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    use super::store::Resolve;
    use super::*;

    use crate::frame::{Reason, StreamId};

    use crate::codec::UserError;
    use crate::codec::UserError::*;

    use bytes::buf::{Buf, Take};
    use std::io;
    use std::task::{Context, Poll, Waker};
    use std::{cmp, fmt, mem};

    /// # Warning
    ///
    /// Queued streams are ordered by stream ID, as we need to ensure that
    /// lower-numbered streams are sent headers before higher-numbered ones.
    /// This is because "idle" stream IDs – those which have been initiated but
    /// have yet to receive frames – will be implicitly closed on receipt of a
    /// frame on a higher stream ID. If these queues was not ordered by stream
    /// IDs, some mechanism would be necessary to ensure that the lowest-numbered]
    /// idle stream is opened first.
    #[derive(Debug)]
    pub(super) struct Prioritize {
        /// Queue of streams waiting for socket capacity to send a frame.
        pending_send: store::Queue<stream::NextSend>,

        /// Queue of streams waiting for window capacity to produce data.
        pending_capacity: store::Queue<stream::NextSendCapacity>,

        /// Streams waiting for capacity due to max concurrency
        ///
        /// The `SendRequest` handle is `Clone`. This enables initiating requests
        /// from many tasks. However, offering this capability while supporting
        /// backpressure at some level is tricky. If there are many `SendRequest`
        /// handles and a single stream becomes available, which handle gets
        /// assigned that stream? Maybe that handle is no longer ready to send a
        /// request.
        ///
        /// The strategy used is to allow each `SendRequest` handle one buffered
        /// request. A `SendRequest` handle is ready to send a request if it has no
        /// associated buffered requests. This is the same strategy as `mpsc` in the
        /// futures library.
        pending_open: store::Queue<stream::NextOpen>,

        /// Connection level flow control governing sent data
        flow: FlowControl,

        /// Stream ID of the last stream opened.
        last_opened_id: StreamId,

        /// What `DATA` frame is currently being sent in the codec.
        in_flight_data_frame: InFlightData,
    }

    #[derive(Debug, Eq, PartialEq)]
    enum InFlightData {
        /// There is no `DATA` frame in flight.
        Nothing,
        /// There is a `DATA` frame in flight belonging to the given stream.
        DataFrame(store::Key),
        /// There was a `DATA` frame, but the stream's queue was since cleared.
        Drop,
    }

    pub(crate) struct Prioritized<B> {
        // The buffer
        inner: Take<B>,

        end_of_stream: bool,

        // The stream that this is associated with
        stream: store::Key,
    }

    // ===== impl Prioritize =====

    impl Prioritize {
        pub fn new(config: &Config) -> Prioritize {
            let mut flow = FlowControl::new();

            flow.inc_window(config.remote_init_window_sz)
                .expect("invalid initial window size");

            flow.assign_capacity(config.remote_init_window_sz);

            tracing::trace!("Prioritize::new; flow={:?}", flow);

            Prioritize {
                pending_send: store::Queue::new(),
                pending_capacity: store::Queue::new(),
                pending_open: store::Queue::new(),
                flow,
                last_opened_id: StreamId::ZERO,
                in_flight_data_frame: InFlightData::Nothing,
            }
        }

        /// Queue a frame to be sent to the remote
        pub fn queue_frame<B>(
            &mut self,
            frame: Frame<B>,
            buffer: &mut Buffer<Frame<B>>,
            stream: &mut store::Ptr,
            task: &mut Option<Waker>,
        ) {
            let span = tracing::trace_span!("Prioritize::queue_frame", ?stream.id);
            let _e = span.enter();
            // Queue the frame in the buffer
            stream.pending_send.push_back(buffer, frame);
            self.schedule_send(stream, task);
        }

        pub fn schedule_send(&mut self, stream: &mut store::Ptr, task: &mut Option<Waker>) {
            // If the stream is waiting to be opened, nothing more to do.
            if stream.is_send_ready() {
                tracing::trace!(?stream.id, "schedule_send");
                // Queue the stream
                self.pending_send.push(stream);

                // Notify the connection.
                if let Some(task) = task.take() {
                    task.wake();
                }
            }
        }

        pub fn queue_open(&mut self, stream: &mut store::Ptr) {
            self.pending_open.push(stream);
        }

