//
// Copyright (c) 2016-2019 Vinnie Falco (vinnie dot falco at gmail 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)
//
// Official repository: https://github.com/boostorg/beast
//

//------------------------------------------------------------------------------
//
// Example: Advanced server
//
//------------------------------------------------------------------------------

#include <boost/beast/core.hpp>
#include <boost/beast/http.hpp>
#include <boost/beast/websocket.hpp>
#include <boost/beast/version.hpp>
#include <boost/asio/bind_executor.hpp>
#include <boost/asio/dispatch.hpp>
#include <boost/asio/signal_set.hpp>
#include <boost/asio/strand.hpp>
#include <boost/make_unique.hpp>
#include <boost/optional.hpp>
#include <algorithm>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <memory>
#include <queue>
#include <string>
#include <thread>
#include <vector>

namespace beast = boost::beast;                 // from <boost/beast.hpp>
namespace http = beast::http;                   // from <boost/beast/http.hpp>
namespace websocket = beast::websocket;         // from <boost/beast/websocket.hpp>
namespace net = boost::asio;                    // from <boost/asio.hpp>
using tcp = boost::asio::ip::tcp;               // from <boost/asio/ip/tcp.hpp>

// Return a reasonable mime type based on the extension of a file.
beast::string_view
mime_type(beast::string_view path)
{
    using beast::iequals;
    auto const ext = [&path]
    {
        auto const pos = path.rfind(".");
        if(pos == beast::string_view::npos)
            return beast::string_view{};
        return path.substr(pos);
    }();
    if(iequals(ext, ".htm"))  return "text/html";
    if(iequals(ext, ".html")) return "text/html";
    if(iequals(ext, ".php"))  return "text/html";
    if(iequals(ext, ".css"))  return "text/css";
    if(iequals(ext, ".txt"))  return "text/plain";
    if(iequals(ext, ".js"))   return "application/javascript";
    if(iequals(ext, ".json")) return "application/json";
    if(iequals(ext, ".xml"))  return "application/xml";
    if(iequals(ext, ".swf"))  return "application/x-shockwave-flash";
    if(iequals(ext, ".flv"))  return "video/x-flv";
    if(iequals(ext, ".png"))  return "image/png";
    if(iequals(ext, ".jpe"))  return "image/jpeg";
    if(iequals(ext, ".jpeg")) return "image/jpeg";
    if(iequals(ext, ".jpg"))  return "image/jpeg";
    if(iequals(ext, ".gif"))  return "image/gif";
    if(iequals(ext, ".bmp"))  return "image/bmp";
    if(iequals(ext, ".ico"))  return "image/vnd.microsoft.icon";
    if(iequals(ext, ".tiff")) return "image/tiff";
    if(iequals(ext, ".tif"))  return "image/tiff";
    if(iequals(ext, ".svg"))  return "image/svg+xml";
    if(iequals(ext, ".svgz")) return "image/svg+xml";
    return "application/text";
}

// Append an HTTP rel-path to a local filesystem path.
// The returned path is normalized for the platform.
std::string
path_cat(
    beast::string_view base,
    beast::string_view path)
{
    if(base.empty())
        return std::string(path);
    std::string result(base);
#ifdef BOOST_MSVC
    char constexpr path_separator = '\\';
    if(result.back() == path_separator)
        result.resize(result.size() - 1);
    result.append(path.data(), path.size());
    for(auto& c : result)
        if(c == '/')
            c = path_separator;
#else
    char constexpr path_separator = '/';
    if(result.back() == path_separator)
        result.resize(result.size() - 1);
    result.append(path.data(), path.size());
#endif
    return result;
}

// Return a response for the given request.
//
// The concrete type of the response message (which depends on the
// request), is type-erased in message_generator.
template <class Body, class Allocator>
http::message_generator
handle_request(
    beast::string_view doc_root,
    http::request<Body, http::basic_fields<Allocator>>&& req)
{
    // Returns a bad request response
    auto const bad_request =
    [&req](beast::string_view why)
    {
        http::response<http::string_body> res{http::status::bad_request, req.version()};
        res.set(http::field::server, BOOST_BEAST_VERSION_STRING);
        res.set(http::field::content_type, "text/html");
        res.keep_alive(req.keep_alive());
        res.body() = std::string(why);
        res.prepare_payload();
        return res;
    };

