olcPGEX_Network.h

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/*
    ASIO Based Networking olcPixelGameEngine Extension v1.0
 
    Videos: 
    Part #1: https://youtu.be/2hNdkYInj4g
    Part #2: https://youtu.be/UbjxGvrDrbw
    Part #3: https://youtu.be/hHowZ3bWsio
    Part #4: https://youtu.be/f_1lt9pfaEo
 
    License (OLC-3)
    ~~~~~~~~~~~~~~~
 
    Copyright 2018 - 2021 OneLoneCoder.com
 
    Redistribution and use in source and binary forms, with or without
    modification, are permitted provided that the following conditions
    are met:
 
    1. Redistributions or derivations of source code must retain the above
    copyright notice, this list of conditions and the following disclaimer.
 
    2. Redistributions or derivative works in binary form must reproduce
    the above copyright notice. This list of conditions and the following
    disclaimer must be reproduced in the documentation and/or other
    materials provided with the distribution.
 
    3. Neither the name of the copyright holder nor the names of its
    contributors may be used to endorse or promote products derived
    from this software without specific prior written permission.
 
    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
    A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
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    Author
    ~~~~~~
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*/
 
#pragma once 
 
#include <memory>
#include <thread>
#include <mutex>
#include <deque>
#include <optional>
#include <vector>
#include <iostream>
#include <algorithm>
#include <chrono>
#include <cstdint>
 
#ifdef _WIN32
#ifndef _WIN32_WINNT
#define _WIN32_WINNT 0x0A00
#endif
#endif
 
#define _WINSOCK_DEPRECATED_NO_WARNINGS
#define ASIO_STANDALONE
#include <asio.hpp>
#include <asio/ts/buffer.hpp>
#include <asio/ts/internet.hpp>
 
namespace olc
{
    namespace net
    {
        // Message
 
        // Message Header is sent at start of all messages. The template allows us
        // to use "enum class" to ensure that the messages are valid at compile time
        template <typename T>
        struct message_header
        {
            T id{};
            uint32_t size = 0;
        };
 
        // Message Body contains a header and a std::vector, containing raw bytes
        // of infomation. This way the message can be variable length, but the size
        // in the header must be updated.
        template <typename T>
        struct message
        {
            // Header & Body vector
            message_header<T> header{};
            std::vector<uint8_t> body;
 
            // returns size of entire message packet in bytes
            size_t size() const
            {
                return body.size();
            }
 
            // Override for std::cout compatibility - produces friendly description of message
            friend std::ostream& operator << (std::ostream& os, const message<T>& msg)
            {
                os << "ID:" << int(msg.header.id) << " Size:" << msg.header.size;
                return os;
            }
 
            // Convenience Operator overloads - These allow us to add and remove stuff from
            // the body vector as if it were a stack, so First in, Last Out. These are a 
            // template in itself, because we dont know what data type the user is pushing or 
            // popping, so lets allow them all. NOTE: It assumes the data type is fundamentally
            // Plain Old Data (POD). TLDR: Serialise & Deserialise into/from a vector
 
            // Pushes any POD-like data into the message buffer
            template<typename DataType>
            friend message<T>& operator << (message<T>& msg, const DataType& data)
            {
                // Check that the type of the data being pushed is trivially copyable
                static_assert(std::is_standard_layout<DataType>::value, "Data is too complex to be pushed into vector");
 
                // Cache current size of vector, as this will be the point we insert the data
                size_t i = msg.body.size();
 
                // Resize the vector by the size of the data being pushed
                msg.body.resize(msg.body.size() + sizeof(DataType));
 
                // Physically copy the data into the newly allocated vector space
                std::memcpy(msg.body.data() + i, &data, sizeof(DataType));
 
                // Recalculate the message size
                msg.header.size = msg.size();
 
                // Return the target message so it can be "chained"
                return msg;
            }
 
            // Pulls any POD-like data form the message buffer
            template<typename DataType>
            friend message<T>& operator >> (message<T>& msg, DataType& data)
            {
                // Check that the type of the data being pushed is trivially copyable
                static_assert(std::is_standard_layout<DataType>::value, "Data is too complex to be pulled from vector");
 
