Unify all mid() functions in QtBase.

Up to now, Qt had at least 3 different implementations of the mid().
Only QString::mid implementation was not crashing on edge cases and
was protected against overflows, therefore I picked that one as the
base implementation, even if it has weird semantics for an invalid
input.

As a side effect QVector::mid was slightly optimized to not detach in
all cases (which follows current QList behavior). Documentation of
QVector::mid and QList::mid was updated to not mention "copy of data"
which could suggest that the mid() result is detached.

QStringRef::mid was fixed and now it follows general Qt behavior, by
returning a null value for a null input.

Change-Id: Ie9ff5d98372bd193d66508e6dd92b6ed1180ad9b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qarraydata.cpp

@@ -130,4 +130,33 @@ void QArrayData::deallocate(QArrayData *data, size_t objectSize,
     ::free(data);
 }
 
+namespace QtPrivate {
+/*!
+  \internal
+*/
+QContainerImplHelper::CutResult QContainerImplHelper::mid(int originalLength, int *_position, int *_length)
+{
+    int &position = *_position;
+    int &length = *_length;
+    if (position > originalLength)
+        return Null;
+
+    if (position < 0) {
+        if (length < 0 || length + position >= originalLength)
+            return Full;
+        if (length + position <= 0)
+            return Null;
+        length += position;
+        position = 0;
+    } else if (uint(length) > uint(originalLength - position)) {
+        length = originalLength - position;
+    }
+
+    if (position == 0 && length == originalLength)
+        return Full;
+
+    return length > 0 ? Subset : Empty;
+}
+}
+
 QT_END_NAMESPACE

src/corelib/tools/qarraydata.h

@@ -361,6 +361,14 @@ namespace QtPrivate {
     QT_PREPEND_NAMESPACE(QtPrivate::qMakeArrayLiteral)<Type>( Array )
 #endif // !defined(Q_ARRAY_LITERAL)
 
+namespace QtPrivate {
+struct Q_CORE_EXPORT QContainerImplHelper
+{
+    enum CutResult { Null, Empty, Full, Subset };
+    static CutResult mid(int originalLength, int *position, int *length);
+};
+}
+
 QT_END_NAMESPACE
 
 #endif // include guard

src/corelib/tools/qbytearray.cpp

@@ -2677,19 +2677,22 @@ QByteArray QByteArray::right(int len) const
 
 QByteArray QByteArray::mid(int pos, int len) const
 {
-    if ((d->size == 0 && d->ref.isStatic()) || pos > d->size)
+    using namespace QtPrivate;
+    switch (QContainerImplHelper::mid(size(), &pos, &len)) {
+    case QContainerImplHelper::Null:
         return QByteArray();
-    if (len < 0)
-        len = d->size - pos;
-    if (pos < 0) {
-        len += pos;
-        pos = 0;
+    case QContainerImplHelper::Empty:
+    {
+        QByteArrayDataPtr empty = { Data::allocate(0) };
+        return QByteArray(empty);
     }
-    if (len + pos > d->size)
-        len = d->size - pos;
-    if (pos == 0 && len == d->size)
+    case QContainerImplHelper::Full:
         return *this;
-    return QByteArray(d->data() + pos, len);
+    case QContainerImplHelper::Subset:
+        return QByteArray(d->data() + pos, len);
+    }
+    Q_UNREACHABLE();
+    return QByteArray();
 }
 
 /*!

src/corelib/tools/qlist.cpp

@@ -491,11 +491,11 @@ void **QListData::erase(void **xi)
 /*!
     \fn QList<T> QList<T>::mid(int pos, int length) const
 
-    Returns a list whose elements are copied from this list,
+    Returns a sub-list which includes elements from this list,
     starting at position \a pos. If \a length is -1 (the default), all
-    elements from \a pos are copied; otherwise \a length elements (or
+    elements from \a pos are included; otherwise \a length elements (or
     all remaining elements if there are less than \a length elements)
-    are copied.
+    are included.
 */
 
 /*! \fn QList::QList()

src/corelib/tools/qlist.h

@@ -45,6 +45,7 @@
 #include <QtCore/qalgorithms.h>
 #include <QtCore/qiterator.h>
 #include <QtCore/qrefcount.h>
+#include <QtCore/qarraydata.h>
 
