Add support for first(n), last(n) and sliced() to QList

This keeps the API symmetric with what we have in our string
classes.

Change-Id: I94c5b39b718ca2472f9ca645e7a42e4314636f67
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

3 files changed, 102 insertions, 0 deletions

diff --git a/src/corelib/tools/qlist.h b/src/corelib/tools/qlist.h
index 6a699aa197..3b9eb154ed 100644
--- a/src/corelib/tools/qlist.h
+++ b/src/corelib/tools/qlist.h
@@ -378,6 +378,29 @@ public:
     inline bool endsWith(const T &t) const { return !isEmpty() && last() == t; }
     QList<T> mid(qsizetype pos, qsizetype len = -1) const;
 
+    QList<T> first(qsizetype n) const
+    {
+        Q_ASSERT(size_t(n) <= size_t(size()));
+        return QList<T>(begin(), begin() + n);
+    }
+    QList<T> last(qsizetype n) const
+    {
+        Q_ASSERT(size_t(n) <= size_t(size()));
+        return QList<T>(end() - n, end());
+    }
+    QList<T> sliced(qsizetype pos) const
+    {
+        Q_ASSERT(size_t(pos) <= size_t(size()));
+        return QList<T>(begin() + pos, end());
+    }
+    QList<T> sliced(qsizetype pos, qsizetype n) const
+    {
+        Q_ASSERT(size_t(pos) <= size_t(size()));
+        Q_ASSERT(n >= 0);
+        Q_ASSERT(pos + n <= size());
+        return QList<T>(begin() + pos, begin() + pos + n);
+    }
+
     T value(qsizetype i) const { return value(i, T()); }
     T value(qsizetype i, const T &defaultValue) const;
 
diff --git a/src/corelib/tools/qlist.qdoc b/src/corelib/tools/qlist.qdoc
index 550663ed0e..047c418469 100644
--- a/src/corelib/tools/qlist.qdoc
+++ b/src/corelib/tools/qlist.qdoc
@@ -202,6 +202,55 @@
     are included.
 */
 
+/*!
+    \fn template <typename T> QList<T> QList<T>::first(qsizetype n) const
+    \since 6.0
+
+    Returns a sub-list that contains the first \a n elements
+    of this list.
+
+    \note The behavior is undefined when \a n < 0 or \a n > size().
+
+    \sa last(), sliced()
+*/
+
+/*!
+    \fn template <typename T> QList<T> QList<T>::last(qsizetype n) const
+    \since 6.0
+
+    Returns a sub-list that contains the last \a n elements of this list.
+
+    \note The behavior is undefined when \a n < 0 or \a n > size().
+
+    \sa first(), sliced()
+*/
+
+/*!
+    \fn template <typename T> QList<T> QList<T>::sliced(qsizetype pos, qsizetype n) const
+    \since 6.0
+
+    Returns a sub-list that contains \a n elements of this list,
+    starting at position \a pos.
+
+    \note The behavior is undefined when \a pos < 0, \a n < 0,
+    or \a pos + \a n > size().
+
+    \sa first(), last()
+*/
+
+/*!
+    \fn template <typename T> QList<T> QList<T>::sliced(qsizetype pos) const
+    \since 6.0
+    \overload
+
+    Returns a sub-list that contains the elements of this list starting at
+    position \a pos and extending to its end.
+
+    \note The behavior is undefined when \a pos < 0 or \a pos > size().
+
+    \sa first(), last()
+*/
+
 
 /*! \fn template <typename T> QList<T>::QList()
 
diff --git a/tests/auto/corelib/tools/qlist/tst_qlist.cpp b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
index ddf5e9c30a..fbd5cf6402 100644
--- a/tests/auto/corelib/tools/qlist/tst_qlist.cpp
+++ b/tests/auto/corelib/tools/qlist/tst_qlist.cpp
@@ -267,6 +267,7 @@ private slots:
     void last() const;
     void lastIndexOf() const;
     void mid() const;
+    void sliced() const;
     void moveInt() const;
     void moveMovable() const;
     void moveCustom() const;
@@ -1287,6 +1288,13 @@ void tst_QList::first() const
     myvec.prepend(23);
     QCOMPARE(myvec.first(), 23);
     QCOMPARE(myvec.constFirst(), 23);
+
+
+    QCOMPARE(QList<int>().first(0), QList<int>());
+    QCOMPARE(myvec.first(0), QList<int>());
+    QCOMPARE(myvec.first(1), (QList<int>{23}));
+    QCOMPARE(myvec.first(2), (QList<int>{23, 42}));
+    QCOMPARE(myvec.first(3), myvec);
 }
 
 void tst_QList::constFirst() const
@@ -1546,6 +1554,12 @@ void tst_QList::last() const
     myvec.remove(3);
     QCOMPARE(myvec.last(), QLatin1String("C"));
     QCOMPARE(myvec.constLast(), QLatin1String("C"));
+
+    QCOMPARE(QList<QString>().last(0), QList<QString>());
+    QCOMPARE(myvec.last(0), QList<QString>());
+    QCOMPARE(myvec.last(1), (QList<QString>{QLatin1String("C")}));
+    QCOMPARE(myvec.last(2), (QList<QString>{QLatin1String("B"), QLatin1String("C")}));
+    QCOMPARE(myvec.last(3), myvec);
 }
 
 void tst_QList::constLast() const
@@ -1648,6 +1662,22 @@ void tst_QList::mid() const
     QCOMPARE(list.mid(4), QList<QString>() << "buck" << "hello" << "kitty");
 }
 
+void tst_QList::sliced() const
+{
+    QList<QString> list;
+    list << "foo" << "bar" << "baz" << "bak" << "buck" << "hello" << "kitty";
+
+    QCOMPARE(QList<QString>().sliced(0), QList<QString>());
+    QCOMPARE(QList<QString>().sliced(0, 0), QList<QString>());
+    QCOMPARE(list.sliced(3, 3), QList<QString>() << "bak" << "buck" << "hello");
+    QCOMPARE(list.sliced(3), QList<QString>() << "bak" << "buck" << "hello" << "kitty");
+    QCOMPARE(list.sliced(6, 1), QList<QString>() << "kitty");
+    QCOMPARE(list.sliced(6), QList<QString>() << "kitty");
+    QCOMPARE(list.sliced(0, list.size()), list);
+    QCOMPARE(list.sliced(0), list);
+    QCOMPARE(list.sliced(4), QList<QString>() << "buck" << "hello" << "kitty");
+}
+
 template <typename T>
 void tst_QList::qhash() const
 {