                // Cache the location towards the end of the vector where the pulled data starts
                size_t i = msg.body.size() - sizeof(DataType);
 
                // Physically copy the data from the vector into the user variable
                std::memcpy(&data, msg.body.data() + i, sizeof(DataType));
 
                // Shrink the vector to remove read bytes, and reset end position
                msg.body.resize(i);
 
                // Recalculate the message size
                msg.header.size = msg.size();
 
                // Return the target message so it can be "chained"
                return msg;
            }           
        };
 
 
        // An "owned" message is identical to a regular message, but it is associated with
        // a connection. On a server, the owner would be the client that sent the message, 
        // on a client the owner would be the server.
 
        // Forward declare the connection
        template <typename T>
        class connection;
 
        template <typename T>
        struct owned_message
        {
            std::shared_ptr<connection<T>> remote = nullptr;
            message<T> msg;
 
            // Again, a friendly string maker
            friend std::ostream& operator<<(std::ostream& os, const owned_message<T>& msg)
            {
                os << msg.msg;
                return os;
            }
        };      
 
        
        // Queue
        template<typename T>
        class tsqueue
        {
        public:
            tsqueue() = default;
            tsqueue(const tsqueue<T>&) = delete;
            virtual ~tsqueue() { clear(); }
 
        public:
            // Returns and maintains item at front of Queue
            const T& front()
            {
                std::scoped_lock lock(muxQueue);
                return deqQueue.front();
            }
 
            // Returns and maintains item at back of Queue
            const T& back()
            {
                std::scoped_lock lock(muxQueue);
                return deqQueue.back();
            }
 
            // Removes and returns item from front of Queue
            T pop_front()
            {
                std::scoped_lock lock(muxQueue);
                auto t = std::move(deqQueue.front());
                deqQueue.pop_front();
                return t;
            }
 
            // Removes and returns item from back of Queue
            T pop_back()
            {
                std::scoped_lock lock(muxQueue);
                auto t = std::move(deqQueue.back());
                deqQueue.pop_back();
                return t;
            }
 
            // Adds an item to back of Queue
            void push_back(const T& item)
            {
                std::scoped_lock lock(muxQueue);
                deqQueue.emplace_back(std::move(item));
 
                std::unique_lock<std::mutex> ul(muxBlocking);
                cvBlocking.notify_one();
            }
 
            // Adds an item to front of Queue
            void push_front(const T& item)
            {
                std::scoped_lock lock(muxQueue);
                deqQueue.emplace_front(std::move(item));
 
                std::unique_lock<std::mutex> ul(muxBlocking);
                cvBlocking.notify_one();
            }
 
            // Returns true if Queue has no items
            bool empty()
            {
                std::scoped_lock lock(muxQueue);
                return deqQueue.empty();
            }
 
            // Returns number of items in Queue
            size_t count()
            {
                std::scoped_lock lock(muxQueue);
                return deqQueue.size();
            }
 
            // Clears Queue
            void clear()
            {
                std::scoped_lock lock(muxQueue);
                deqQueue.clear();
            }
 
            void wait()
            {
                while (empty())
                {
                    std::unique_lock<std::mutex> ul(muxBlocking);
                    cvBlocking.wait(ul);
                }
            }
 
        protected:
            std::mutex muxQueue;
            std::deque<T> deqQueue;
            std::condition_variable cvBlocking;
            std::mutex muxBlocking;
        };
        
        // Connection
        // Forward declare
        template<typename T>
        class server_interface;
 
        template<typename T>
        class connection : public std::enable_shared_from_this<connection<T>>
        {
        public:
            // A connection is "owned" by either a server or a client, and its
            // behaviour is slightly different bewteen the two.
            enum class owner
            {
                server,
                client
            };
 
        public:
            // Constructor: Specify Owner, connect to context, transfer the socket
            //              Provide reference to incoming message queue
            connection(owner parent, asio::io_context& asioContext, asio::ip::tcp::socket socket, tsqueue<owned_message<T>>& qIn)
                : m_asioContext(asioContext), m_socket(std::move(socket)), m_qMessagesIn(qIn)
            {
                m_nOwnerType = parent;
 
