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    use crate::codec::{RecvError, UserError};
    use crate::frame::{Reason, StreamId};
    use crate::{client, frame, proto, server};

    use crate::frame::DEFAULT_INITIAL_WINDOW_SIZE;
    use crate::proto::*;

    use bytes::{Buf, Bytes};
    use futures_core::Stream;
    use std::io;
    use std::marker::PhantomData;
    use std::pin::Pin;
    use std::task::{Context, Poll};
    use std::time::Duration;
    use tokio::io::{AsyncRead, AsyncWrite};

    /// An H2 connection
    #[derive(Debug)]
    pub(crate) struct Connection<T, P, B: Buf = Bytes>
    where
        P: Peer,
    {
        /// Read / write frame values
        codec: Codec<T, Prioritized<B>>,

        inner: ConnectionInner<P, B>,
    }

    // Extracted part of `Connection` which does not depend on `T`. Reduces the amount of duplicated
    // method instantiations.
    #[derive(Debug)]
    struct ConnectionInner<P, B: Buf = Bytes>
    where
        P: Peer,
    {
        /// Tracks the connection level state transitions.
        state: State,

        /// An error to report back once complete.
        ///
        /// This exists separately from State in order to support
        /// graceful shutdown.
        error: Option<Reason>,

        /// Pending GOAWAY frames to write.
        go_away: GoAway,

        /// Ping/pong handler
        ping_pong: PingPong,

        /// Connection settings
        settings: Settings,

        /// Stream state handler
        streams: Streams<B, P>,

        /// A `tracing` span tracking the lifetime of the connection.
        span: tracing::Span,

        /// Client or server
        _phantom: PhantomData<P>,
    }

    struct DynConnection<'a, B: Buf = Bytes> {
        state: &'a mut State,

        go_away: &'a mut GoAway,

        streams: DynStreams<'a, B>,

        error: &'a mut Option<Reason>,

        ping_pong: &'a mut PingPong,
    }

    #[derive(Debug, Clone)]
    pub(crate) struct Config {
        pub next_stream_id: StreamId,
        pub initial_max_send_streams: usize,
        pub reset_stream_duration: Duration,
        pub reset_stream_max: usize,
        pub settings: frame::Settings,
    }

    #[derive(Debug)]
    enum State {
        /// Currently open in a sane state
        Open,

        /// The codec must be flushed
        Closing(Reason),

        /// In a closed state
        Closed(Reason),
    }

    impl<T, P, B> Connection<T, P, B>
    where
        T: AsyncRead + AsyncWrite + Unpin,
        P: Peer,
        B: Buf,
    {
        pub fn new(codec: Codec<T, Prioritized<B>>, config: Config) -> Connection<T, P, B> {
            fn streams_config(config: &Config) -> streams::Config {
                streams::Config {
                    local_init_window_sz: config
                        .settings
                        .initial_window_size()
                        .unwrap_or(DEFAULT_INITIAL_WINDOW_SIZE),
                    initial_max_send_streams: config.initial_max_send_streams,
                    local_next_stream_id: config.next_stream_id,
                    local_push_enabled: config.settings.is_push_enabled().unwrap_or(true),
                    local_reset_duration: config.reset_stream_duration,
                    local_reset_max: config.reset_stream_max,
                    remote_init_window_sz: DEFAULT_INITIAL_WINDOW_SIZE,
                    remote_max_initiated: config
                        .settings
                        .max_concurrent_streams()
                        .map(|max| max as usize),
                }
            }
            let streams = Streams::new(streams_config(&config));
            Connection {
                codec,
                inner: ConnectionInner {
                    state: State::Open,
                    error: None,
                    go_away: GoAway::new(),
                    ping_pong: PingPong::new(),
                    settings: Settings::new(config.settings),
                    streams,
                    span: tracing::debug_span!("Connection", peer = %P::NAME),
                    _phantom: PhantomData,
                },
            }
        }

        /// connection flow control
        pub(crate) fn set_target_window_size(&mut self, size: WindowSize) {
            self.inner.streams.set_target_connection_window_size(size);
        }

        /// Send a new SETTINGS frame with an updated initial window size.
        pub(crate) fn set_initial_window_size(&mut self, size: WindowSize) -> Result<(), UserError> {
            let mut settings = frame::Settings::default();
            settings.set_initial_window_size(Some(size));
            self.inner.settings.send_settings(settings)
        }

        /// Returns the maximum number of concurrent streams that may be initiated
        /// by this peer.
        pub(crate) fn max_send_streams(&self) -> usize {
            self.inner.streams.max_send_streams()
        }

