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    use crate::codec::Codec;
    use crate::frame::Ping;
    use crate::proto::{self, PingPayload};

    use bytes::Buf;
    use futures_util::task::AtomicWaker;
    use std::io;
    use std::sync::atomic::{AtomicUsize, Ordering};
    use std::sync::Arc;
    use std::task::{Context, Poll};
    use tokio::io::AsyncWrite;

    /// Acknowledges ping requests from the remote.
    #[derive(Debug)]
    pub(crate) struct PingPong {
        pending_ping: Option<PendingPing>,
        pending_pong: Option<PingPayload>,
        user_pings: Option<UserPingsRx>,
    }

    #[derive(Debug)]
    pub(crate) struct UserPings(Arc<UserPingsInner>);

    #[derive(Debug)]
    struct UserPingsRx(Arc<UserPingsInner>);

    #[derive(Debug)]
    struct UserPingsInner {
        state: AtomicUsize,
        /// Task to wake up the main `Connection`.
        ping_task: AtomicWaker,
        /// Task to wake up `share::PingPong::poll_pong`.
        pong_task: AtomicWaker,
    }

    #[derive(Debug)]
    struct PendingPing {
        payload: PingPayload,
        sent: bool,
    }

    /// Status returned from `PingPong::recv_ping`.
    #[derive(Debug)]
    pub(crate) enum ReceivedPing {
        MustAck,
        Unknown,
        Shutdown,
    }

    /// No user ping pending.
    const USER_STATE_EMPTY: usize = 0;
    /// User has called `send_ping`, but PING hasn't been written yet.
    const USER_STATE_PENDING_PING: usize = 1;
    /// User PING has been written, waiting for PONG.
    const USER_STATE_PENDING_PONG: usize = 2;
    /// We've received user PONG, waiting for user to `poll_pong`.
    const USER_STATE_RECEIVED_PONG: usize = 3;
    /// The connection is closed.
    const USER_STATE_CLOSED: usize = 4;

    // ===== impl PingPong =====

    impl PingPong {
        pub(crate) fn new() -> Self {
            PingPong {
                pending_ping: None,
                pending_pong: None,
                user_pings: None,
            }
        }

        /// Can only be called once. If called a second time, returns `None`.
        pub(crate) fn take_user_pings(&mut self) -> Option<UserPings> {
            if self.user_pings.is_some() {
                return None;
            }

            let user_pings = Arc::new(UserPingsInner {
                state: AtomicUsize::new(USER_STATE_EMPTY),
                ping_task: AtomicWaker::new(),
                pong_task: AtomicWaker::new(),
            });
            self.user_pings = Some(UserPingsRx(user_pings.clone()));
            Some(UserPings(user_pings))
        }

        pub(crate) fn ping_shutdown(&mut self) {
            assert!(self.pending_ping.is_none());

            self.pending_ping = Some(PendingPing {
                payload: Ping::SHUTDOWN,
                sent: false,
            });
        }

        /// Process a ping
        pub(crate) fn recv_ping(&mut self, ping: Ping) -> ReceivedPing {
            // The caller should always check that `send_pongs` returns ready before
            // calling `recv_ping`.
            assert!(self.pending_pong.is_none());

            if ping.is_ack() {
                if let Some(pending) = self.pending_ping.take() {
                    if &pending.payload == ping.payload() {
                        assert_eq!(
                            &pending.payload,
                            &Ping::SHUTDOWN,
                            "pending_ping should be for shutdown",
                        );
                        tracing::trace!("recv PING SHUTDOWN ack");
                        return ReceivedPing::Shutdown;
                    }

                    // if not the payload we expected, put it back.
                    self.pending_ping = Some(pending);
                }

                if let Some(ref users) = self.user_pings {
                    if ping.payload() == &Ping::USER && users.receive_pong() {
                        tracing::trace!("recv PING USER ack");
                        return ReceivedPing::Unknown;
                    }
                }

                // else we were acked a ping we didn't send?
                // The spec doesn't require us to do anything about this,
                // so for resiliency, just ignore it for now.
                tracing::warn!("recv PING ack that we never sent: {:?}", ping);
                ReceivedPing::Unknown
            } else {
                // Save the ping's payload to be sent as an acknowledgement.
                self.pending_pong = Some(ping.into_payload());
                ReceivedPing::MustAck
            }
        }

