text: //
  // chat_server.cpp
  // ~~~~~~~~~~~~~~~
  //
  // Copyright (c) 2003-2020 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 <algorithm>
  #include <cstdlib>
  #include <deque>
  #include <iostream>
  #include <list>
  #include <set>
  #include <boost/bind/bind.hpp>
  #include <boost/shared_ptr.hpp>
  #include <boost/enable_shared_from_this.hpp>
  #include <boost/asio.hpp>
  #include "chat_message.hpp"
  
  using boost::asio::ip::tcp;
  
  //----------------------------------------------------------------------
  
  typedef std::deque<chat_message> chat_message_queue;
  
  //----------------------------------------------------------------------
  
  class chat_participant
  {
  public:
    virtual ~chat_participant() {}
    virtual void deliver(const chat_message& msg) = 0;
  };
  
  typedef boost::shared_ptr<chat_participant> chat_participant_ptr;
  
  //----------------------------------------------------------------------
  
  class chat_room
  {
  public:
    void join(chat_participant_ptr participant)
    {
      participants_.insert(participant);
      std::for_each(recent_msgs_.begin(), recent_msgs_.end(),
          boost::bind(&chat_participant::deliver,
            participant, boost::placeholders::_1));
    }
  
    void leave(chat_participant_ptr participant)
    {
      participants_.erase(participant);
    }
  
    void deliver(const chat_message& msg)
    {
      recent_msgs_.push_back(msg);
      while (recent_msgs_.size() > max_recent_msgs)
        recent_msgs_.pop_front();
  
      std::for_each(participants_.begin(), participants_.end(),
          boost::bind(&chat_participant::deliver,
            boost::placeholders::_1, boost::ref(msg)));
    }
  
  private:
    std::set<chat_participant_ptr> participants_;
    enum { max_recent_msgs = 100 };
    chat_message_queue recent_msgs_;
  };
  
  //----------------------------------------------------------------------
  
  class chat_session
    : public chat_participant,
      public boost::enable_shared_from_this<chat_session>
  {
  public:
    chat_session(boost::asio::io_context& io_context, chat_room& room)
      : socket_(io_context),
        room_(room)
    {
    }
  
    tcp::socket& socket()
    {
      return socket_;
    }
  
    void start()
    {
      room_.join(shared_from_this());
      boost::asio::async_read(socket_,
          boost::asio::buffer(read_msg_.data(), chat_message::header_length),
          boost::bind(
            &chat_session::handle_read_header, shared_from_this(),
            boost::asio::placeholders::error));
    }
  
    void deliver(const chat_message& msg)
    {
      bool write_in_progress = !write_msgs_.empty();
      write_msgs_.push_back(msg);
      if (!write_in_progress)
      {
        boost::asio::async_write(socket_,
            boost::asio::buffer(write_msgs_.front().data(),
              write_msgs_.front().length()),
            boost::bind(&chat_session::handle_write, shared_from_this(),
              boost::asio::placeholders::error));
      }
    }
  
    void handle_read_header(const boost::system::error_code& error)
    {
      if (!error && read_msg_.decode_header())
      {
        boost::asio::async_read(socket_,
            boost::asio::buffer(read_msg_.body(), read_msg_.body_length()),
            boost::bind(&chat_session::handle_read_body, shared_from_this(),
              boost::asio::placeholders::error));
      }
      else
      {
        room_.leave(shared_from_this());
      }
    }
  
    void handle_read_body(const boost::system::error_code& error)
    {
      if (!error)
      {
        room_.deliver(read_msg_);
        boost::asio::async_read(socket_,
            boost::asio::buffer(read_msg_.data(), chat_message::header_length),
            boost::bind(&chat_session::handle_read_header, shared_from_this(),
              boost::asio::placeholders::error));
      }
      else
      {
        room_.leave(shared_from_this());
      }
    }
  
    void handle_write(const boost::system::error_code& error)
    {
      if (!error)
      {
        write_msgs_.pop_front();
        if (!write_msgs_.empty())
        {
          boost::asio::async_write(socket_,
              boost::asio::buffer(write_msgs_.front().data(),
                write_msgs_.front().length()),
              boost::bind(&chat_session::handle_write, shared_from_this(),
                boost::asio::placeholders::error));
        }
      }
      else
      {
        room_.leave(shared_from_this());
      }
    }
  
  private:
    tcp::socket socket_;
    chat_room& room_;
    chat_message read_msg_;
    chat_message_queue write_msgs_;
  };
  
  typedef boost::shared_ptr<chat_session> chat_session_ptr;
  
  //----------------------------------------------------------------------
  
  class chat_server
  {
  public:
    chat_server(boost::asio::io_context& io_context,
        const tcp::endpoint& endpoint)
      : io_context_(io_context),
        acceptor_(io_context, endpoint)
    {
      start_accept();
    }
  
    void start_accept()
    {
      chat_session_ptr new_session(new chat_session(io_context_, room_));
      acceptor_.async_accept(new_session->socket(),
          boost::bind(&chat_server::handle_accept, this, new_session,
            boost::asio::placeholders::error));
    }
  
    void handle_accept(chat_session_ptr session,
        const boost::system::error_code& error)
    {
      if (!error)
      {
        session->start();
      }
  
      start_accept();
    }
  
  private:
    boost::asio::io_context& io_context_;
    tcp::acceptor acceptor_;
    chat_room room_;
  };
  
  typedef boost::shared_ptr<chat_server> chat_server_ptr;
  typedef std::list<chat_server_ptr> chat_server_list;
  
  //----------------------------------------------------------------------
  
  int main(int argc, char* argv[])
  {
    try
    {
      if (argc < 2)
      {
        std::cerr << "Usage: chat_server <port> [<port> ...]\n";
        return 1;
      }
  
      boost::asio::io_context io_context;
  
      chat_server_list servers;
      for (int i = 1; i < argc; ++i)
      {
        using namespace std; // For atoi.
        tcp::endpoint endpoint(tcp::v4(), atoi(argv[i]));
        chat_server_ptr server(new chat_server(io_context, endpoint));
        servers.push_back(server);
      }
  
      io_context.run();
    }
    catch (std::exception& e)
    {
      std::cerr << "Exception: " << e.what() << "\n";
    }
  
    return 0;
  }
  