                // Construct validation check data
                if (m_nOwnerType == owner::server)
                {
                    // Connection is Server -> Client, construct random data for the client
                    // to transform and send back for validation
                    m_nHandshakeOut = uint64_t(std::chrono::system_clock::now().time_since_epoch().count());
 
                    // Pre-calculate the result for checking when the client responds
                    m_nHandshakeCheck = scramble(m_nHandshakeOut);
                }
                else
                {
                    // Connection is Client -> Server, so we have nothing to define, 
                    m_nHandshakeIn = 0;
                    m_nHandshakeOut = 0;
                }
            }
 
            virtual ~connection()
            {}
 
            // This ID is used system wide - its how clients will understand other clients
            // exist across the whole system.
            uint32_t GetID() const
            {
                return id;
            }
 
        public:
            void ConnectToClient(olc::net::server_interface<T>* server, uint32_t uid = 0)
            {
                if (m_nOwnerType == owner::server)
                {
                    if (m_socket.is_open())
                    {
                        id = uid;
 
                        // Was: ReadHeader();
 
                        // A client has attempted to connect to the server, but we wish
                        // the client to first validate itself, so first write out the
                        // handshake data to be validated
                        WriteValidation();
 
                        // Next, issue a task to sit and wait asynchronously for precisely
                        // the validation data sent back from the client
                        ReadValidation(server);
                    }
                }
            }
 
            void ConnectToServer(const asio::ip::tcp::resolver::results_type& endpoints)
            {
                // Only clients can connect to servers
                if (m_nOwnerType == owner::client)
                {
                    // Request asio attempts to connect to an endpoint
                    asio::async_connect(m_socket, endpoints,
                        [this](std::error_code ec, asio::ip::tcp::endpoint endpoint)
                        {
                            if (!ec)
                            {
                                // Was: ReadHeader();
 
                                // First thing server will do is send packet to be validated
                                // so wait for that and respond
                                ReadValidation();
                            }
                        });
                }
            }
 
 
            void Disconnect()
            {
                if (IsConnected())
                    asio::post(m_asioContext, [this]() { m_socket.close(); });
            }
 
            bool IsConnected() const
            {
                return m_socket.is_open();
            }
 
            // Prime the connection to wait for incoming messages
            void StartListening()
            {
                
            }
 
        public:
            // ASYNC - Send a message, connections are one-to-one so no need to specifiy
            // the target, for a client, the target is the server and vice versa
            void Send(const message<T>& msg)
            {
                asio::post(m_asioContext,
                    [this, msg]()
                    {
                        // If the queue has a message in it, then we must 
                        // assume that it is in the process of asynchronously being written.
                        // Either way add the message to the queue to be output. If no messages
                        // were available to be written, then start the process of writing the
                        // message at the front of the queue.
                        bool bWritingMessage = !m_qMessagesOut.empty();
                        m_qMessagesOut.push_back(msg);
                        if (!bWritingMessage)
                        {
                            WriteHeader();
                        }
                    });
            }
 
 
 
        private:
            // ASYNC - Prime context to write a message header
            void WriteHeader()
            {
                // If this function is called, we know the outgoing message queue must have 
                // at least one message to send. So allocate a transmission buffer to hold
                // the message, and issue the work - asio, send these bytes
                asio::async_write(m_socket, asio::buffer(&m_qMessagesOut.front().header, sizeof(message_header<T>)),
                    [this](std::error_code ec, std::size_t length)
                    {
                        // asio has now sent the bytes - if there was a problem
                        // an error would be available...
                        if (!ec)
                        {
                            // ... no error, so check if the message header just sent also
                            // has a message body...
                            if (m_qMessagesOut.front().body.size() > 0)
                            {
                                // ...it does, so issue the task to write the body bytes
                                WriteBody();
                            }
                            else
                            {
                                // ...it didnt, so we are done with this message. Remove it from 
                                // the outgoing message queue
                                m_qMessagesOut.pop_front();
 
