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    use std::io;

    use crate::codec::UserError::*;
    use crate::codec::{RecvError, UserError};
    use crate::frame::{self, Reason};
    use crate::proto::{self, PollReset};

    use self::Inner::*;
    use self::Peer::*;

    /// Represents the state of an H2 stream
    ///
    /// ```not_rust
    ///                              +--------+
    ///                      send PP |        | recv PP
    ///                     ,--------|  idle  |--------.
    ///                    /         |        |         \
    ///                   v          +--------+          v
    ///            +----------+          |           +----------+
    ///            |          |          | send H /  |          |
    ///     ,------| reserved |          | recv H    | reserved |------.
    ///     |      | (local)  |          |           | (remote) |      |
    ///     |      +----------+          v           +----------+      |
    ///     |          |             +--------+             |          |
    ///     |          |     recv ES |        | send ES     |          |
    ///     |   send H |     ,-------|  open  |-------.     | recv H   |
    ///     |          |    /        |        |        \    |          |
    ///     |          v   v         +--------+         v   v          |
    ///     |      +----------+          |           +----------+      |
    ///     |      |   half   |          |           |   half   |      |
    ///     |      |  closed  |          | send R /  |  closed  |      |
    ///     |      | (remote) |          | recv R    | (local)  |      |
    ///     |      +----------+          |           +----------+      |
    ///     |           |                |                 |           |
    ///     |           | send ES /      |       recv ES / |           |
    ///     |           | send R /       v        send R / |           |
    ///     |           | recv R     +--------+   recv R   |           |
    ///     | send R /  `----------->|        |<-----------'  send R / |
    ///     | recv R                 | closed |               recv R   |
    ///     `----------------------->|        |<----------------------'
    ///                              +--------+
    ///
    ///        send:   endpoint sends this frame
    ///        recv:   endpoint receives this frame
    ///
    ///        H:  HEADERS frame (with implied CONTINUATIONs)
    ///        PP: PUSH_PROMISE frame (with implied CONTINUATIONs)
    ///        ES: END_STREAM flag
    ///        R:  RST_STREAM frame
    /// ```
    #[derive(Debug, Clone)]
    pub struct State {
        inner: Inner,
    }

    #[derive(Debug, Clone, Copy)]
    enum Inner {
        Idle,
        // TODO: these states shouldn't count against concurrency limits:
        ReservedLocal,
        ReservedRemote,
        Open { local: Peer, remote: Peer },
        HalfClosedLocal(Peer), // TODO: explicitly name this value
        HalfClosedRemote(Peer),
        Closed(Cause),
    }

    #[derive(Debug, Copy, Clone)]
    enum Peer {
        AwaitingHeaders,
        Streaming,
    }

    #[derive(Debug, Copy, Clone)]
    enum Cause {
        EndStream,
        Proto(Reason),
        LocallyReset(Reason),
        Io,

        /// This indicates to the connection that a reset frame must be sent out
        /// once the send queue has been flushed.
        ///
        /// Examples of when this could happen:
        /// - User drops all references to a stream, so we want to CANCEL the it.
        /// - Header block size was too large, so we want to REFUSE, possibly
        ///   after sending a 431 response frame.
        Scheduled(Reason),
    }

    impl State {
        /// Opens the send-half of a stream if it is not already open.
        pub fn send_open(&mut self, eos: bool) -> Result<(), UserError> {
            let local = Streaming;

            self.inner = match self.inner {
                Idle => {
                    if eos {
                        HalfClosedLocal(AwaitingHeaders)
                    } else {
                        Open {
                            local,
                            remote: AwaitingHeaders,
                        }
                    }
                }
                Open {
                    local: AwaitingHeaders,
                    remote,
                } => {
                    if eos {
                        HalfClosedLocal(remote)
                    } else {
                        Open { local, remote }
                    }
                }
                HalfClosedRemote(AwaitingHeaders) | ReservedLocal => {
                    if eos {
                        Closed(Cause::EndStream)
                    } else {
                        HalfClosedRemote(local)
                    }
                }
                _ => {
                    // All other transitions result in a protocol error
                    return Err(UnexpectedFrameType);
                }
            };

