3 files changed, 61 insertions, 12 deletions

diff --git a/src/corelib/io/qbuffer.cpp b/src/corelib/io/qbuffer.cpp
index c1ff353..73c71ea 100644
--- a/src/corelib/io/qbuffer.cpp
+++ b/src/corelib/io/qbuffer.cpp
@@ -62,6 +62,9 @@ public:
     QByteArray defaultBuf;
     int ioIndex;
 
+    virtual qint64 peek(char *data, qint64 maxSize);
+    virtual QByteArray peek(qint64 maxSize);
+
 #ifndef QT_NO_QOBJECT
     // private slots
     void _q_emitSignals();
@@ -83,6 +86,21 @@ void QBufferPrivate::_q_emitSignals()
 }
 #endif
 
+qint64 QBufferPrivate::peek(char *data, qint64 maxSize)
+{
+    qint64 readBytes = qMin(maxSize, static_cast<qint64>(buf->size()) - pos);
+    memcpy(data, buf->constData() + pos, readBytes);
+    return readBytes;
+}
+
+QByteArray QBufferPrivate::peek(qint64 maxSize)
+{
+    qint64 readBytes = qMin(maxSize, static_cast<qint64>(buf->size()) - pos);
+    if (pos == 0 && maxSize >= buf->size())
+        return *buf;
+    return QByteArray(buf->constData() + pos, readBytes);
+}
+
 /*!
     \class QBuffer
     \reentrant
diff --git a/src/corelib/io/qiodevice.cpp b/src/corelib/io/qiodevice.cpp
index 223df9b..ea60792 100644
--- a/src/corelib/io/qiodevice.cpp
+++ b/src/corelib/io/qiodevice.cpp
@@ -1442,6 +1442,35 @@ bool QIODevicePrivate::putCharHelper(char c)
     return q_func()->write(&c, 1) == 1;
 }
 
+/*!
+    \internal
+*/
+qint64 QIODevicePrivate::peek(char *data, qint64 maxSize)
+{
+    qint64 readBytes = q_func()->read(data, maxSize);
+    if (readBytes <= 0)
+        return readBytes;
+
+    buffer.ungetBlock(data, readBytes);
+    pos -= readBytes;
+    return readBytes;
+}
+
+/*!
+    \internal
+*/
+QByteArray QIODevicePrivate::peek(qint64 maxSize)
+{
+    QByteArray result = q_func()->read(maxSize);
+
+    if (result.isEmpty())
+        return result;
+
+    buffer.ungetBlock(result.constData(), result.size());
+    pos -= result.size();
+    return result;
+}
+
 /*! \fn bool QIODevice::getChar(char *c)
 
     Reads one character from the device and stores it in \a c. If \a c
@@ -1476,11 +1505,7 @@ bool QIODevice::getChar(char *c)
 */
 qint64 QIODevice::peek(char *data, qint64 maxSize)
 {
-    qint64 readBytes = read(data, maxSize);
-    int i = readBytes;
-    while (i > 0)
-        ungetChar(data[i-- - 1]);
-    return readBytes;
+    return d_func()->peek(data, maxSize);
 }
 
 /*!
@@ -1502,12 +1527,7 @@ qint64 QIODevice::peek(char *data, qint64 maxSize)
 */
 QByteArray QIODevice::peek(qint64 maxSize)
 {
-    QByteArray result = read(maxSize);
-    int i = result.size();
-    const char *data = result.constData();
-    while (i > 0)
-        ungetChar(data[i-- - 1]);
-    return result;
+    return d_func()->peek(maxSize);
 }
 
 /*!
diff --git a/src/corelib/io/qiodevice_p.h b/src/corelib/io/qiodevice_p.h
index 94dadca..4a25562 100644
--- a/src/corelib/io/qiodevice_p.h
+++ b/src/corelib/io/qiodevice_p.h
@@ -151,6 +151,15 @@ public:
         len++;
         *first = c;
     }
+    void ungetBlock(const char* block, int size) {
+        if ((first - buf) < size) {
+            // underflow, the existing valid data needs to move to the end of the (potentially bigger) buffer
+            makeSpace(len + size, freeSpaceAtStart);
+            memcpy(first - size, block, size);
+        }
+        first -= size;
+        len += size;
+    }
 
 private:
     enum FreeSpacePos {freeSpaceAtStart, freeSpaceAtEnd};
@@ -222,7 +231,9 @@ public:
             accessMode = q_func()->isSequential() ? Sequential : RandomAccess;
         return accessMode == Sequential;
     }
-    
+
+    virtual qint64 peek(char *data, qint64 maxSize);
+    virtual QByteArray peek(qint64 maxSize);
 
 #ifdef QT_NO_QOBJECT
     QIODevice *q_ptr;