backbuffer.cpp

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/*
  Zipios -- a small C++ library that provides easy access to .zip files.

  Copyright (C) 2000-2007  Thomas Sondergaard
  Copyright (C) 2015-2019  Made to Order Software Corporation

  This library is free software; you can redistribute it and/or
  modify it under the terms of the GNU Lesser General Public
  License as published by the Free Software Foundation; either
  version 2.1 of the License, or (at your option) any later version.

  This library is distributed in the hope that it will be useful,
  but WITHOUT ANY WARRANTY; without even the implied warranty of
  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  Lesser General Public License for more details.

  You should have received a copy of the GNU Lesser General Public
  License along with this library; if not, write to the Free Software
  Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
*/

#include "backbuffer.hpp"

#include "zipios/zipiosexceptions.hpp"

#include <algorithm>

namespace zipios
{

BackBuffer::BackBuffer(std::istream & is, VirtualSeeker const & vs, ssize_t const chunk_size)
    : m_vs(vs)
    , m_chunk_size(chunk_size)
    , m_is(is)
    , m_file_pos(0)
{
    if(!m_is)
    {
        throw InvalidException("BackBuffer() initialized with an invalid input stream.");
    }
    if(m_chunk_size <= 0)
    {
        throw InvalidException("Invalid chunk_size parameter, it has to be larger than zero.");
    }

    m_vs.vseekg(m_is, 0, std::ios::end);
    m_file_pos = m_vs.vtellg(m_is);

    // The following should only happens when m_vs.startOffset() is a
    // position in the file that lies after m_vs.endOffset(), which
    // is clearly not a valid situation. However, vtellg() may just
    // fail too.
    if(m_file_pos < 0)
    {
        throw IOException("Invalid virtual file endings.");
    }
}


ssize_t BackBuffer::readChunk(ssize_t& read_pointer)
{
    // Update m_chunk_size and file position
    m_chunk_size = std::min<ssize_t>(static_cast<ssize_t>(m_file_pos), m_chunk_size);
    m_file_pos -= m_chunk_size;
    m_vs.vseekg(m_is, m_file_pos, std::ios::beg);

    // Make space for m_chunk_size new bytes
    insert(begin(), m_chunk_size, static_cast<char>(0));

    // Read in the next m_chunk_size bytes
    zipRead(m_is, *this, m_chunk_size);
    read_pointer += m_chunk_size;

    return m_is.good() ? m_chunk_size : 0;
}


} // zipios namespace
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