//    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/>.

// Define the UDP header

#ifndef OPENVPN_IP_UDP_H
#define OPENVPN_IP_UDP_H

#include <openvpn/ip/ipcommon.hpp>

namespace openvpn {

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

  struct UDPHeader {
    std::uint16_t   source;
    std::uint16_t   dest;
    std::uint16_t   len;
    std::uint16_t   check;
  };

#pragma pack(pop)

  inline std::uint16_t udp_checksum (const std::uint8_t *buf,
				     const unsigned int len_udp,
				     const std::uint8_t *src_addr,
				     const std::uint8_t *dest_addr)
  {
    std::uint32_t sum = 0;

    /* make 16 bit words out of every two adjacent 8 bit words and  */
    /* calculate the sum of all 16 bit words */
    for (unsigned int i = 0; i < len_udp; i += 2)
      {
	std::uint16_t word16 = ((buf[i] << 8) & 0xFF00) + ((i + 1 < len_udp) ? (buf[i+1] & 0xFF) : 0);
	sum += word16;
      }

    /* add the UDP pseudo header which contains the IP source and destination addresses */
    for (unsigned int i = 0; i < 4; i += 2)
      {
	std::uint16_t word16 =((src_addr[i] << 8) & 0xFF00) + (src_addr[i+1] & 0xFF);
	sum += word16;
      }
    for (unsigned int i = 0; i < 4; i += 2)
      {
	std::uint16_t word16 =((dest_addr[i] << 8) & 0xFF00) + (dest_addr[i+1] & 0xFF);
	sum += word16;
      }

    /* the protocol number and the length of the UDP packet */
    sum += (std::uint16_t)IPCommon::UDP + (std::uint16_t)len_udp;

    /* keep only the last 16 bits of the 32 bit calculated sum and add the carries */
    while (sum >> 16)
      sum = (sum & 0xFFFF) + (sum >> 16);

    /* take the one's complement of sum */
    return std::uint16_t(~sum);
  }

}

#endif