        /// Send any pending pongs.
        pub(crate) fn send_pending_pong<T, B>(
            &mut self,
            cx: &mut Context,
            dst: &mut Codec<T, B>,
        ) -> Poll<io::Result<()>>
        where
            T: AsyncWrite + Unpin,
            B: Buf,
        {
            if let Some(pong) = self.pending_pong.take() {
                if !dst.poll_ready(cx)?.is_ready() {
                    self.pending_pong = Some(pong);
                    return Poll::Pending;
                }

                dst.buffer(Ping::pong(pong).into())
                    .expect("invalid pong frame");
            }

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

        /// Send any pending pings.
        pub(crate) fn send_pending_ping<T, B>(
            &mut self,
            cx: &mut Context,
            dst: &mut Codec<T, B>,
        ) -> Poll<io::Result<()>>
        where
            T: AsyncWrite + Unpin,
            B: Buf,
        {
            if let Some(ref mut ping) = self.pending_ping {
                if !ping.sent {
                    if !dst.poll_ready(cx)?.is_ready() {
                        return Poll::Pending;
                    }

                    dst.buffer(Ping::new(ping.payload).into())
                        .expect("invalid ping frame");
                    ping.sent = true;
                }
            } else if let Some(ref users) = self.user_pings {
                if users.0.state.load(Ordering::Acquire) == USER_STATE_PENDING_PING {
                    if !dst.poll_ready(cx)?.is_ready() {
                        return Poll::Pending;
                    }

                    dst.buffer(Ping::new(Ping::USER).into())
                        .expect("invalid ping frame");
                    users
                        .0
                        .state
                        .store(USER_STATE_PENDING_PONG, Ordering::Release);
                } else {
                    users.0.ping_task.register(cx.waker());
                }
            }

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

    impl ReceivedPing {
        pub(crate) fn is_shutdown(&self) -> bool {
            match *self {
                ReceivedPing::Shutdown => true,
                _ => false,
            }
        }
    }

    // ===== impl UserPings =====

    impl UserPings {
        pub(crate) fn send_ping(&self) -> Result<(), Option<proto::Error>> {
            let prev = self
                .0
                .state
                .compare_exchange(
                    USER_STATE_EMPTY,        // current
                    USER_STATE_PENDING_PING, // new
                    Ordering::AcqRel,
                    Ordering::Acquire,
                )
                .unwrap_or_else(|v| v);

            match prev {
                USER_STATE_EMPTY => {
                    self.0.ping_task.wake();
                    Ok(())
                }
                USER_STATE_CLOSED => Err(Some(broken_pipe().into())),
                _ => {
                    // Was already pending, user error!
                    Err(None)
                }
            }
        }

        pub(crate) fn poll_pong(&self, cx: &mut Context) -> Poll<Result<(), proto::Error>> {
            // Must register before checking state, in case state were to change
            // before we could register, and then the ping would just be lost.
            self.0.pong_task.register(cx.waker());
            let prev = self
                .0
                .state
                .compare_exchange(
                    USER_STATE_RECEIVED_PONG, // current
                    USER_STATE_EMPTY,         // new
                    Ordering::AcqRel,
                    Ordering::Acquire,
                )
                .unwrap_or_else(|v| v);

            match prev {
                USER_STATE_RECEIVED_PONG => Poll::Ready(Ok(())),
                USER_STATE_CLOSED => Poll::Ready(Err(broken_pipe().into())),
                _ => Poll::Pending,
            }
        }
    }

    // ===== impl UserPingsRx =====

    impl UserPingsRx {
        fn receive_pong(&self) -> bool {
            let prev = self
                .0
                .state
                .compare_exchange(
                    USER_STATE_PENDING_PONG,  // current
                    USER_STATE_RECEIVED_PONG, // new
                    Ordering::AcqRel,
                    Ordering::Acquire,
                )
                .unwrap_or_else(|v| v);

            if prev == USER_STATE_PENDING_PONG {
                self.0.pong_task.wake();
                true
            } else {
                false
            }
        }
    }

    impl Drop for UserPingsRx {
        fn drop(&mut self) {
            self.0.state.store(USER_STATE_CLOSED, Ordering::Release);
            self.0.pong_task.wake();
        }
    }

    fn broken_pipe() -> io::Error {
        io::ErrorKind::BrokenPipe.into()
    }