        /// Returns the maximum number of concurrent streams that may be initiated
        /// by the remote peer.
        pub(crate) fn max_recv_streams(&self) -> usize {
            self.inner.streams.max_recv_streams()
        }

        /// Returns `Ready` when the connection is ready to receive a frame.
        ///
        /// Returns `RecvError` as this may raise errors that are caused by delayed
        /// processing of received frames.
        fn poll_ready(&mut self, cx: &mut Context) -> Poll<Result<(), RecvError>> {
            let _e = self.inner.span.enter();
            let span = tracing::trace_span!("poll_ready");
            let _e = span.enter();
            // The order of these calls don't really matter too much
            ready!(self.inner.ping_pong.send_pending_pong(cx, &mut self.codec))?;
            ready!(self.inner.ping_pong.send_pending_ping(cx, &mut self.codec))?;
            ready!(self
                .inner
                .settings
                .poll_send(cx, &mut self.codec, &mut self.inner.streams))?;
            ready!(self.inner.streams.send_pending_refusal(cx, &mut self.codec))?;

            Poll::Ready(Ok(()))
        }

        /// Send any pending GOAWAY frames.
        ///
        /// This will return `Some(reason)` if the connection should be closed
        /// afterwards. If this is a graceful shutdown, this returns `None`.
        fn poll_go_away(&mut self, cx: &mut Context) -> Poll<Option<io::Result<Reason>>> {
            self.inner.go_away.send_pending_go_away(cx, &mut self.codec)
        }

        pub fn go_away_from_user(&mut self, e: Reason) {
            self.inner.as_dyn().go_away_from_user(e)
        }

        fn take_error(&mut self, ours: Reason) -> Poll<Result<(), proto::Error>> {
            let reason = if let Some(theirs) = self.inner.error.take() {
                match (ours, theirs) {
                    // If either side reported an error, return that
                    // to the user.
                    (Reason::NO_ERROR, err) | (err, Reason::NO_ERROR) => err,
                    // If both sides reported an error, give their
                    // error back to th user. We assume our error
                    // was a consequence of their error, and less
                    // important.
                    (_, theirs) => theirs,
                }
            } else {
                ours
            };

            if reason == Reason::NO_ERROR {
                Poll::Ready(Ok(()))
            } else {
                Poll::Ready(Err(proto::Error::Proto(reason)))
            }
        }

        /// Closes the connection by transitioning to a GOAWAY state
        /// iff there are no streams or references
        pub fn maybe_close_connection_if_no_streams(&mut self) {
            // If we poll() and realize that there are no streams or references
            // then we can close the connection by transitioning to GOAWAY
            if !self.inner.streams.has_streams_or_other_references() {
                self.inner.as_dyn().go_away_now(Reason::NO_ERROR);
            }
        }

        pub(crate) fn take_user_pings(&mut self) -> Option<UserPings> {
            self.inner.ping_pong.take_user_pings()
        }

        /// Advances the internal state of the connection.
        pub fn poll(&mut self, cx: &mut Context) -> Poll<Result<(), proto::Error>> {
            // XXX(eliza): cloning the span is unfortunately necessary here in
            // order to placate the borrow checker — `self` is mutably borrowed by
            // `poll2`, which means that we can't borrow `self.span` to enter it.
            // The clone is just an atomic ref bump.
            let span = self.inner.span.clone();
            let _e = span.enter();
            let span = tracing::trace_span!("poll");
            let _e = span.enter();

            loop {
                tracing::trace!(connection.state = ?self.inner.state);
                // TODO: probably clean up this glob of code
                match self.inner.state {
                    // When open, continue to poll a frame
                    State::Open => {
                        let result = match self.poll2(cx) {
                            Poll::Ready(result) => result,
                            // The connection is not ready to make progress
                            Poll::Pending => {
                                // Ensure all window updates have been sent.
                                //
                                // This will also handle flushing `self.codec`
                                ready!(self.inner.streams.poll_complete(cx, &mut self.codec))?;

                                if (self.inner.error.is_some()
                                    || self.inner.go_away.should_close_on_idle())
                                    && !self.inner.streams.has_streams()
                                {
                                    self.inner.as_dyn().go_away_now(Reason::NO_ERROR);
                                    continue;
                                }

                                return Poll::Pending;
                            }
                        };

                        self.inner.as_dyn().handle_poll2_result(result)?
                    }
                    State::Closing(reason) => {
                        tracing::trace!("connection closing after flush");
                        // Flush/shutdown the codec
                        ready!(self.codec.shutdown(cx))?;

