get download speed

void DownloadData::calcSpeed(quint64 bytesReceived)
{
	time_t nCurrentTime = time(NULL);
	quint64 nNewReceiveSize = bytesReceived;
	if(nCurrentTime - m_nOldTime > 5)
	{
		m_nSpeed = (nNewReceiveSize - m_nOldReceiveSize)/(nCurrentTime - m_nOldTime);

		m_nOldTime = nCurrentTime;
		m_nOldReceiveSize = nNewReceiveSize;
	}
	if(m_nSpeed > 0)
		m_nTimeLeft = (getSize() - nNewReceiveSize)/m_nSpeed;
	else
		m_nTimeLeft = INT_MAX;
}