    // Returns a not found response
    auto const not_found =
    [&req](beast::string_view target)
    {
        http::response<http::string_body> res{http::status::not_found, req.version()};
        res.set(http::field::server, BOOST_BEAST_VERSION_STRING);
        res.set(http::field::content_type, "text/html");
        res.keep_alive(req.keep_alive());
        res.body() = "The resource '" + std::string(target) + "' was not found.";
        res.prepare_payload();
        return res;
    };

    // Returns a server error response
    auto const server_error =
    [&req](beast::string_view what)
    {
        http::response<http::string_body> res{http::status::internal_server_error, req.version()};
        res.set(http::field::server, BOOST_BEAST_VERSION_STRING);
        res.set(http::field::content_type, "text/html");
        res.keep_alive(req.keep_alive());
        res.body() = "An error occurred: '" + std::string(what) + "'";
        res.prepare_payload();
        return res;
    };

    // Make sure we can handle the method
    if( req.method() != http::verb::get &&
        req.method() != http::verb::head)
        return bad_request("Unknown HTTP-method");

    // Request path must be absolute and not contain "..".
    if( req.target().empty() ||
        req.target()[0] != '/' ||
        req.target().find("..") != beast::string_view::npos)
        return bad_request("Illegal request-target");

    // Build the path to the requested file
    std::string path = path_cat(doc_root, req.target());
    if(req.target().back() == '/')
        path.append("index.html");

    // Attempt to open the file
    beast::error_code ec;
    http::file_body::value_type body;
    body.open(path.c_str(), beast::file_mode::scan, ec);

    // Handle the case where the file doesn't exist
    if(ec == beast::errc::no_such_file_or_directory)
        return not_found(req.target());

    // Handle an unknown error
    if(ec)
        return server_error(ec.message());

    // Cache the size since we need it after the move
    auto const size = body.size();

    // Respond to HEAD request
    if(req.method() == http::verb::head)
    {
        http::response<http::empty_body> res{http::status::ok, req.version()};
        res.set(http::field::server, BOOST_BEAST_VERSION_STRING);
        res.set(http::field::content_type, mime_type(path));
        res.content_length(size);
        res.keep_alive(req.keep_alive());
        return res;
    }

    // Respond to GET request
    http::response<http::file_body> res{
        std::piecewise_construct,
        std::make_tuple(std::move(body)),
        std::make_tuple(http::status::ok, req.version())};
    res.set(http::field::server, BOOST_BEAST_VERSION_STRING);
    res.set(http::field::content_type, mime_type(path));
    res.content_length(size);
    res.keep_alive(req.keep_alive());
    return res;
}

//------------------------------------------------------------------------------

// Report a failure
void
fail(beast::error_code ec, char const* what)
{
    std::cerr << what << ": " << ec.message() << "\n";
}

// Echoes back all received WebSocket messages
class websocket_session : public std::enable_shared_from_this<websocket_session>
{
    websocket::stream<beast::tcp_stream> ws_;
    beast::flat_buffer buffer_;

public:
    // Take ownership of the socket
    explicit
    websocket_session(tcp::socket&& socket)
        : ws_(std::move(socket))
    {
    }

    // Start the asynchronous accept operation
    template<class Body, class Allocator>
    void
    do_accept(http::request<Body, http::basic_fields<Allocator>> req)
    {
        // Set suggested timeout settings for the websocket
        ws_.set_option(
            websocket::stream_base::timeout::suggested(
                beast::role_type::server));

        // Set a decorator to change the Server of the handshake
        ws_.set_option(websocket::stream_base::decorator(
            [](websocket::response_type& res)
            {
                res.set(http::field::server,
                    std::string(BOOST_BEAST_VERSION_STRING) +
                        " advanced-server");
            }));

        // Accept the websocket handshake
        ws_.async_accept(
            req,
            beast::bind_front_handler(
                &websocket_session::on_accept,
                shared_from_this()));
    }

private:
    void
    on_accept(beast::error_code ec)
    {
        if(ec)
            return fail(ec, "accept");

        // Read a message
        do_read();
    }

    void
    do_read()
    {
        // Read a message into our buffer
        ws_.async_read(
            buffer_,
            beast::bind_front_handler(
                &websocket_session::on_read,
                shared_from_this()));
    }

    void
    on_read(
        beast::error_code ec,
        std::size_t bytes_transferred)
    {
        boost::ignore_unused(bytes_transferred);