                        // Transition the state to error
                        self.inner.state = State::Closed(reason);
                    }
                    State::Closed(reason) => return self.take_error(reason),
                }
            }
        }

        fn poll2(&mut self, cx: &mut Context) -> Poll<Result<(), RecvError>> {
            // This happens outside of the loop to prevent needing to do a clock
            // check and then comparison of the queue possibly multiple times a
            // second (and thus, the clock wouldn't have changed enough to matter).
            self.clear_expired_reset_streams();

            loop {
                // First, ensure that the `Connection` is able to receive a frame
                //
                // The order here matters:
                // - poll_go_away may buffer a graceful shutdown GOAWAY frame
                // - If it has, we've also added a PING to be sent in poll_ready
                if let Some(reason) = ready!(self.poll_go_away(cx)?) {
                    if self.inner.go_away.should_close_now() {
                        if self.inner.go_away.is_user_initiated() {
                            // A user initiated abrupt shutdown shouldn't return
                            // the same error back to the user.
                            return Poll::Ready(Ok(()));
                        } else {
                            return Poll::Ready(Err(RecvError::Connection(reason)));
                        }
                    }
                    // Only NO_ERROR should be waiting for idle
                    debug_assert_eq!(
                        reason,
                        Reason::NO_ERROR,
                        "graceful GOAWAY should be NO_ERROR"
                    );
                }
                ready!(self.poll_ready(cx))?;

                match self
                    .inner
                    .as_dyn()
                    .recv_frame(ready!(Pin::new(&mut self.codec).poll_next(cx)?))?
                {
                    ReceivedFrame::Settings(frame) => {
                        self.inner.settings.recv_settings(
                            frame,
                            &mut self.codec,
                            &mut self.inner.streams,
                        )?;
                    }
                    ReceivedFrame::Continue => (),
                    ReceivedFrame::Done => {
                        return Poll::Ready(Ok(()));
                    }
                }
            }
        }

        fn clear_expired_reset_streams(&mut self) {
            self.inner.streams.clear_expired_reset_streams();
        }
    }

    impl<P, B> ConnectionInner<P, B>
    where
        P: Peer,
        B: Buf,
    {
        fn as_dyn(&mut self) -> DynConnection<'_, B> {
            let ConnectionInner {
                state,
                go_away,
                streams,
                error,
                ping_pong,
                ..
            } = self;
            let streams = streams.as_dyn();
            DynConnection {
                state,
                go_away,
                streams,
                error,
                ping_pong,
            }
        }
    }

    impl<B> DynConnection<'_, B>
    where
        B: Buf,
    {
        fn go_away(&mut self, id: StreamId, e: Reason) {
            let frame = frame::GoAway::new(id, e);
            self.streams.send_go_away(id);
            self.go_away.go_away(frame);
        }

        fn go_away_now(&mut self, e: Reason) {
            let last_processed_id = self.streams.last_processed_id();
            let frame = frame::GoAway::new(last_processed_id, e);
            self.go_away.go_away_now(frame);
        }

        fn go_away_from_user(&mut self, e: Reason) {
            let last_processed_id = self.streams.last_processed_id();
            let frame = frame::GoAway::new(last_processed_id, e);
            self.go_away.go_away_from_user(frame);

            // Notify all streams of reason we're abruptly closing.
            self.streams.recv_err(&proto::Error::Proto(e));
        }

        fn handle_poll2_result(&mut self, result: Result<(), RecvError>) -> Result<(), Error> {
            use crate::codec::RecvError::*;
            match result {
                // The connection has shutdown normally
                Ok(()) => {
                    *self.state = State::Closing(Reason::NO_ERROR);
                    Ok(())
                }
                // Attempting to read a frame resulted in a connection level
                // error. This is handled by setting a GOAWAY frame followed by
                // terminating the connection.
                Err(Connection(e)) => {
                    tracing::debug!(error = ?e, "Connection::poll; connection error");

                    // We may have already sent a GOAWAY for this error,
                    // if so, don't send another, just flush and close up.
                    if let Some(reason) = self.go_away.going_away_reason() {
                        if reason == e {
                            tracing::trace!("    -> already going away");
                            *self.state = State::Closing(e);
                            return Ok(());
                        }
                    }

                    // Reset all active streams
                    self.streams.recv_err(&e.into());
                    self.go_away_now(e);
                    Ok(())
                }
                // Attempting to read a frame resulted in a stream level error.
                // This is handled by resetting the frame then trying to read
                // another frame.
                Err(Stream { id, reason }) => {
                    tracing::trace!(?id, ?reason, "stream error");
                    self.streams.send_reset(id, reason);
                    Ok(())
                }
                // Attempting to read a frame resulted in an I/O error. All
                // active streams must be reset.
                //
                // TODO: Are I/O errors recoverable?
                Err(Io(e)) => {
                    tracing::debug!(error = ?e, "Connection::poll; IO error");
                    let e = e.into();

