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

#include "./helpers/helpers.h"

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>
using namespace uad;

// Return a reasonable mime type based on the extension of a file.


// 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;
}

// This function produces an HTTP response for the given
// request. The type of the response object depends on the
// contents of the request, so the interface requires the
// caller to pass a generic lambda for receiving the response.
template <class Body, class Allocator, class Send>
void handle_request(beast::string_view doc_root,
                    http::request<Body, http::basic_fields<Allocator>> &&req,
                    Send &&send) {
  // 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 send(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 send(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 send(not_found(req.target()));

  // Handle an unknown error
  if (ec)
    return send(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, MimeType(path));
    res.content_length(size);
    res.keep_alive(req.keep_alive());
    return send(std::move(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, MimeType(path));
  res.content_length(size);
  res.keep_alive(req.keep_alive());
  return send(std::move(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)
      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> {
  // This queue is used for HTTP pipelining.
  class queue {
    enum {
      // Maximum number of responses we will queue
      limit = 8
    };

    // The type-erased, saved work item
    struct work {
      virtual ~work() = default;
      virtual void operator()() = 0;
    };

    http_session &self_;
    std::vector<std::unique_ptr<work>> items_;

  public:
    explicit queue(http_session &self) : self_(self) {
      static_assert(limit > 0, "queue limit must be positive");
      items_.reserve(limit);
    }

    // Returns `true` if we have reached the queue limit
    bool is_full() const { return items_.size() >= limit; }

    // Called when a message finishes sending
    // Returns `true` if the caller should initiate a read
    bool on_write() {
      BOOST_ASSERT(!items_.empty());
      auto const was_full = is_full();
      items_.erase(items_.begin());
      if (!items_.empty())
        (*items_.front())();
      return was_full;
    }

    // Called by the HTTP handler to send a response.
    template <bool isRequest, class Body, class Fields>
    void operator()(http::message<isRequest, Body, Fields> &&msg) {
      // This holds a work item
      struct work_impl : work {
        http_session &self_;
        http::message<isRequest, Body, Fields> msg_;

        work_impl(http_session &self,
                  http::message<isRequest, Body, Fields> &&msg)
            : self_(self), msg_(std::move(msg)) {}

        void operator()() {
          http::async_write(self_.stream_, msg_,
                            beast::bind_front_handler(&http_session::on_write,
                                                      self_.shared_from_this(),
                                                      msg_.need_eof()));
        }
      };

      // Allocate and store the work
      items_.push_back(boost::make_unique<work_impl>(self_, std::move(msg)));

      // If there was no previous work, start this one
      if (items_.size() == 1)
        (*items_.front())();
    }
  };

  beast::tcp_stream stream_;
  beast::flat_buffer buffer_;
  std::shared_ptr<std::string const> doc_root_;
  queue 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), queue_(*this) {}

  // 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
    handle_request(*doc_root_, parser_->release(), queue_);

    // If we aren't at the queue limit, try to pipeline another request
    if (!queue_.is_full())
      do_read();
  }

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

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

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

    // Inform the queue that a write completed
    if (queue_.on_write()) {
      // 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() {
    // 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;
}