                                // If the queue is not empty, there are more messages to send, so
                                // make this happen by issuing the task to send the next header.
                                if (!m_qMessagesOut.empty())
                                {
                                    WriteHeader();
                                }
                            }
                        }
                        else
                        {
                            // ...asio failed to write the message, we could analyse why but 
                            // for now simply assume the connection has died by closing the
                            // socket. When a future attempt to write to this client fails due
                            // to the closed socket, it will be tidied up.
                            std::cout << "[" << id << "] Write Header Fail.\n";
                            m_socket.close();
                        }
                    });
            }
 
            // ASYNC - Prime context to write a message body
            void WriteBody()
            {
                // If this function is called, a header has just been sent, and that header
                // indicated a body existed for this message. Fill a transmission buffer
                // with the body data, and send it!
                asio::async_write(m_socket, asio::buffer(m_qMessagesOut.front().body.data(), m_qMessagesOut.front().body.size()),
                    [this](std::error_code ec, std::size_t length)
                    {
                        if (!ec)
                        {
                            // Sending was successful, so we are done with the message
                            // and remove it from the queue
                            m_qMessagesOut.pop_front();
 
                            // If the queue still has messages in it, then issue the task to 
                            // send the next messages' header.
                            if (!m_qMessagesOut.empty())
                            {
                                WriteHeader();
                            }
                        }
                        else
                        {
                            // Sending failed, see WriteHeader() equivalent for description :P
                            std::cout << "[" << id << "] Write Body Fail.\n";
                            m_socket.close();
                        }
                    });
            }
 
            // ASYNC - Prime context ready to read a message header
            void ReadHeader()
            {
                // If this function is called, we are expecting asio to wait until it receives
                // enough bytes to form a header of a message. We know the headers are a fixed
                // size, so allocate a transmission buffer large enough to store it. In fact, 
                // we will construct the message in a "temporary" message object as it's 
                // convenient to work with.
                asio::async_read(m_socket, asio::buffer(&m_msgTemporaryIn.header, sizeof(message_header<T>)),
                    [this](std::error_code ec, std::size_t length)
                    {                       
                        if (!ec)
                        {
                            // A complete message header has been read, check if this message
                            // has a body to follow...
                            if (m_msgTemporaryIn.header.size > 0)
                            {
                                // ...it does, so allocate enough space in the messages' body
                                // vector, and issue asio with the task to read the body.
                                m_msgTemporaryIn.body.resize(m_msgTemporaryIn.header.size);
                                ReadBody();
                            }
                            else
                            {
                                // it doesn't, so add this bodyless message to the connections
                                // incoming message queue
                                AddToIncomingMessageQueue();
                            }
                        }
                        else
                        {
                            // Reading form the client went wrong, most likely a disconnect
                            // has occurred. Close the socket and let the system tidy it up later.
                            std::cout << "[" << id << "] Read Header Fail.\n";
                            m_socket.close();
                        }
                    });
            }
 
            // ASYNC - Prime context ready to read a message body
            void ReadBody()
            {
                // If this function is called, a header has already been read, and that header
                // request we read a body, The space for that body has already been allocated
                // in the temporary message object, so just wait for the bytes to arrive...
                asio::async_read(m_socket, asio::buffer(m_msgTemporaryIn.body.data(), m_msgTemporaryIn.body.size()),
                    [this](std::error_code ec, std::size_t length)
                    {                       
                        if (!ec)
                        {
                            // ...and they have! The message is now complete, so add
                            // the whole message to incoming queue
                            AddToIncomingMessageQueue();
                        }
                        else
                        {
                            // As above!
                            std::cout << "[" << id << "] Read Body Fail.\n";
                            m_socket.close();
                        }
                    });
            }
 
            // "Encrypt" data
            uint64_t scramble(uint64_t nInput)
            {
                uint64_t out = nInput ^ 0xDEADBEEFC0DECAFE;
                out = (out & 0xF0F0F0F0F0F0F0) >> 4 | (out & 0x0F0F0F0F0F0F0F) << 4;
                return out ^ 0xC0DEFACE12345678;
            }
 