        /// Send a data frame
        pub fn send_data<B>(
            &mut self,
            frame: frame::Data<B>,
            buffer: &mut Buffer<Frame<B>>,
            stream: &mut store::Ptr,
            counts: &mut Counts,
            task: &mut Option<Waker>,
        ) -> Result<(), UserError>
        where
            B: Buf,
        {
            let sz = frame.payload().remaining();

            if sz > MAX_WINDOW_SIZE as usize {
                return Err(UserError::PayloadTooBig);
            }

            let sz = sz as WindowSize;

            if !stream.state.is_send_streaming() {
                if stream.state.is_closed() {
                    return Err(InactiveStreamId);
                } else {
                    return Err(UnexpectedFrameType);
                }
            }

            // Update the buffered data counter
            stream.buffered_send_data += sz;

            let span =
                tracing::trace_span!("send_data", sz, requested = stream.requested_send_capacity);
            let _e = span.enter();
            tracing::trace!(buffered = stream.buffered_send_data);

            // Implicitly request more send capacity if not enough has been
            // requested yet.
            if stream.requested_send_capacity < stream.buffered_send_data {
                // Update the target requested capacity
                stream.requested_send_capacity = stream.buffered_send_data;

                self.try_assign_capacity(stream);
            }

            if frame.is_end_stream() {
                stream.state.send_close();
                self.reserve_capacity(0, stream, counts);
            }

            tracing::trace!(
                available = %stream.send_flow.available(),
                buffered = stream.buffered_send_data,
            );

            // The `stream.buffered_send_data == 0` check is here so that, if a zero
            // length data frame is queued to the front (there is no previously
            // queued data), it gets sent out immediately even if there is no
            // available send window.
            //
            // Sending out zero length data frames can be done to signal
            // end-of-stream.
            //
            if stream.send_flow.available() > 0 || stream.buffered_send_data == 0 {
                // The stream currently has capacity to send the data frame, so
                // queue it up and notify the connection task.
                self.queue_frame(frame.into(), buffer, stream, task);
            } else {
                // The stream has no capacity to send the frame now, save it but
                // don't notify the connection task. Once additional capacity
                // becomes available, the frame will be flushed.
                stream.pending_send.push_back(buffer, frame.into());
            }

            Ok(())
        }

        /// Request capacity to send data
        pub fn reserve_capacity(
            &mut self,
            capacity: WindowSize,
            stream: &mut store::Ptr,
            counts: &mut Counts,
        ) {
            let span = tracing::trace_span!(
                "reserve_capacity",
                ?stream.id,
                requested = capacity,
                effective = capacity + stream.buffered_send_data,
                curr = stream.requested_send_capacity
            );
            let _e = span.enter();

            // Actual capacity is `capacity` + the current amount of buffered data.
            // If it were less, then we could never send out the buffered data.
            let capacity = capacity + stream.buffered_send_data;

            if capacity == stream.requested_send_capacity {
                // Nothing to do
            } else if capacity < stream.requested_send_capacity {
                // Update the target requested capacity
                stream.requested_send_capacity = capacity;

                // Currently available capacity assigned to the stream
                let available = stream.send_flow.available().as_size();

                // If the stream has more assigned capacity than requested, reclaim
                // some for the connection
                if available > capacity {
                    let diff = available - capacity;

                    stream.send_flow.claim_capacity(diff);

                    self.assign_connection_capacity(diff, stream, counts);
                }
            } else {
                // If trying to *add* capacity, but the stream send side is closed,
                // there's nothing to be done.
                if stream.state.is_send_closed() {
                    return;
                }

                // Update the target requested capacity
                stream.requested_send_capacity = capacity;

                // Try to assign additional capacity to the stream. If none is
                // currently available, the stream will be queued to receive some
                // when more becomes available.
                self.try_assign_capacity(stream);
            }
        }

        pub fn recv_stream_window_update(
            &mut self,
            inc: WindowSize,
            stream: &mut store::Ptr,
        ) -> Result<(), Reason> {
            let span = tracing::trace_span!(
                "recv_stream_window_update",
                ?stream.id,
                ?stream.state,
                inc,
                flow = ?stream.send_flow
            );
            let _e = span.enter();

            if stream.state.is_send_closed() && stream.buffered_send_data == 0 {
                // We can't send any data, so don't bother doing anything else.
                return Ok(());
            }

            // Update the stream level flow control.
            stream.send_flow.inc_window(inc)?;

            // If the stream is waiting on additional capacity, then this will
            // assign it (if available on the connection) and notify the producer
            self.try_assign_capacity(stream);

