QJsonObject::iterator: add keyView()

Unlike CBOR maps, JSON objects can only have string keys, but, like
CBOR, they are internally stored as either US-ASCII (ie. Latin-1),
UTF-8, or UTF-16, so in order to return a view on the internal
storage, QAnyStringView is the perfect fit.

Add (const_)iterator::keyView() and add a benchmark, prepared for
testing a similar change to the value side of things.

Results (fastest each of ten runs) on my machine suggest a 40%
speedup:

  PASS   : BenchmarkQtJson::iteratorKey()
  RESULT : BenchmarkQtJson::iteratorKey():
       0.071 msecs per iteration (total: 73, iterations: 1024)
  PASS   : BenchmarkQtJson::iteratorKeyView()
  RESULT : BenchmarkQtJson::iteratorKeyView():
       0.042 msecs per iteration (total: 87, iterations: 2048)

[ChangeLog][QtCore][QJsonObject] Added keyView() methods to iterator
and const_iterator, allowing zero-copy inspection of the key().

Task-number: QTBUG-133688
Change-Id: I0ccedaf8a4fa41125b12bdbab5bea3bd2468d9a5
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/serialization/qcborvalue_p.h

@@ -286,6 +286,18 @@ public:
             return data->asLatin1();
         return data->toUtf8String();
     }
+    QAnyStringView anyStringViewAt(qsizetype idx) const
+    {
+        const auto &e = elements.at(idx);
+        const auto data = byteData(e);
+        if (!data)
+            return nullptr;
+        if (e.flags & QtCbor::Element::StringIsUtf16)
+            return data->asStringView();
+        if (e.flags & QtCbor::Element::StringIsAscii)
+            return data->asLatin1();
+        return data->asUtf8StringView();
+    }
 
     static void resetValue(QCborValue &v)
     {

src/corelib/serialization/qjsonobject.cpp

@@ -914,7 +914,24 @@ QJsonObject::const_iterator QJsonObject::constFindImpl(T key) const
     iterator, although it can be done by calling QJsonObject::erase()
     followed by QJsonObject::insert().
 
-    \sa value()
+    \sa value(), keyView()
+*/
+
+/*!
+    \fn QAnyStringView QJsonObject::iterator::keyView() const
+    \since 6.10
+
+    Returns the current item's key as a QAnyStringView. This function does not
+    allocate memory.
+
+    Since QJsonObject stores keys in US-ASCII, UTF-8 or UTF-16, the returned
+    QAnyStringView may be in any of these encodings.
+
+    There is no direct way of changing an item's key through an
+    iterator, although it can be done by calling QJsonObject::erase()
+    followed by QJsonObject::insert().
+
+    \sa key(), value()
 */
 
 /*! \fn QJsonValueRef QJsonObject::iterator::value() const
@@ -930,7 +947,7 @@ QJsonObject::const_iterator QJsonObject::constFindImpl(T key) const
     the assignment will apply to the element in the QJsonArray or QJsonObject
     from which you got the reference.
 
-    \sa key(), operator*()
+    \sa key(), keyView(), operator*()
 */
 
 /*! \fn QJsonValueRef QJsonObject::iterator::operator*() const
@@ -945,7 +962,7 @@ QJsonObject::const_iterator QJsonObject::constFindImpl(T key) const
     the assignment will apply to the element in the QJsonArray or QJsonObject
     from which you got the reference.
 
-    \sa key()
+    \sa key(), keyView()
 */
 
 /*! \fn QJsonValueRef *QJsonObject::iterator::operator->()
@@ -1189,14 +1206,27 @@ QJsonObject::const_iterator QJsonObject::constFindImpl(T key) const
 
     Returns the current item's key.
 
-    \sa value()
+    \sa value(), keyView()
+*/
+
+/*!
+    \fn QAnyStringView QJsonObject::const_iterator::keyView() const
+    \since 6.10
+
+    Returns the current item's key as a QAnyStringView. This function does not
+    allocate.
+
+    Since QJsonObject stores keys in US-ASCII, UTF-8 or UTF-16, the returned
+    QAnyStringView may be in any of these encodings.
+
+    \sa value(), key()
 */
 
 /*! \fn QJsonValueConstRef QJsonObject::const_iterator::value() const
 
     Returns the current item's value.
 