            // ASYNC - Used by both client and server to write validation packet
            void WriteValidation()
            {
                asio::async_write(m_socket, asio::buffer(&m_nHandshakeOut, sizeof(uint64_t)),
                    [this](std::error_code ec, std::size_t length)
                    {
                        if (!ec)
                        {
                            // Validation data sent, clients should sit and wait
                            // for a response (or a closure)
                            if (m_nOwnerType == owner::client)
                                ReadHeader();
                        }
                        else
                        {
                            m_socket.close();
                        }
                    });
            }
 
            void ReadValidation(olc::net::server_interface<T>* server = nullptr)
            {
                asio::async_read(m_socket, asio::buffer(&m_nHandshakeIn, sizeof(uint64_t)),
                    [this, server](std::error_code ec, std::size_t length)
                    {
                        if (!ec)
                        {
                            if (m_nOwnerType == owner::server)
                            {
                                // Connection is a server, so check response from client
 
                                // Compare sent data to actual solution
                                if (m_nHandshakeIn == m_nHandshakeCheck)
                                {
                                    // Client has provided valid solution, so allow it to connect properly
                                    std::cout << "Client Validated" << std::endl;
                                    server->OnClientValidated(this->shared_from_this());
 
                                    // Sit waiting to receive data now
                                    ReadHeader();
                                }
                                else
                                {
                                    // Client gave incorrect data, so disconnect
                                    std::cout << "Client Disconnected (Fail Validation)" << std::endl;
                                    m_socket.close();
                                }
                            }
                            else
                            {
                                // Connection is a client, so solve puzzle
                                m_nHandshakeOut = scramble(m_nHandshakeIn);
 
                                // Write the result
                                WriteValidation();
                            }
                        }
                        else
                        {
                            // Some biggerfailure occured
                            std::cout << "Client Disconnected (ReadValidation)" << std::endl;
                            m_socket.close();
                        }
                    });
            }
 
            // Once a full message is received, add it to the incoming queue
            void AddToIncomingMessageQueue()
            {               
                // Shove it in queue, converting it to an "owned message", by initialising
                // with the a shared pointer from this connection object
                if(m_nOwnerType == owner::server)
                    m_qMessagesIn.push_back({ this->shared_from_this(), m_msgTemporaryIn });
                else
                    m_qMessagesIn.push_back({ nullptr, m_msgTemporaryIn });
 
                // We must now prime the asio context to receive the next message. It 
                // wil just sit and wait for bytes to arrive, and the message construction
                // process repeats itself. Clever huh?
                ReadHeader();
            }
 
        protected:
            // Each connection has a unique socket to a remote 
            asio::ip::tcp::socket m_socket;
 
            // This context is shared with the whole asio instance
            asio::io_context& m_asioContext;
 
            // This queue holds all messages to be sent to the remote side
            // of this connection
            tsqueue<message<T>> m_qMessagesOut;
 
            // This references the incoming queue of the parent object
            tsqueue<owned_message<T>>& m_qMessagesIn;
 
            // Incoming messages are constructed asynchronously, so we will
            // store the part assembled message here, until it is ready
            message<T> m_msgTemporaryIn;
 
            // The "owner" decides how some of the connection behaves
            owner m_nOwnerType = owner::server;
 
            // Handshake Validation         
            uint64_t m_nHandshakeOut = 0;
            uint64_t m_nHandshakeIn = 0;
            uint64_t m_nHandshakeCheck = 0;
 
 
            bool m_bValidHandshake = false;
            bool m_bConnectionEstablished = false;
 
            uint32_t id = 0;
 
        };
        
        // Client
        template <typename T>
        class client_interface
        {
        public:
            client_interface() 
            {}
 
            virtual ~client_interface()
            {
                // If the client is destroyed, always try and disconnect from server
                Disconnect();
            }
 
        public:
            // Connect to server with hostname/ip-address and port
            bool Connect(const std::string& host, const uint16_t port)
            {
                try
                {
                    // Resolve hostname/ip-address into tangiable physical address
                    asio::ip::tcp::resolver resolver(m_context);
                    asio::ip::tcp::resolver::results_type endpoints = resolver.resolve(host, std::to_string(port));
 
                    // Create connection
                    m_connection = std::make_unique<connection<T>>(connection<T>::owner::client, m_context, asio::ip::tcp::socket(m_context), m_qMessagesIn);
                    
                    // Tell the connection object to connect to server
                    m_connection->ConnectToServer(endpoints);
 