            Ok(())
        }

        /// Opens the receive-half of the stream when a HEADERS frame is received.
        ///
        /// is_informational: whether received a 1xx status code
        ///
        /// Returns true if this transitions the state to Open.
        pub fn recv_open(&mut self, frame: &frame::Headers) -> Result<bool, RecvError> {
            let remote = Streaming;
            let mut initial = false;
            let eos = frame.is_end_stream();

            self.inner = match self.inner {
                Idle => {
                    initial = true;

                    if eos {
                        HalfClosedRemote(AwaitingHeaders)
                    } else {
                        Open {
                            local: AwaitingHeaders,
                            remote,
                        }
                    }
                }
                ReservedRemote => {
                    initial = true;

                    if eos {
                        Closed(Cause::EndStream)
                    } else {
                        HalfClosedLocal(Streaming)
                    }
                }
                Open {
                    local,
                    remote: AwaitingHeaders,
                } => {
                    if eos {
                        HalfClosedRemote(local)
                    } else {
                        Open { local, remote }
                    }
                }
                HalfClosedLocal(AwaitingHeaders) => {
                    if eos {
                        Closed(Cause::EndStream)
                    } else if frame.is_informational() {
                        tracing::trace!("skipping 1xx response headers");
                        HalfClosedLocal(AwaitingHeaders)
                    } else {
                        HalfClosedLocal(remote)
                    }
                }
                state => {
                    // All other transitions result in a protocol error
                    proto_err!(conn: "recv_open: in unexpected state {:?}", state);
                    return Err(RecvError::Connection(Reason::PROTOCOL_ERROR));
                }
            };

            Ok(initial)
        }

        /// Transition from Idle -> ReservedRemote
        pub fn reserve_remote(&mut self) -> Result<(), RecvError> {
            match self.inner {
                Idle => {
                    self.inner = ReservedRemote;
                    Ok(())
                }
                state => {
                    proto_err!(conn: "reserve_remote: in unexpected state {:?}", state);
                    Err(RecvError::Connection(Reason::PROTOCOL_ERROR))
                }
            }
        }

        /// Transition from Idle -> ReservedLocal
        pub fn reserve_local(&mut self) -> Result<(), UserError> {
            match self.inner {
                Idle => {
                    self.inner = ReservedLocal;
                    Ok(())
                }
                _ => Err(UserError::UnexpectedFrameType),
            }
        }

        /// Indicates that the remote side will not send more data to the local.
        pub fn recv_close(&mut self) -> Result<(), RecvError> {
            match self.inner {
                Open { local, .. } => {
                    // The remote side will continue to receive data.
                    tracing::trace!("recv_close: Open => HalfClosedRemote({:?})", local);
                    self.inner = HalfClosedRemote(local);
                    Ok(())
                }
                HalfClosedLocal(..) => {
                    tracing::trace!("recv_close: HalfClosedLocal => Closed");
                    self.inner = Closed(Cause::EndStream);
                    Ok(())
                }
                state => {
                    proto_err!(conn: "recv_close: in unexpected state {:?}", state);
                    Err(RecvError::Connection(Reason::PROTOCOL_ERROR))
                }
            }
        }

        /// The remote explicitly sent a RST_STREAM.
        ///
        /// # Arguments
        /// - `reason`: the reason field of the received RST_STREAM frame.
        /// - `queued`: true if this stream has frames in the pending send queue.
        pub fn recv_reset(&mut self, reason: Reason, queued: bool) {
            match self.inner {
                // If the stream is already in a `Closed` state, do nothing,
                // provided that there are no frames still in the send queue.
                Closed(..) if !queued => {}
                // A notionally `Closed` stream may still have queued frames in
                // the following cases:
                //
                // - if the cause is `Cause::Scheduled(..)` (i.e. we have not
                //   actually closed the stream yet).
                // - if the cause is `Cause::EndStream`: we transition to this
                //   state when an EOS frame is *enqueued* (so that it's invalid
                //   to enqueue more frames), not when the EOS frame is *sent*;
                //   therefore, there may still be frames ahead of the EOS frame
                //   in the send queue.
                //
                // In either of these cases, we want to overwrite the stream's
                // previous state with the received RST_STREAM, so that the queue
                // will be cleared by `Prioritize::pop_frame`.
                state => {
                    tracing::trace!(
                        "recv_reset; reason={:?}; state={:?}; queued={:?}",
                        reason,
                        state,
                        queued
                    );
                    self.inner = Closed(Cause::Proto(reason));
                }
            }
        }

        /// We noticed a protocol error.
        pub fn recv_err(&mut self, err: &proto::Error) {
            use crate::proto::Error::*;

            match self.inner {
                Closed(..) => {}
                _ => {
                    tracing::trace!("recv_err; err={:?}", err);
                    self.inner = Closed(match *err {
                        Proto(reason) => Cause::LocallyReset(reason),
                        Io(..) => Cause::Io,
                    });
                }
            }
        }

        pub fn recv_eof(&mut self) {
            match self.inner {
                Closed(..) => {}
                s => {
                    tracing::trace!("recv_eof; state={:?}", s);
                    self.inner = Closed(Cause::Io);
                }
            }
        }