-    \sa key(), operator*()
+    \sa key(), keyView(), operator*()
 */
 
 /*! \fn const QJsonValueConstRef QJsonObject::const_iterator::operator*() const
@@ -1205,7 +1235,7 @@ QJsonObject::const_iterator QJsonObject::constFindImpl(T key) const
 
     Same as value().
 
-    \sa key()
+    \sa key(), keyView()
 */
 
 /*! \fn const QJsonValueConstRef *QJsonObject::const_iterator::operator->() const

src/corelib/serialization/qjsonobject.h

@@ -102,6 +102,7 @@ public:
         }
 
         inline QString key() const { return item.objectKey(); }
+        QAnyStringView keyView() const { return item.objectKeyView(); }
         inline QJsonValueRef value() const { return item; }
         inline QJsonValueRef operator*() const { return item; }
         inline const QJsonValueConstRef *operator->() const { return &item; }
@@ -215,6 +216,7 @@ public:
         }
 
         inline QString key() const { return item.objectKey(); }
+        QAnyStringView keyView() const { return item.objectKeyView(); }
         inline QJsonValueConstRef value() const { return item; }
         inline const QJsonValueConstRef operator*() const { return item; }
         inline const QJsonValueConstRef *operator->() const { return &item; }

src/corelib/serialization/qjsonvalue.cpp

@@ -1073,6 +1073,17 @@ QJsonValue QJsonValueConstRef::concrete(QJsonValueConstRef self) noexcept
     return QJsonPrivate::Value::fromTrustedCbor(d->valueAt(index));
 }
 
+QAnyStringView QJsonValueConstRef::objectKeyView(QJsonValueConstRef self)
+{
+    Q_ASSERT(self.is_object);
+    const QCborContainerPrivate *d = QJsonPrivate::Value::container(self);
+    const qsizetype index = QJsonPrivate::Value::indexHelper(self);
+
+    Q_ASSERT(d);
+    Q_ASSERT(index < d->elements.size());
+    return d->anyStringViewAt(index - 1);
+}
+
 QString QJsonValueConstRef::objectKey(QJsonValueConstRef self)
 {
     Q_ASSERT(self.is_object);

src/corelib/serialization/qjsonvalue.h

@@ -201,6 +201,9 @@ protected:
     Q_CORE_EXPORT static QString objectKey(QJsonValueConstRef self);
     QString objectKey() const { return objectKey(*this); }
 
+    Q_CORE_EXPORT static QAnyStringView objectKeyView(QJsonValueConstRef self);
+    QAnyStringView objectKeyView() const { return objectKeyView(*this); }
+
 #if QT_VERSION < QT_VERSION_CHECK(7, 0, 0) && !defined(QT_BOOTSTRAPPED)
     QJsonValueConstRef(QJsonArray *array, qsizetype idx)
         : a(array), is_object(false), index(static_cast<quint64>(idx)) {}

tests/auto/corelib/serialization/json/tst_qtjson.cpp

@@ -1135,6 +1135,7 @@ void tst_QtJson::testObjectIteration()
 
     for (QJsonObject::iterator it = object.begin(); it != object.end(); ++it) {
         QJsonValue value = it.value();
+        QCOMPARE(it.keyView(), QString::number(it.value().toInteger()));
         QCOMPARE((double)it.key().toInt(), value.toDouble());
         QT_TEST_EQUALITY_OPS(it, QJsonObject::iterator(), false);
     }
@@ -1600,8 +1601,10 @@ void tst_QtJson::keySorting()
     QCOMPARE(o.keys(), sortedKeys);
     QJsonObject::const_iterator it = o.constBegin();
     QStringList::const_iterator it2 = sortedKeys.constBegin();
-    for ( ; it != o.constEnd(); ++it, ++it2)
+    for ( ; it != o.constEnd(); ++it, ++it2) {
         QCOMPARE(it.key(), *it2);
+        QCOMPARE(it.keyView(), *it2);
+    }
 }
 
