//    OpenVPN -- An application to securely tunnel IP networks
//               over a single port, with support for SSL/TLS-based
//               session authentication and key exchange,
//               packet encryption, packet authentication, and
//               packet compression.
//
//    Copyright (C) 2012-2020 OpenVPN Inc.
//
//    This program is free software: you can redistribute it and/or modify
//    it under the terms of the GNU Affero General Public License Version 3
//    as published by the Free Software Foundation.
//
//    This program 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 Affero General Public License for more details.
//
//    You should have received a copy of the GNU Affero General Public License
//    along with this program in the COPYING file.
//    If not, see <http://www.gnu.org/licenses/>.

#pragma once

#include <openvpn/common/endian.hpp>
#include <openvpn/ip/ipcommon.hpp>

namespace openvpn {

#pragma pack(push)
#pragma pack(1)

  struct TCPHeader {
    static unsigned int length(const std::uint8_t doff_res)
    {
      return ((doff_res) & 0xF0) >> 2;
    }

    std::uint16_t   source;
    std::uint16_t   dest;
    std::uint32_t   seq;
    std::uint32_t   ack_seq;
    std::uint8_t    doff_res;
    std::uint8_t    flags;
    std::uint16_t   window;
    std::uint16_t   check;
    std::uint16_t   urgent_p;

    // helper enum to parse options in TCP header
    enum {
      OPT_EOL       = 0,
      OPT_NOP       = 1,
      OPT_MAXSEG    = 2,
      OPTLEN_MAXSEG = 4
    };

    enum {
      FLAG_SYN      = 1 << 1
    };
  };

#pragma pack(pop)

  /*
   * The following routine is used to update an
   * internet checksum.  "acc" is a 32-bit
   * accumulation of all the changes to the
   * checksum (adding in old 16-bit words and
   * subtracting out new words), and "cksum"
   * is the checksum value to be updated.
   */
  inline void tcp_adjust_checksum(int acc, std::uint16_t& cksum)
  {
    int _acc = acc;
    _acc += cksum;
    if (_acc < 0)
      {
	_acc = -_acc;
	_acc = (_acc >> 16) + (_acc & 0xffff);
	_acc += _acc >> 16;
	cksum = (uint16_t)~_acc;
      }
    else
      {
	_acc = (_acc >> 16) + (_acc & 0xffff);
	_acc += _acc >> 16;
	cksum = (uint16_t)_acc;
      }
  }
}