//
// icmp_header.hpp
// ~~~~~~~~~~~~~~~
//
// Copyright (c) 2003-2008 Christopher M. Kohlhoff (chris at kohlhoff dot com)
//
// Distributed under the Boost Software License, Version 1.0. (See accompanying
// file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
//

#ifndef ICMP_HEADER_HPP
#define ICMP_HEADER_HPP

#include <istream>
#include <ostream>
#include <algorithm>

// ICMP header for both IPv4 and IPv6.
//
// The wire format of an ICMP header is:
// 
// 0               8               16                             31
// +---------------+---------------+------------------------------+      ---
// |               |               |                              |       ^
// |     type      |     code      |          checksum            |       |
// |               |               |                              |       |
// +---------------+---------------+------------------------------+    8 bytes
// |                               |                              |       |
// |          identifier           |       sequence number        |       |
// |                               |                              |       v
// +-------------------------------+------------------------------+      ---

class icmp_header
{
public:
    enum { echo_reply = 0, destination_unreachable = 3, source_quench = 4,
           redirect = 5, echo_request = 8, time_exceeded = 11, parameter_problem = 12,
           timestamp_request = 13, timestamp_reply = 14, info_request = 15,
           info_reply = 16, address_request = 17, address_reply = 18 };

    icmp_header() { std::fill(rep_, rep_ + sizeof(rep_), 0); }

    unsigned char type() const { return rep_[0]; }
    unsigned char code() const { return rep_[1]; }
    unsigned short checksum() const { return decode(2, 3); }
    unsigned short identifier() const { return decode(4, 5); }
    unsigned short sequence_number() const { return decode(6, 7); }

    void type(unsigned char n) { rep_[0] = n; }
    void code(unsigned char n) { rep_[1] = n; }
    void checksum(unsigned short n) { encode(2, 3, n); }
    void identifier(unsigned short n) { encode(4, 5, n); }
    void sequence_number(unsigned short n) { encode(6, 7, n); }

    friend std::istream& operator>>(std::istream& is, icmp_header& header)
    { return is.read(reinterpret_cast<char*>(header.rep_), 8); }

    friend std::ostream& operator<<(std::ostream& os, const icmp_header& header)
    { return os.write(reinterpret_cast<const char*>(header.rep_), 8); }

private:
    unsigned short decode(int a, int b) const
    { return (rep_[a] << 8) + rep_[b]; }

    void encode(int a, int b, unsigned short n)
    {
        rep_[a] = static_cast<unsigned char>(n >> 8);
        rep_[b] = static_cast<unsigned char>(n & 0xFF);
    }

    unsigned char rep_[8];
};

template <typename Iterator>
void compute_checksum(icmp_header& header,
                      Iterator body_begin, Iterator body_end)
{
    unsigned int sum = (header.type() << 8) + header.code()
        + header.identifier() + header.sequence_number();

    Iterator body_iter = body_begin;
    while (body_iter != body_end)
        {
            sum += (static_cast<unsigned char>(*body_iter++) << 8);
            if (body_iter != body_end)
                sum += static_cast<unsigned char>(*body_iter++);
        }

    sum = (sum >> 16) + (sum & 0xFFFF);
    sum += (sum >> 16);
    header.checksum(static_cast<unsigned short>(~sum));
}

#endif // ICMP_HEADER_HPP

