src/corelib/json/qjsonobject.cpp

@@ -545,8 +545,8 @@ bool QJsonObject::operator==(const QJsonObject &other) const
 
     for (uint i = 0; i < o->length; ++i) {
         QJsonPrivate::Entry *e = o->entryAt(i);
-        QJsonValue v(d, o, e->value);
-        if (other.value(e->key()) != v)
+        QJsonPrivate::Entry *oe = other.o->entryAt(i);
+        if (*e != *oe || QJsonValue(d, o, e->value) != QJsonValue(other.d, other.o, oe->value))
             return false;
     }