        // This indicates that the websocket_session was closed
        if(ec == websocket::error::closed)
            return;

        if(ec)
            return fail(ec, "read");

        // Echo the message
        ws_.text(ws_.got_text());
        ws_.async_write(
            buffer_.data(),
            beast::bind_front_handler(
                &websocket_session::on_write,
                shared_from_this()));
    }

    void
    on_write(
        beast::error_code ec,
        std::size_t bytes_transferred)
    {
        boost::ignore_unused(bytes_transferred);

        if(ec)
            return fail(ec, "write");

        // Clear the buffer
        buffer_.consume(buffer_.size());

        // Do another read
        do_read();
    }
};

//------------------------------------------------------------------------------

// Handles an HTTP server connection
class http_session : public std::enable_shared_from_this<http_session>
{
    beast::tcp_stream stream_;
    beast::flat_buffer buffer_;
    std::shared_ptr<std::string const> doc_root_;

    static constexpr std::size_t queue_limit = 8; // max responses
    std::queue<http::message_generator> response_queue_;

    // The parser is stored in an optional container so we can
    // construct it from scratch it at the beginning of each new message.
    boost::optional<http::request_parser<http::string_body>> parser_;

public:
    // Take ownership of the socket
    http_session(
        tcp::socket&& socket,
        std::shared_ptr<std::string const> const& doc_root)
        : stream_(std::move(socket))
        , doc_root_(doc_root)
    {
        static_assert(queue_limit > 0,
                      "queue limit must be positive");
    }

    // Start the session
    void
    run()
    {
        // We need to be executing within a strand to perform async operations
        // on the I/O objects in this session. Although not strictly necessary
        // for single-threaded contexts, this example code is written to be
        // thread-safe by default.
        net::dispatch(
            stream_.get_executor(),
            beast::bind_front_handler(
                &http_session::do_read,
                this->shared_from_this()));
    }

private:
    void
    do_read()
    {
        // Construct a new parser for each message
        parser_.emplace();

        // Apply a reasonable limit to the allowed size
        // of the body in bytes to prevent abuse.
        parser_->body_limit(10000);

        // Set the timeout.
        stream_.expires_after(std::chrono::seconds(30));

        // Read a request using the parser-oriented interface
        http::async_read(
            stream_,
            buffer_,
            *parser_,
            beast::bind_front_handler(
                &http_session::on_read,
                shared_from_this()));
    }

    void
    on_read(beast::error_code ec, std::size_t bytes_transferred)
    {
        boost::ignore_unused(bytes_transferred);

        // This means they closed the connection
        if(ec == http::error::end_of_stream)
            return do_close();

        if(ec)
            return fail(ec, "read");

        // See if it is a WebSocket Upgrade
        if(websocket::is_upgrade(parser_->get()))
        {
            // Create a websocket session, transferring ownership
            // of both the socket and the HTTP request.
            std::make_shared<websocket_session>(
                stream_.release_socket())->do_accept(parser_->release());
            return;
        }

        // Send the response
        queue_write(handle_request(*doc_root_, parser_->release()));

        // If we aren't at the queue limit, try to pipeline another request
        if (response_queue_.size() < queue_limit)
            do_read();
    }

    void
    queue_write(http::message_generator response)
    {
        // Allocate and store the work
        response_queue_.push(std::move(response));

        // If there was no previous work, start the write loop
        if (response_queue_.size() == 1)
            do_write();
    }

    // Called to start/continue the write-loop. Should not be called when
    // write_loop is already active.
    void
    do_write()
    {
        if(! response_queue_.empty())
        {
            bool keep_alive = response_queue_.front().keep_alive();

            beast::async_write(
                stream_,
                std::move(response_queue_.front()),
                beast::bind_front_handler(
                    &http_session::on_write,
                    shared_from_this(),
                    keep_alive));
        }
    }

    void
    on_write(
        bool keep_alive,
        beast::error_code ec,
        std::size_t bytes_transferred)
    {
        boost::ignore_unused(bytes_transferred);

        if(ec)
            return fail(ec, "write");

        if(! keep_alive)
        {
            // This means we should close the connection, usually because
            // the response indicated the "Connection: close" semantic.
            return do_close();
        }