                    // Start Context Thread
                    thrContext = std::thread([this]() { m_context.run(); });
                }
                catch (std::exception& e)
                {
                    std::cerr << "Client Exception: " << e.what() << "\n";
                    return false;
                }
                return true;
            }
 
            // Disconnect from server
            void Disconnect()
            {
                // If connection exists, and it's connected then...
                if(IsConnected())
                {
                    // ...disconnect from server gracefully
                    m_connection->Disconnect();
                }
 
                // Either way, we're also done with the asio context...             
                m_context.stop();
                // ...and its thread
                if (thrContext.joinable())
                    thrContext.join();
 
                // Destroy the connection object
                m_connection.release();
            }
 
            // Check if client is actually connected to a server
            bool IsConnected()
            {
                if (m_connection)
                    return m_connection->IsConnected();
                else
                    return false;
            }
 
        public:
            // Send message to server
            void Send(const message<T>& msg)
            {
                if (IsConnected())
                     m_connection->Send(msg);
            }
 
            // Retrieve queue of messages from server
            tsqueue<owned_message<T>>& Incoming()
            { 
                return m_qMessagesIn;
            }
 
        protected:
            // asio context handles the data transfer...
            asio::io_context m_context;
            // ...but needs a thread of its own to execute its work commands
            std::thread thrContext;
            // The client has a single instance of a "connection" object, which handles data transfer
            std::unique_ptr<connection<T>> m_connection;
            
        private:
            // This is the thread safe queue of incoming messages from server
            tsqueue<owned_message<T>> m_qMessagesIn;
        };
        
        // Server
        template<typename T>
        class server_interface
        {
        public:
            // Create a server, ready to listen on specified port
            server_interface(uint16_t port)
                : m_asioAcceptor(m_asioContext, asio::ip::tcp::endpoint(asio::ip::tcp::v4(), port))
            {
 
            }
 
            virtual ~server_interface()
            {
                // May as well try and tidy up
                Stop();
            }
 
            // Starts the server!
            bool Start()
            {
                try
                {
                    // Issue a task to the asio context - This is important
                    // as it will prime the context with "work", and stop it
                    // from exiting immediately. Since this is a server, we 
                    // want it primed ready to handle clients trying to
                    // connect.
                    WaitForClientConnection();
 
                    // Launch the asio context in its own thread
                    m_threadContext = std::thread([this]() { m_asioContext.run(); });
                }
                catch (std::exception& e)
                {
                    // Something prohibited the server from listening
                    std::cerr << "[SERVER] Exception: " << e.what() << "\n";
                    return false;
                }
 
                std::cout << "[SERVER] Started!\n";
                return true;
            }
 
            // Stops the server!
            void Stop()
            {
                // Request the context to close
                m_asioContext.stop();
 
                // Tidy up the context thread
                if (m_threadContext.joinable()) m_threadContext.join();
 
                // Inform someone, anybody, if they care...
                std::cout << "[SERVER] Stopped!\n";
            }
 
            // ASYNC - Instruct asio to wait for connection
            void WaitForClientConnection()
            {
                // Prime context with an instruction to wait until a socket connects. This
                // is the purpose of an "acceptor" object. It will provide a unique socket
                // for each incoming connection attempt
                m_asioAcceptor.async_accept(
                    [this](std::error_code ec, asio::ip::tcp::socket socket)
                    {
                        // Triggered by incoming connection request
                        if (!ec)
                        {
                            // Display some useful(?) information
                            std::cout << "[SERVER] New Connection: " << socket.remote_endpoint() << "\n";
 
                            // Create a new connection to handle this client 
                            std::shared_ptr<connection<T>> newconn = 
                                std::make_shared<connection<T>>(connection<T>::owner::server, 
                                    m_asioContext, std::move(socket), m_qMessagesIn);
                            
                            
 
                            // Give the user server a chance to deny connection
                            if (OnClientConnect(newconn))
                            {                               
                                // Connection allowed, so add to container of new connections
                                m_deqConnections.push_back(std::move(newconn));
 
                                // And very important! Issue a task to the connection's
                                // asio context to sit and wait for bytes to arrive!
                                m_deqConnections.back()->ConnectToClient(this, nIDCounter++);
 
