//
// server.cpp
// ~~~~~~~~~~
//
// Copyright (c) 2003-2013 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)
//
/*
*该文件编写了一个简单的echo服务器。采用异步I/O的形式。特点在于自己编写了内存池，和会话管理。
**/

#include <cstdlib>
#include <iostream>
#include <boost/aligned_storage.hpp>
#include <boost/array.hpp>
#include <boost/bind.hpp>
#include <boost/enable_shared_from_this.hpp>
#include <boost/noncopyable.hpp>
#include <boost/shared_ptr.hpp>
#include <boost/asio.hpp>

using boost::asio::ip::tcp;

// Class to manage the memory to be used for handler-based custom allocation.
// It contains a single block of memory which may be returned for allocation
// requests. If the memory is in use when an allocation request is made, the
// allocator delegates allocation to the global heap.
//相当于内存池功能，避免大量异步I/O时内存new delete的开销。
class handler_allocator
  : private boost::noncopyable
{
public:
//每次session只做了一次实例化。可以在该构造函数加入输出验证。
  handler_allocator()
    : in_use_(false)
  {
  }

  void* allocate(std::size_t size)
  {
    if (!in_use_ && size < storage_.size)
    {
      in_use_ = true;
      return storage_.address();
    }
    else
    {
      return ::operator new(size);
    }
  }

  void deallocate(void* pointer)
  {
    if (pointer == storage_.address())
    {
      in_use_ = false;
    }
    else
    {
      ::operator delete(pointer);
    }
  }

private:
  // Storage space used for handler-based custom memory allocation.
  //保证对齐的内存空间。
  boost::aligned_storage<1024> storage_;

  // Whether the handler-based custom allocation storage has been used.
  bool in_use_;
};

// Wrapper class template for handler objects to allow handler memory
// allocation to be customised. Calls to operator() are forwarded to the
// encapsulated handler.
//函数对象，handle的再次封装，封装了内存分配和释放
template <typename Handler>
class custom_alloc_handler
{
//通过该构造函数可以发现，每次异步读写操作会实例化一次
public:
  custom_alloc_handler(handler_allocator& a, Handler h)
    : allocator_(a),
      handler_(h)
  {
  }
//实际上由内部的handle执行。外部只是一层函数对象进行包装。
  template <typename Arg1>
  void operator()(Arg1 arg1)
  {
    handler_(arg1);
  }

  template <typename Arg1, typename Arg2>
  void operator()(Arg1 arg1, Arg2 arg2)
  {
    handler_(arg1, arg2);
  }
    /*为handler默认的分配函数http://www.boost.org/doc/libs/1_55_0/doc/html/boost_asio/reference/asio_handler_allocate.html
    *异步操作需要分配一个临时对象，这对象和handle相对应。这些对象默认实现方式是new delete
    *所有handle相关的临时对象，将在调用该handle之前执行deallocated解分配。所以允许对对象的内存重用。
    **/
  friend void* asio_handler_allocate(std::size_t size,
      custom_alloc_handler<Handler>* this_handler)
  {
    return this_handler->allocator_.allocate(size);//自定义分配对异步临时对象内存
  }
//注意参数
  friend void asio_handler_deallocate(void* pointer, std::size_t /*size*/,
      custom_alloc_handler<Handler>* this_handler)
  {
    this_handler->allocator_.deallocate(pointer);//释放内存
  }

private:
//注意这里是引用，而不是对象。
  handler_allocator& allocator_;
  Handler handler_;
};

// Helper function to wrap a handler object to add custom allocation.工厂类，模板参数独立于任何类型
template <typename Handler>
inline custom_alloc_handler<Handler> make_custom_alloc_handler(
    handler_allocator& a, Handler h)
{
  return custom_alloc_handler<Handler>(a, h);//返回的是一个函数对象。
}

//会话类
//有关enable_shared_from_this当类对象被 shared_ptr 管理时。需要在类自己定义的函数里把当前类对象作为参数传给其他函数时，
//不能返回裸指针这时需要传递一个 shared_ptr ，否则就不能保持 
//shared_ptr 管理这个类对象的语义。http://www.2cto.com/kf/201212/175430.html http://blog.sina.com.cn/s/blog_62cd38470100g3dd.html
class session
  : public boost::enable_shared_from_this<session>
{
public:
  session(boost::asio::io_service& io_service)
    : socket_(io_service)
  {
  }

  tcp::socket& socket()
  {
    return socket_;
  }

  void start()
  {
  //当数据读取完成后，执行后面的handle。第二个参数为一个函数handle。这个make_custom_alloc_handler怎么使用模板的参数？
    socket_.async_read_some(boost::asio::buffer(data_),
        make_custom_alloc_handler(allocator_,
          boost::bind(&session::handle_read,// bind会返回函数对象.主要函数成员是这样绑定的。要有个函数成员取地址和一个对象的占位符
            shared_from_this(),
            boost::asio::placeholders::error,
            boost::asio::placeholders::bytes_transferred)));//占位符，从该参数传递错误码和已读个数。
  }

  void handle_read(const boost::system::error_code& error,
      size_t bytes_transferred)
  {
    if (!error)//如果没有发生错误，那么就异步写
    {
    //sleep(10);经测试好像还是存在排队等待的现象
      boost::asio::async_write(socket_,
          boost::asio::buffer(data_, bytes_transferred),
          make_custom_alloc_handler(allocator_,
            boost::bind(&session::handle_write,
              shared_from_this(),
              boost::asio::placeholders::error)));
    }
  }

  void handle_write(const boost::system::error_code& error)
  {
  //如果不发生错误，那么就执行异步写。
    if (!error)
    {
      socket_.async_read_some(boost::asio::buffer(data_),
          make_custom_alloc_handler(allocator_,
            boost::bind(&session::handle_read,
              shared_from_this(),
              boost::asio::placeholders::error,
              boost::asio::placeholders::bytes_transferred)));
    }
  }

private:
  // The socket used to communicate with the client.
  tcp::socket socket_;

  // Buffer used to store data received from the client.缓冲区
  boost::array<char, 1024> data_;

  // The allocator to use for handler-based custom memory allocation.
  //每次会话所分配的内存池
  handler_allocator allocator_;
};

typedef boost::shared_ptr<session> session_ptr;

class server
{
public:
  server(boost::asio::io_service& io_service, short port)
    : io_service_(io_service),
      acceptor_(io_service, tcp::endpoint(tcp::v4(), port))
  {
    session_ptr new_session(new session(io_service_));
    acceptor_.async_accept(new_session->socket(),
        boost::bind(&server::handle_accept, this, new_session,
          boost::asio::placeholders::error));
  }

  void handle_accept(session_ptr new_session,
      const boost::system::error_code& error)
  {
    if (!error)
    {
      new_session->start();
    }

    new_session.reset(new session(io_service_));//重置指针
    acceptor_.async_accept(new_session->socket(),
        boost::bind(&server::handle_accept, this, new_session,
          boost::asio::placeholders::error));
  }

private:
  boost::asio::io_service& io_service_;
  tcp::acceptor acceptor_;
};

int main(int argc, char* argv[])
{
  try
  {
    if (argc != 2)
    {
      std::cerr << "Usage: server <port>\n";
      return 1;
    }

    boost::asio::io_service io_service;

    using namespace std; // For atoi.
    server s(io_service, atoi(argv[1]));

    io_service.run();
  }
  catch (std::exception& e)
  {
    std::cerr << "Exception: " << e.what() << "\n";
  }

  return 0;
}