                    // Reset all active streams
                    self.streams.recv_err(&e);

                    // Return the error
                    Err(e)
                }
            }
        }

        fn recv_frame(&mut self, frame: Option<Frame>) -> Result<ReceivedFrame, RecvError> {
            use crate::frame::Frame::*;
            match frame {
                Some(Headers(frame)) => {
                    tracing::trace!(?frame, "recv HEADERS");
                    self.streams.recv_headers(frame)?;
                }
                Some(Data(frame)) => {
                    tracing::trace!(?frame, "recv DATA");
                    self.streams.recv_data(frame)?;
                }
                Some(Reset(frame)) => {
                    tracing::trace!(?frame, "recv RST_STREAM");
                    self.streams.recv_reset(frame)?;
                }
                Some(PushPromise(frame)) => {
                    tracing::trace!(?frame, "recv PUSH_PROMISE");
                    self.streams.recv_push_promise(frame)?;
                }
                Some(Settings(frame)) => {
                    tracing::trace!(?frame, "recv SETTINGS");
                    return Ok(ReceivedFrame::Settings(frame));
                }
                Some(GoAway(frame)) => {
                    tracing::trace!(?frame, "recv GOAWAY");
                    // This should prevent starting new streams,
                    // but should allow continuing to process current streams
                    // until they are all EOS. Once they are, State should
                    // transition to GoAway.
                    self.streams.recv_go_away(&frame)?;
                    *self.error = Some(frame.reason());
                }
                Some(Ping(frame)) => {
                    tracing::trace!(?frame, "recv PING");
                    let status = self.ping_pong.recv_ping(frame);
                    if status.is_shutdown() {
                        assert!(
                            self.go_away.is_going_away(),
                            "received unexpected shutdown ping"
                        );

                        let last_processed_id = self.streams.last_processed_id();
                        self.go_away(last_processed_id, Reason::NO_ERROR);
                    }
                }
                Some(WindowUpdate(frame)) => {
                    tracing::trace!(?frame, "recv WINDOW_UPDATE");
                    self.streams.recv_window_update(frame)?;
                }
                Some(Priority(frame)) => {
                    tracing::trace!(?frame, "recv PRIORITY");
                    // TODO: handle
                }
                None => {
                    tracing::trace!("codec closed");
                    self.streams.recv_eof(false).expect("mutex poisoned");
                    return Ok(ReceivedFrame::Done);
                }
            }
            Ok(ReceivedFrame::Continue)
        }
    }

    enum ReceivedFrame {
        Settings(frame::Settings),
        Continue,
        Done,
    }

    impl<T, B> Connection<T, client::Peer, B>
    where
        T: AsyncRead + AsyncWrite,
        B: Buf,
    {
        pub(crate) fn streams(&self) -> &Streams<B, client::Peer> {
            &self.inner.streams
        }
    }

    impl<T, B> Connection<T, server::Peer, B>
    where
        T: AsyncRead + AsyncWrite + Unpin,
        B: Buf,
    {
        pub fn next_incoming(&mut self) -> Option<StreamRef<B>> {
            self.inner.streams.next_incoming()
        }

        // Graceful shutdown only makes sense for server peers.
        pub fn go_away_gracefully(&mut self) {
            if self.inner.go_away.is_going_away() {
                // No reason to start a new one.
                return;
            }

            // According to http://httpwg.org/specs/rfc7540.html#GOAWAY:
            //
            // > A server that is attempting to gracefully shut down a connection
            // > SHOULD send an initial GOAWAY frame with the last stream
            // > identifier set to 2^31-1 and a NO_ERROR code. This signals to the
            // > client that a shutdown is imminent and that initiating further
            // > requests is prohibited. After allowing time for any in-flight
            // > stream creation (at least one round-trip time), the server can
            // > send another GOAWAY frame with an updated last stream identifier.
            // > This ensures that a connection can be cleanly shut down without
            // > losing requests.
            self.inner.as_dyn().go_away(StreamId::MAX, Reason::NO_ERROR);

            // We take the advice of waiting 1 RTT literally, and wait
            // for a pong before proceeding.
            self.inner.ping_pong.ping_shutdown();
        }
    }

    impl<T, P, B> Drop for Connection<T, P, B>
    where
        P: Peer,
        B: Buf,
    {
        fn drop(&mut self) {
            // Ignore errors as this indicates that the mutex is poisoned.
            let _ = self.inner.streams.recv_eof(true);
        }
    }