 #include <iterator>
 #include <list>
@@ -646,10 +647,17 @@ inline void QList<T>::move(int from, int to)
 template<typename T>
 Q_OUTOFLINE_TEMPLATE QList<T> QList<T>::mid(int pos, int alength) const
 {
-    if (alength < 0 || pos > size() - alength)
-        alength = size() - pos;
-    if (pos == 0 && alength == size())
+    using namespace QtPrivate;
+    switch (QContainerImplHelper::mid(size(), &pos, &alength)) {
+    case QContainerImplHelper::Null:
+    case QContainerImplHelper::Empty:
+        return QList<T>();
+    case QContainerImplHelper::Full:
         return *this;
+    case QContainerImplHelper::Subset:
+        break;
+    }
+
     QList<T> cpy;
     if (alength <= 0)
         return cpy;

src/corelib/tools/qstring.cpp

@@ -4090,21 +4090,22 @@ QString QString::right(int n) const
 
 QString QString::mid(int position, int n) const
 {
-    if (position > d->size)
+    using namespace QtPrivate;
+    switch (QContainerImplHelper::mid(d->size, &position, &n)) {
+    case QContainerImplHelper::Null:
         return QString();
-    if (position < 0) {
-        if (n < 0 || n + position >= d->size)
-            return *this;
-        if (n + position <= 0)
-            return QString();
-
-        n += position;
-        position = 0;
-    } else if (uint(n) > uint(d->size - position))
-        n = d->size - position;
-    if (position == 0 && n == d->size)
+    case QContainerImplHelper::Empty:
+    {
+        QStringDataPtr empty = { Data::allocate(0) };
+        return QString(empty);
+    }
+    case QContainerImplHelper::Full:
         return *this;
-    return QString((const QChar*) d->data() + position, n);
+    case QContainerImplHelper::Subset:
+        return QString((const QChar*)d->data() + position, n);
+    }
+    Q_UNREACHABLE();
+    return QString();
 }
 
 /*!
@@ -8794,19 +8795,19 @@ QStringRef QString::rightRef(int n) const
 */
 QStringRef QStringRef::mid(int pos, int n) const
 {
-    if (pos > m_size)
+    using namespace QtPrivate;
+    switch (QContainerImplHelper::mid(m_size, &pos, &n)) {
+    case QContainerImplHelper::Null:
         return QStringRef();
-    if (pos < 0) {
-        if (n < 0 || n + pos >= m_size)
-            return QStringRef(m_string, m_position, m_size);
-        if (n + pos <= 0)
-            return QStringRef();
-        n += pos;
-        pos = 0;
-    } else if (uint(n) > uint(m_size - pos)) {
-        n = m_size - pos;
+    case QContainerImplHelper::Empty:
+        return QStringRef(m_string, 0, 0);
+    case QContainerImplHelper::Full:
+        return *this;
+    case QContainerImplHelper::Subset:
+        return QStringRef(m_string, pos + m_position, n);
     }
-    return QStringRef(m_string, pos + m_position, n);
+    Q_UNREACHABLE();
+    return QStringRef();
 }
 
 /*!
@@ -8831,19 +8832,19 @@ QStringRef QStringRef::mid(int pos, int n) const
 */
 QStringRef QString::midRef(int position, int n) const
 {
-    if (position > d->size)
+    using namespace QtPrivate;
+    switch (QContainerImplHelper::mid(d->size, &position, &n)) {
+    case QContainerImplHelper::Null:
         return QStringRef();
-    if (position < 0) {
-        if (n < 0 || n + position >= d->size)
-            return QStringRef(this, 0, d->size);
-        if (n + position <= 0)
-            return QStringRef();
-
-        n += position;
-        position = 0;
-    } else if (uint(n) > uint(d->size - position))
-        n = d->size - position;
-    return QStringRef(this, position, n);
+    case QContainerImplHelper::Empty:
+        return QStringRef(this, 0, 0);
+    case QContainerImplHelper::Full:
+        return QStringRef(this, 0, d->size);
+    case QContainerImplHelper::Subset:
+        return QStringRef(this, position, n);
+    }
+    Q_UNREACHABLE();
+    return QStringRef();
 }
 
 /*!

src/corelib/tools/qvector.cpp

@@ -180,11 +180,11 @@
 /*!
     \fn QVector<T> QVector::mid(int pos, int length = -1) const
 
-    Returns a vector whose elements are copied from this vector,
+    Returns a sub-vector which contains elements from this vector,
     starting at position \a pos. If \a length is -1 (the default), all
-    elements after \a pos are copied; otherwise \a length elements (or
+    elements after \a pos are included; otherwise \a length elements (or
     all remaining elements if there are less than \a length elements)
-    are copied.
+    are included.
 */
 
 

src/corelib/tools/qvector.h

@@ -822,12 +822,17 @@ int QVector<T>::count(const T &t) const
 template <typename T>
 Q_OUTOFLINE_TEMPLATE QVector<T> QVector<T>::mid(int pos, int len) const
 {
-    if (len < 0)
-        len = size() - pos;
-    if (pos == 0 && len == size())
+    using namespace QtPrivate;
+    switch (QContainerImplHelper::mid(d->size, &pos, &len)) {
+    case QContainerImplHelper::Null:
+    case QContainerImplHelper::Empty:
+        return QVector<T>();
+    case QContainerImplHelper::Full:
         return *this;
-    if (pos + len > size())
-        len = size() - pos;
+    case QContainerImplHelper::Subset:
+        break;
+    }
+
     QVector<T> copy;
     copy.reserve(len);
     for (int i = pos; i < pos + len; ++i)