//
// server.cpp
// ~~~~~~~~~~
//
// Copyright (c) 2003-2025 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 <array>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <memory>
#include <type_traits>
#include <utility>
#include <boost/asio.hpp>

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

// Class to manage the memory to be used for allocating objects that are
// associated with an execution context (such as services and internal state of
// I/O objects). It contains a single block of memory from which objects are
// monotonically allocated (similar to std::pmr::monotonic_resource). If no
// more space is available it delegates allocation to the global heap.
class context_memory
{
public:
 explicit context_memory(std::size_t preallocated)
 : preallocated_(preallocated),
 next_allocation_(0),
 storage_(new unsigned char[preallocated_])
 {
 }

 ~context_memory()
 {
 delete[] storage_;
 }

 context_memory(const context_memory&) = delete;
 context_memory& operator=(const context_memory&) = delete;

 void* allocate(std::size_t size, std::size_t align)
 {
 // Since this program is single-threaded there is no need to perform any
 // synchronisation when modifying next_allocation_. Use an atomic or other
 // form of synchronisation when using an exeution context from multiple
 // threads.
 std::size_t space = size + align;
 if (next_allocation_ + space < preallocated_)
 {
 void* ptr = storage_ + next_allocation_;
 next_allocation_ += space;
 return std::align(align, size, ptr, space);
 }
 else
 {
 return ::operator new(size);
 }
 }

 void deallocate(void* ptr)
 {
 auto* ucptr = static_cast<unsigned char*>(ptr);
 if (std::less_equal<unsigned char*>{}(storage_, ucptr)
 && std::less<unsigned char*>{}(ucptr, storage_ + preallocated_))
 {
 // Nothing to do.
 }
 else
 {
 ::operator delete(ptr);
 }
 }

private:
 std::size_t preallocated_;
 std::size_t next_allocation_;
 unsigned char* storage_;
};

// The allocator to be associated with the execution context. This allocatoro
// only needs to satisfy the C++11 minimal allocator requirements.
template <typename T>
class context_allocator
{
public:
 using value_type = T;

 explicit context_allocator(context_memory& mem)
 : memory_(mem)
 {
 }

 template <typename U>
 context_allocator(const context_allocator<U>& other) noexcept
 : memory_(other.memory_)
 {
 }

 bool operator==(const context_allocator& other) const noexcept
 {
 return &memory_ == &other.memory_;
 }

 bool operator!=(const context_allocator& other) const noexcept
 {
 return &memory_ != &other.memory_;
 }

 T* allocate(std::size_t n) const
 {
 return static_cast<T*>(memory_.allocate(sizeof(T) * n, alignof(T)));
 }

 void deallocate(T* p, std::size_t /*n*/) const
 {
 return memory_.deallocate(p);
 }

private:
 template <typename> friend class context_allocator;

 // The underlying memory.
 context_memory& memory_;
};

// 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.
class handler_memory
{
public:
 handler_memory()
 : in_use_(false)
 {
 }

 handler_memory(const handler_memory&) = delete;
 handler_memory& operator=(const handler_memory&) = delete;

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

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

private:
 // Storage space used for handler-based custom memory allocation.
 typename std::aligned_storage<1024>::type storage_;

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

// The allocator to be associated with the handler objects. This allocator only
// needs to satisfy the C++11 minimal allocator requirements.
template <typename T>
class handler_allocator
{
public:
 using value_type = T;

 explicit handler_allocator(handler_memory& mem)
 : memory_(mem)
 {
 }

 template <typename U>
 handler_allocator(const handler_allocator<U>& other) noexcept
 : memory_(other.memory_)
 {
 }

 bool operator==(const handler_allocator& other) const noexcept
 {
 return &memory_ == &other.memory_;
 }

 bool operator!=(const handler_allocator& other) const noexcept
 {
 return &memory_ != &other.memory_;
 }

 T* allocate(std::size_t n) const
 {
 return static_cast<T*>(memory_.allocate(sizeof(T) * n));
 }

 void deallocate(T* p, std::size_t /*n*/) const
 {
 return memory_.deallocate(p);
 }

private:
 template <typename> friend class handler_allocator;

 // The underlying memory.
 handler_memory& memory_;
};

class session
 : public std::enable_shared_from_this<session>
{
public:
 session(tcp::socket socket)
 : socket_(std::move(socket))
 {
 }

 void start()
 {
 do_read();
 }

private:
 void do_read()
 {
 auto self(shared_from_this());
 socket_.async_read_some(boost::asio::buffer(data_),
 boost::asio::bind_allocator(
 handler_allocator<int>(handler_memory_),
 [this, self](boost::system::error_code ec, std::size_t length)
 {
 if (!ec)
 {
 do_write(length);
 }
 }));
 }

 void do_write(std::size_t length)
 {
 auto self(shared_from_this());
 boost::asio::async_write(socket_, boost::asio::buffer(data_, length),
 boost::asio::bind_allocator(
 handler_allocator<int>(handler_memory_),
 [this, self](boost::system::error_code ec, std::size_t /*length*/)
 {
 if (!ec)
 {
 do_read();
 }
 }));
 }

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

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

 // The memory to use for handler-based custom memory allocation.
 handler_memory handler_memory_;
};

class server
{
public:
 server(boost::asio::io_context& io_context, short port)
 : acceptor_(io_context, tcp::endpoint(tcp::v4(), port))
 {
 do_accept();
 }

private:
 void do_accept()
 {
 acceptor_.async_accept(
 [this](boost::system::error_code ec, tcp::socket socket)
 {
 if (!ec)
 {
 std::make_shared<session>(std::move(socket))->start();
 }

 do_accept();
 });
 }

 tcp::acceptor acceptor_;
};

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

 context_memory memory(4096);
 context_allocator<void> allocator(memory);
 boost::asio::io_context io_context(std::allocator_arg, allocator);
 server s(io_context, std::atoi(argv[1]));
 io_context.run();
 }
 catch (std::exception& e)
 {
 std::cerr << "Exception: " << e.what() << "\n";
 }

 return 0;
}