//
// ping.cpp
// ~~~~~~~~
//
// Copyright (c) 2003-2022 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)
//

#include <asio.hpp>
#include <boost/bind/bind.hpp>
#include <istream>
#include <iostream>
#include <ostream>

#include "icmp_header.hpp"
#include "ipv4_header.hpp"

using asio::ip::icmp;
using asio::steady_timer;
namespace chrono = asio::chrono;

class pinger
{
public:
  pinger(asio::io_context& io_context, const char* destination)
    : resolver_(io_context), socket_(io_context, icmp::v4()),
      timer_(io_context), sequence_number_(0), num_replies_(0)
  {
    destination_ = *resolver_.resolve(icmp::v4(), destination, "").begin();

    start_send();
    start_receive();
  }

private:
  void start_send()
  {
    std::string body("\"Hello!\" from Asio ping.");

    // Create an ICMP header for an echo request.
    icmp_header echo_request;
    echo_request.type(icmp_header::echo_request);
    echo_request.code(0);
    echo_request.identifier(get_identifier());
    echo_request.sequence_number(++sequence_number_);
    compute_checksum(echo_request, body.begin(), body.end());

    // Encode the request packet.
    asio::streambuf request_buffer;
    std::ostream os(&request_buffer);
    os << echo_request << body;

    // Send the request.
    time_sent_ = steady_timer::clock_type::now();
    socket_.send_to(request_buffer.data(), destination_);

    // Wait up to five seconds for a reply.
    num_replies_ = 0;
    timer_.expires_at(time_sent_ + chrono::seconds(5));
    timer_.async_wait(boost::bind(&pinger::handle_timeout, this));
  }

  void handle_timeout()
  {
    if (num_replies_ == 0)
      std::cout << "Request timed out" << std::endl;

    // Requests must be sent no less than one second apart.
    timer_.expires_at(time_sent_ + chrono::seconds(1));
    timer_.async_wait(boost::bind(&pinger::start_send, this));
  }

  void start_receive()
  {
    // Discard any data already in the buffer.
    reply_buffer_.consume(reply_buffer_.size());

    // Wait for a reply. We prepare the buffer to receive up to 64KB.
    socket_.async_receive(reply_buffer_.prepare(65536),
        boost::bind(&pinger::handle_receive, this, boost::placeholders::_2));
  }

  void handle_receive(std::size_t length)
  {
    // The actual number of bytes received is committed to the buffer so that we
    // can extract it using a std::istream object.
    reply_buffer_.commit(length);

    // Decode the reply packet.
    std::istream is(&reply_buffer_);
    ipv4_header ipv4_hdr;
    icmp_header icmp_hdr;
    is >> ipv4_hdr >> icmp_hdr;

    // We can receive all ICMP packets received by the host, so we need to
    // filter out only the echo replies that match the our identifier and
    // expected sequence number.
    if (is && icmp_hdr.type() == icmp_header::echo_reply
          && icmp_hdr.identifier() == get_identifier()
          && icmp_hdr.sequence_number() == sequence_number_)
    {
      // If this is the first reply, interrupt the five second timeout.
      if (num_replies_++ == 0)
        timer_.cancel();

      // Print out some information about the reply packet.
      chrono::steady_clock::time_point now = chrono::steady_clock::now();
      chrono::steady_clock::duration elapsed = now - time_sent_;
      std::cout << length - ipv4_hdr.header_length()
        << " bytes from " << ipv4_hdr.source_address()
        << ": icmp_seq=" << icmp_hdr.sequence_number()
        << ", ttl=" << ipv4_hdr.time_to_live()
        << ", time="
        << chrono::duration_cast<chrono::milliseconds>(elapsed).count()
        << std::endl;
    }

    start_receive();
  }

  static unsigned short get_identifier()
  {
#if defined(ASIO_WINDOWS)
    return static_cast<unsigned short>(::GetCurrentProcessId());
#else
    return static_cast<unsigned short>(::getpid());
#endif
  }

  icmp::resolver resolver_;
  icmp::endpoint destination_;
  icmp::socket socket_;
  steady_timer timer_;
  unsigned short sequence_number_;
  chrono::steady_clock::time_point time_sent_;
  asio::streambuf reply_buffer_;
  std::size_t num_replies_;
};

int main(int argc, char* argv[])
{
  try
  {
    if (argc != 2)
    {
      std::cerr << "Usage: ping <host>" << std::endl;
#if !defined(ASIO_WINDOWS)
      std::cerr << "(You may need to run this program as root.)" << std::endl;
#endif
      return 1;
    }

    asio::io_context io_context;
    pinger p(io_context, argv[1]);
    io_context.run();
  }
  catch (std::exception& e)
  {
    std::cerr << "Exception: " << e.what() << std::endl;
  }
}
