#include <algorithm>
#include <boost/asio/dispatch.hpp>
#include <boost/asio/strand.hpp>
#include <boost/beast/core.hpp>
#include <boost/beast/http.hpp>
#include <boost/beast/version.hpp>
#include <boost/config.hpp>
#include <cstdlib>
#include <functional>
#include <iostream>
#include <memory>
#include <string>
#include <thread>
#include <vector>

namespace beast = boost::beast;   // from <boost/beast.hpp>
namespace http = beast::http;     // from <boost/beast/http.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";
}

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

public:
    // Take ownership of the stream
    session(tcp::socket&& socket,
            std::shared_ptr<std::string const> const& doc_root)
            : stream_(std::move(socket))
            , doc_root_(doc_root) {}

    // Start the asynchronous operation
    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(
                        &session::do_read, shared_from_this()));
    }

    void do_read() {
        // Make the request empty before reading,
        // otherwise the operation behavior is undefined.
        req_ = {};

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

        // Read a request
        http::async_read(stream_, buffer_, req_,
                beast::bind_front_handler(
                        &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");

        // Send the response
        send_response(handle_request(*doc_root_, std::move(req_)));
    }

    void send_response(http::message_generator&& msg) {
        bool keep_alive = msg.keep_alive();

        // Write the response
        beast::async_write(stream_, std::move(msg),
                beast::bind_front_handler(
                        &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();
        }

        // Read another request
        do_read();
    }

    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() { do_accept(); }

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");
            return; // To avoid infinite loop
        } else {
            // Create the session and run it
            std::make_shared<session>(std::move(socket), doc_root_)->run();
        }

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

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