 void tst_QtJson::undefinedValues()

tests/auto/corelib/serialization/qcborvalue_json/tst_qcborvalue_json.cpp

@@ -306,6 +306,7 @@ void tst_QCborValue_Json::nonStringKeysInMaps()
         QJsonObject o = m.toJsonObject();
         auto it = o.begin();
         QVERIFY(it != o.end());
+        QCOMPARE(it.keyView(), converted);
         QCOMPARE(it.key(), converted);
         QCOMPARE(it.value(), 0);
         QCOMPARE(++it, o.end());

tests/benchmarks/corelib/json/tst_bench_qtjson.cpp

@@ -2,6 +2,8 @@
 // SPDX-License-Identifier: LicenseRef-Qt-Commercial OR GPL-3.0-only
 
 #include <QTest>
+
+#include <QtCore/qjsonarray.h>
 #include <QVariantMap>
 #include <qjsondocument.h>
 #include <qjsonobject.h>
@@ -24,6 +26,21 @@ private Q_SLOTS:
 
     void jsonObjectInsert();
     void variantMapInsert();
+
+    void iteratorKey()
+    {
+        iteratorKeyImpl([](auto it) { return it.key(); },
+                        [](auto it) { return it->toString(); });
+    }
+    void iteratorKeyView()
+    {
+        iteratorKeyImpl([](auto it) { return it.keyView(); },
+                        [](auto it) { return it->toString(); });
+    }
+
+private:
+    template <typename KeyFunc, typename ValueFunc>
+    void iteratorKeyImpl(KeyFunc key, ValueFunc val);
 };
 
 BenchmarkQtJson::BenchmarkQtJson(QObject *parent) : QObject(parent)
@@ -117,6 +134,55 @@ void BenchmarkQtJson::variantMapInsert()
     }
 }
 
+namespace Iteration {
+template <typename KeyFunc, typename ValueFunc>
+void visitObject(const QJsonObject &o, KeyFunc key, ValueFunc val);
+template <typename KeyFunc, typename ValueFunc>
+void visitArray(const QJsonArray &a, KeyFunc key, ValueFunc val) {
+    for (const auto &value : a) {
+        if (value.isObject())
+            visitObject(value.toObject(), key, val);
+        else if (value.isArray())
+            visitArray(value.toArray(), key, val);
+    }
+}
+template <typename KeyFunc, typename ValueFunc>
+void visitObject(const QJsonObject &o, KeyFunc key, ValueFunc val) {
+    for (auto it = o.begin(), end = o.end(); it != end; ++it) {
+        [[maybe_unused]] const auto k = key(it);
+        [[maybe_unused]] const auto v = val(it);
+        if (it->isObject())
+            visitObject(it->toObject(), key, val);
+        else if (it->isArray())
+            visitArray(it->toArray(), key, val);
+    }
+}
+} // namespace Iteration
+
+template <typename KeyFunc, typename ValueFunc>
+void BenchmarkQtJson::iteratorKeyImpl(KeyFunc key, ValueFunc val)
+{
+    const QString testFile = QFINDTESTDATA("test.json");
+    QVERIFY2(!testFile.isEmpty(), "cannot find test file test.json!");
+    QFile file(testFile);
+    QVERIFY2(file.open(QFile::ReadOnly), qPrintable(file.errorString()));
+    const QByteArray content = file.readAll();
+    QVERIFY(!content.isEmpty());
+    QJsonParseError error;
+    const auto doc = QJsonDocument::fromJson(content, &error);
+    QVERIFY2(!error.error, qPrintable(QString::asprintf("at offset %d: %ls",
+                                                        error.offset,
+                                                        qUtf16Printable(error.errorString()))));
+    QVERIFY(!doc.isNull());
+    QVERIFY(doc.isArray());
+    const auto array = doc.array();
+    QCOMPARE_GT(array.size(), 0);
+
+    QBENCHMARK {
+        Iteration::visitArray(array, key, val);
+    }
+}
+
 QTEST_MAIN(BenchmarkQtJson)
 #include "tst_bench_qtjson.moc"