        // Resume the read if it has been paused
        if(response_queue_.size() == queue_limit)
            do_read();

        response_queue_.pop();

        do_write();
    }

    void
    do_close()
    {
        // Send a TCP shutdown
        beast::error_code ec;
        stream_.socket().shutdown(tcp::socket::shutdown_send, ec);

        // At this point the connection is closed gracefully
    }
};

//------------------------------------------------------------------------------

// Accepts incoming connections and launches the sessions
class listener : public std::enable_shared_from_this<listener>
{
    net::io_context& ioc_;
    tcp::acceptor acceptor_;
    std::shared_ptr<std::string const> doc_root_;

public:
    listener(
        net::io_context& ioc,
        tcp::endpoint endpoint,
        std::shared_ptr<std::string const> const& doc_root)
        : ioc_(ioc)
        , acceptor_(net::make_strand(ioc))
        , doc_root_(doc_root)
    {
        beast::error_code ec;

        // Open the acceptor
        acceptor_.open(endpoint.protocol(), ec);
        if(ec)
        {
            fail(ec, "open");
            return;
        }

        // Allow address reuse
        acceptor_.set_option(net::socket_base::reuse_address(true), ec);
        if(ec)
        {
            fail(ec, "set_option");
            return;
        }

        // Bind to the server address
        acceptor_.bind(endpoint, ec);
        if(ec)
        {
            fail(ec, "bind");
            return;
        }

        // Start listening for connections
        acceptor_.listen(
            net::socket_base::max_listen_connections, ec);
        if(ec)
        {
            fail(ec, "listen");
            return;
        }
    }

    // Start accepting incoming connections
    void
    run()
    {
        // We need to be executing within a strand to perform async operations
        // on the I/O objects in this session. Although not strictly necessary
        // for single-threaded contexts, this example code is written to be
        // thread-safe by default.
        net::dispatch(
            acceptor_.get_executor(),
            beast::bind_front_handler(
                &listener::do_accept,
                this->shared_from_this()));
    }

private:
    void
    do_accept()
    {
        // The new connection gets its own strand
        acceptor_.async_accept(
            net::make_strand(ioc_),
            beast::bind_front_handler(
                &listener::on_accept,
                shared_from_this()));
    }

    void
    on_accept(beast::error_code ec, tcp::socket socket)
    {
        if(ec)
        {
            fail(ec, "accept");
        }
        else
        {
            // Create the http session and run it
            std::make_shared<http_session>(
                std::move(socket),
                doc_root_)->run();
        }

        // Accept another connection
        do_accept();
    }
};

//------------------------------------------------------------------------------

int main(int argc, char* argv[])
{
    // Check command line arguments.
    if (argc != 5)
    {
        std::cerr <<
            "Usage: advanced-server <address> <port> <doc_root> <threads>\n" <<
            "Example:\n" <<
            "    advanced-server 0.0.0.0 8080 . 1\n";
        return EXIT_FAILURE;
    }
    auto const address = net::ip::make_address(argv[1]);
    auto const port = static_cast<unsigned short>(std::atoi(argv[2]));
    auto const doc_root = std::make_shared<std::string>(argv[3]);
    auto const threads = std::max<int>(1, std::atoi(argv[4]));

    // The io_context is required for all I/O
    net::io_context ioc{threads};

    // Create and launch a listening port
    std::make_shared<listener>(
        ioc,
        tcp::endpoint{address, port},
        doc_root)->run();

    // Capture SIGINT and SIGTERM to perform a clean shutdown
    net::signal_set signals(ioc, SIGINT, SIGTERM);
    signals.async_wait(
        [&](beast::error_code const&, int)
        {
            // Stop the `io_context`. This will cause `run()`
            // to return immediately, eventually destroying the
            // `io_context` and all of the sockets in it.
            ioc.stop();
        });

    // Run the I/O service on the requested number of threads
    std::vector<std::thread> v;
    v.reserve(threads - 1);
    for(auto i = threads - 1; i > 0; --i)
        v.emplace_back(
        [&ioc]
        {
            ioc.run();
        });
    ioc.run();

    // (If we get here, it means we got a SIGINT or SIGTERM)

    // Block until all the threads exit
    for(auto& t : v)
        t.join();

    return EXIT_SUCCESS;
}