QByteDataBuffer: add readPointer functionality using QByteArrayView

While it could be done before it's nice to not have a custom "local"
struct or the size in an out-parameter.

Change-Id: Ie910f7060b1dadf037312d45e922f8e2deafe3ec
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/text/qbytedata_p.h

@@ -193,6 +193,55 @@ public:
         return originalAmount;
     }
 
+    /*!
+        \internal
+        Returns a view into the first QByteArray contained inside,
+        ignoring any already read data. Call advanceReadPointer()
+        to advance the view forward. When a QByteArray is exhausted
+        the view returned by this function will view into another
+        QByteArray if any. Returns a default constructed view if
+        no data is available.
+
+        \sa advanceReadPointer
+    */
+    QByteArrayView readPointer() const
+    {
+        if (isEmpty())
+            return {};
+        return { buffers.first().constData() + qsizetype(firstPos),
+                 buffers.first().size() - qsizetype(firstPos) };
+    }
+
+    /*!
+        \internal
+        Advances the read pointer by \a distance.
+
+        \sa readPointer
+    */
+    void advanceReadPointer(qint64 distance)
+    {
+        qint64 newPos = firstPos + distance;
+        if (isEmpty()) {
+            newPos = 0;
+        } else if (auto size = buffers.first().size(); newPos >= size) {
+            while (newPos >= size) {
+                bufferCompleteSize -= (size - firstPos);
+                newPos -= size;
+                buffers.pop_front();
+                if (isEmpty()) {
+                    size = 0;
+                    newPos = 0;
+                    break;
+                }
+                size = buffers.front().size();
+            }
+            bufferCompleteSize -= newPos;
+        } else {
+            bufferCompleteSize -= newPos - firstPos;
+        }
+        firstPos = newPos;
+    }
+
     inline char getChar()
     {
         char c;

tests/auto/corelib/text/qbytedatabuffer/tst_qbytedatabuffer.cpp

@@ -43,6 +43,7 @@ private Q_SLOTS:
     void readCompleteBuffer();
     void readPartialBuffer_data();
     void readPartialBuffer();
+    void readPointer();
 private:
     void readBuffer(int size, int readSize);
 };
@@ -170,5 +171,59 @@ void tst_QByteDataBuffer::readPartialBuffer()
     readBuffer(size, QIODEVICE_BUFFERSIZE);
 }
 
+void tst_QByteDataBuffer::readPointer()
+{
+    QByteDataBuffer buffer;
+
+    auto view = buffer.readPointer();
+    QCOMPARE(view.size(), 0);
+    QCOMPARE(view, "");
+
+    buffer.append("Hello");
+    buffer.append("World");
+
+    qint64 initialSize = buffer.byteAmount();
+    view = buffer.readPointer();
+
+    QCOMPARE(initialSize, buffer.byteAmount());
+    QCOMPARE(view.size(), 5);
+    QCOMPARE(view, "Hello");
+
+    buffer.advanceReadPointer(2);
+    view = buffer.readPointer();
+
+    QCOMPARE(initialSize - 2, buffer.byteAmount());
+    QCOMPARE(view.size(), 3);
+    QCOMPARE(view, "llo");
+
+    buffer.advanceReadPointer(3);
+    view = buffer.readPointer();
+
+    QCOMPARE(initialSize - 5, buffer.byteAmount());
+    QCOMPARE(view.size(), 5);
+    QCOMPARE(view, "World");
+
+    buffer.advanceReadPointer(5);
+    view = buffer.readPointer();
+
+    QVERIFY(buffer.isEmpty());
+    QCOMPARE(view.size(), 0);
+    QCOMPARE(view, "");
+
+    // Advance past the current view's size
+    buffer.append("Hello");
+    buffer.append("World");
+
+    buffer.advanceReadPointer(6);
+    view = buffer.readPointer();
+    QCOMPARE(view, "orld");
+    QCOMPARE(buffer.byteAmount(), 4);
+
+    // Advance past the end of all contained data
+    buffer.advanceReadPointer(6);
+    view = buffer.readPointer();
+    QCOMPARE(view, "");
+}
+
 QTEST_MAIN(tst_QByteDataBuffer)
 #include "tst_qbytedatabuffer.moc"