        /// Indicates that the local side will not send more data to the local.
        pub fn send_close(&mut self) {
            match self.inner {
                Open { remote, .. } => {
                    // The remote side will continue to receive data.
                    tracing::trace!("send_close: Open => HalfClosedLocal({:?})", remote);
                    self.inner = HalfClosedLocal(remote);
                }
                HalfClosedRemote(..) => {
                    tracing::trace!("send_close: HalfClosedRemote => Closed");
                    self.inner = Closed(Cause::EndStream);
                }
                state => panic!("send_close: unexpected state {:?}", state),
            }
        }

        /// Set the stream state to reset locally.
        pub fn set_reset(&mut self, reason: Reason) {
            self.inner = Closed(Cause::LocallyReset(reason));
        }

        /// Set the stream state to a scheduled reset.
        pub fn set_scheduled_reset(&mut self, reason: Reason) {
            debug_assert!(!self.is_closed());
            self.inner = Closed(Cause::Scheduled(reason));
        }

        pub fn get_scheduled_reset(&self) -> Option<Reason> {
            match self.inner {
                Closed(Cause::Scheduled(reason)) => Some(reason),
                _ => None,
            }
        }

        pub fn is_scheduled_reset(&self) -> bool {
            match self.inner {
                Closed(Cause::Scheduled(..)) => true,
                _ => false,
            }
        }

        pub fn is_local_reset(&self) -> bool {
            match self.inner {
                Closed(Cause::LocallyReset(_)) => true,
                Closed(Cause::Scheduled(..)) => true,
                _ => false,
            }
        }

        /// Returns true if the stream is already reset.
        pub fn is_reset(&self) -> bool {
            match self.inner {
                Closed(Cause::EndStream) => false,
                Closed(_) => true,
                _ => false,
            }
        }

        pub fn is_send_streaming(&self) -> bool {
            match self.inner {
                Open {
                    local: Streaming, ..
                } => true,
                HalfClosedRemote(Streaming) => true,
                _ => false,
            }
        }

        /// Returns true when the stream is in a state to receive headers
        pub fn is_recv_headers(&self) -> bool {
            match self.inner {
                Idle => true,
                Open {
                    remote: AwaitingHeaders,
                    ..
                } => true,
                HalfClosedLocal(AwaitingHeaders) => true,
                ReservedRemote => true,
                _ => false,
            }
        }

        pub fn is_recv_streaming(&self) -> bool {
            match self.inner {
                Open {
                    remote: Streaming, ..
                } => true,
                HalfClosedLocal(Streaming) => true,
                _ => false,
            }
        }

        pub fn is_closed(&self) -> bool {
            match self.inner {
                Closed(_) => true,
                _ => false,
            }
        }

        pub fn is_recv_closed(&self) -> bool {
            match self.inner {
                Closed(..) | HalfClosedRemote(..) | ReservedLocal => true,
                _ => false,
            }
        }

        pub fn is_send_closed(&self) -> bool {
            match self.inner {
                Closed(..) | HalfClosedLocal(..) | ReservedRemote => true,
                _ => false,
            }
        }

        pub fn is_idle(&self) -> bool {
            match self.inner {
                Idle => true,
                _ => false,
            }
        }

        pub fn ensure_recv_open(&self) -> Result<bool, proto::Error> {
            // TODO: Is this correct?
            match self.inner {
                Closed(Cause::Proto(reason))
                | Closed(Cause::LocallyReset(reason))
                | Closed(Cause::Scheduled(reason)) => Err(proto::Error::Proto(reason)),
                Closed(Cause::Io) => Err(proto::Error::Io(io::ErrorKind::BrokenPipe.into())),
                Closed(Cause::EndStream) | HalfClosedRemote(..) | ReservedLocal => Ok(false),
                _ => Ok(true),
            }
        }

        /// Returns a reason if the stream has been reset.
        pub(super) fn ensure_reason(&self, mode: PollReset) -> Result<Option<Reason>, crate::Error> {
            match self.inner {
                Closed(Cause::Proto(reason))
                | Closed(Cause::LocallyReset(reason))
                | Closed(Cause::Scheduled(reason)) => Ok(Some(reason)),
                Closed(Cause::Io) => Err(proto::Error::Io(io::ErrorKind::BrokenPipe.into()).into()),
                Open {
                    local: Streaming, ..
                }
                | HalfClosedRemote(Streaming) => match mode {
                    PollReset::AwaitingHeaders => Err(UserError::PollResetAfterSendResponse.into()),
                    PollReset::Streaming => Ok(None),
                },
                _ => Ok(None),
            }
        }
    }

    impl Default for State {
        fn default() -> State {
            State { inner: Inner::Idle }
        }
    }

    impl Default for Peer {
        fn default() -> Self {
            AwaitingHeaders
        }
    }