QRingBuffer: allow to peek bytes from any position

Add an ordinary peek() function which also allows retrieving data from
a specified position. We need this functionality in several places.

Change-Id: Ia4a1b6fe1d7f76cb8f6f1ea34b3e4b89e05a2a68
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qringbuffer_p.h

@@ -293,6 +293,30 @@ public:
         return qba;
     }
 
+    // peek the bytes from a specified position
+    inline qint64 peek(char *data, qint64 maxLength, qint64 pos = 0) const
+    {
+        qint64 readSoFar = 0;
+
+        if (pos >= 0) {
+            pos += head;
+            for (int i = 0; readSoFar < maxLength && i < buffers.size(); ++i) {
+                qint64 blockLength = (i == tailBuffer ? tail : buffers[i].size());
+
+                if (pos < blockLength) {
+                    blockLength = qMin(blockLength - pos, maxLength - readSoFar);
+                    memcpy(data + readSoFar, buffers[i].constData() + pos, blockLength);
+                    readSoFar += blockLength;
+                    pos = 0;
+                } else {
+                    pos -= blockLength;
+                }
+            }
+        }
+
+        return readSoFar;
+    }
+
     // append a new buffer to the end
     inline void append(const QByteArray &qba) {
         if (tail == 0) {

tests/auto/corelib/tools/qringbuffer/tst_qringbuffer.cpp

@@ -52,6 +52,7 @@ private slots:
     void ungetChar();
     void indexOf();
     void appendAndRead();
+    void peek();
     void readLine();
 };
 
@@ -280,6 +281,31 @@ void tst_QRingBuffer::appendAndRead()
     QVERIFY(ringBuffer.read() == ba3);
 }
 
+void tst_QRingBuffer::peek()
+{
+    QRingBuffer ringBuffer;
+    QByteArray testBuffer;
+    // fill buffer with an arithmetic progression
+    for (int i = 1; i < 256; ++i) {
+        char *ringPos = ringBuffer.reserve(i);
+        QVERIFY(ringPos);
+        memset(ringPos, i, i);
+        testBuffer.append(ringPos, i);
+    }
+
+    // check stored data
+    QByteArray resultBuffer;
+    int peekPosition = testBuffer.size();
+    for (int i = 1; i < 256; ++i) {
+        QByteArray ba(i, 0);
+        peekPosition -= i;
+        qint64 thisPeek = ringBuffer.peek(ba.data(), i, peekPosition);
+        QCOMPARE(thisPeek, qint64(i));
+        resultBuffer.prepend(ba);
+    }
+    QCOMPARE(testBuffer, resultBuffer);
+}
+
 void tst_QRingBuffer::readLine()
 {
     QRingBuffer ringBuffer;