                                std::cout << "[" << m_deqConnections.back()->GetID() << "] Connection Approved\n";
                            }
                            else
                            {
                                std::cout << "[-----] Connection Denied\n";
 
                                // Connection will go out of scope with no pending tasks, so will
                                // get destroyed automagically due to the wonder of smart pointers
                            }
                        }
                        else
                        {
                            // Error has occurred during acceptance
                            std::cout << "[SERVER] New Connection Error: " << ec.message() << "\n";
                        }
 
                        // Prime the asio context with more work - again simply wait for
                        // another connection...
                        WaitForClientConnection();
                    });
            }
 
            // Send a message to a specific client
            void MessageClient(std::shared_ptr<connection<T>> client, const message<T>& msg)
            {
                // Check client is legitimate...
                if (client && client->IsConnected())
                {
                    // ...and post the message via the connection
                    client->Send(msg);
                }
                else
                {
                    // If we cant communicate with client then we may as 
                    // well remove the client - let the server know, it may
                    // be tracking it somehow
                    OnClientDisconnect(client);
 
                    // Off you go now, bye bye!
                    client.reset();
 
                    // Then physically remove it from the container
                    m_deqConnections.erase(
                        std::remove(m_deqConnections.begin(), m_deqConnections.end(), client), m_deqConnections.end());
                }
            }
            
            // Send message to all clients
            void MessageAllClients(const message<T>& msg, std::shared_ptr<connection<T>> pIgnoreClient = nullptr)
            {
                bool bInvalidClientExists = false;
 
                // Iterate through all clients in container
                for (auto& client : m_deqConnections)
                {
                    // Check client is connected...
                    if (client && client->IsConnected())
                    {
                        // ..it is!
                        if(client != pIgnoreClient)
                            client->Send(msg);
                    }
                    else
                    {
                        // The client couldnt be contacted, so assume it has
                        // disconnected.
                        OnClientDisconnect(client);
                        client.reset();
 
                        // Set this flag to then remove dead clients from container
                        bInvalidClientExists = true;
                    }
                }
 
                // Remove dead clients, all in one go - this way, we dont invalidate the
                // container as we iterated through it.
                if (bInvalidClientExists)
                    m_deqConnections.erase(
                        std::remove(m_deqConnections.begin(), m_deqConnections.end(), nullptr), m_deqConnections.end());
            }
 
            // Force server to respond to incoming messages
            void Update(size_t nMaxMessages = -1, bool bWait = false)
            {
                if (bWait) m_qMessagesIn.wait();
 
                // Process as many messages as you can up to the value
                // specified
                size_t nMessageCount = 0;
                while (nMessageCount < nMaxMessages && !m_qMessagesIn.empty())
                {
                    // Grab the front message
                    auto msg = m_qMessagesIn.pop_front();
 
                    // Pass to message handler
                    OnMessage(msg.remote, msg.msg);
 
                    nMessageCount++;
                }
            }
 
        protected:
            // This server class should override thse functions to implement
            // customised functionality
 
            // Called when a client connects, you can veto the connection by returning false
            virtual bool OnClientConnect(std::shared_ptr<connection<T>> client)
            {
                return false;
            }
 
            // Called when a client appears to have disconnected
            virtual void OnClientDisconnect(std::shared_ptr<connection<T>> client)
            {
 
            }
 
            // Called when a message arrives
            virtual void OnMessage(std::shared_ptr<connection<T>> client, message<T>& msg)
            {
 
            }
 
        public:
            // Called when a client is validated
            virtual void OnClientValidated(std::shared_ptr<connection<T>> client)
            {
 
            }
 
 
        protected:
            // Thread Safe Queue for incoming message packets
            tsqueue<owned_message<T>> m_qMessagesIn;
 
            // Container of active validated connections
            std::deque<std::shared_ptr<connection<T>>> m_deqConnections;
 
            // Order of declaration is important - it is also the order of initialisation
            asio::io_context m_asioContext;
            std::thread m_threadContext;
 
            // These things need an asio context
            asio::ip::tcp::acceptor m_asioAcceptor; // Handles new incoming connection attempts...
 
            // Clients will be identified in the "wider system" via an ID
            uint32_t nIDCounter = 10000;
        };
    }
}