            Ok(())
        }

        pub fn recv_connection_window_update(
            &mut self,
            inc: WindowSize,
            store: &mut Store,
            counts: &mut Counts,
        ) -> Result<(), Reason> {
            // Update the connection's window
            self.flow.inc_window(inc)?;

            self.assign_connection_capacity(inc, store, counts);
            Ok(())
        }

        /// Reclaim all capacity assigned to the stream and re-assign it to the
        /// connection
        pub fn reclaim_all_capacity(&mut self, stream: &mut store::Ptr, counts: &mut Counts) {
            let available = stream.send_flow.available().as_size();
            stream.send_flow.claim_capacity(available);
            // Re-assign all capacity to the connection
            self.assign_connection_capacity(available, stream, counts);
        }

        /// Reclaim just reserved capacity, not buffered capacity, and re-assign
        /// it to the connection
        pub fn reclaim_reserved_capacity(&mut self, stream: &mut store::Ptr, counts: &mut Counts) {
            // only reclaim requested capacity that isn't already buffered
            if stream.requested_send_capacity > stream.buffered_send_data {
                let reserved = stream.requested_send_capacity - stream.buffered_send_data;

                stream.send_flow.claim_capacity(reserved);
                self.assign_connection_capacity(reserved, stream, counts);
            }
        }

        pub fn clear_pending_capacity(&mut self, store: &mut Store, counts: &mut Counts) {
            let span = tracing::trace_span!("clear_pending_capacity");
            let _e = span.enter();
            while let Some(stream) = self.pending_capacity.pop(store) {
                counts.transition(stream, |_, stream| {
                    tracing::trace!(?stream.id, "clear_pending_capacity");
                })
            }
        }

        pub fn assign_connection_capacity<R>(
            &mut self,
            inc: WindowSize,
            store: &mut R,
            counts: &mut Counts,
        ) where
            R: Resolve,
        {
            let span = tracing::trace_span!("assign_connection_capacity", inc);
            let _e = span.enter();

            self.flow.assign_capacity(inc);

            // Assign newly acquired capacity to streams pending capacity.
            while self.flow.available() > 0 {
                let stream = match self.pending_capacity.pop(store) {
                    Some(stream) => stream,
                    None => return,
                };

                // Streams pending capacity may have been reset before capacity
                // became available. In that case, the stream won't want any
                // capacity, and so we shouldn't "transition" on it, but just evict
                // it and continue the loop.
                if !(stream.state.is_send_streaming() || stream.buffered_send_data > 0) {
                    continue;
                }

                counts.transition(stream, |_, mut stream| {
                    // Try to assign capacity to the stream. This will also re-queue the
                    // stream if there isn't enough connection level capacity to fulfill
                    // the capacity request.
                    self.try_assign_capacity(&mut stream);
                })
            }
        }

        /// Request capacity to send data
        fn try_assign_capacity(&mut self, stream: &mut store::Ptr) {
            let total_requested = stream.requested_send_capacity;

            // Total requested should never go below actual assigned
            // (Note: the window size can go lower than assigned)
            debug_assert!(total_requested >= stream.send_flow.available());

            // The amount of additional capacity that the stream requests.
            // Don't assign more than the window has available!
            let additional = cmp::min(
                total_requested - stream.send_flow.available().as_size(),
                // Can't assign more than what is available
                stream.send_flow.window_size() - stream.send_flow.available().as_size(),
            );
            let span = tracing::trace_span!("try_assign_capacity", ?stream.id);
            let _e = span.enter();
            tracing::trace!(
                requested = total_requested,
                additional,
                buffered = stream.buffered_send_data,
                window = stream.send_flow.window_size(),
                conn = %self.flow.available()
            );

            if additional == 0 {
                // Nothing more to do
                return;
            }

            // If the stream has requested capacity, then it must be in the
            // streaming state (more data could be sent) or there is buffered data
            // waiting to be sent.
            debug_assert!(
                stream.state.is_send_streaming() || stream.buffered_send_data > 0,
                "state={:?}",
                stream.state
            );

            // The amount of currently available capacity on the connection
            let conn_available = self.flow.available().as_size();

            // First check if capacity is immediately available
            if conn_available > 0 {
                // The amount of capacity to assign to the stream
                // TODO: Should prioritization factor into this?
                let assign = cmp::min(conn_available, additional);

                tracing::trace!(capacity = assign, "assigning");

                // Assign the capacity to the stream
                stream.assign_capacity(assign);

                // Claim the capacity from the connection
                self.flow.claim_capacity(assign);
            }

            tracing::trace!(
                available = %stream.send_flow.available(),
                requested = stream.requested_send_capacity,
                buffered = stream.buffered_send_data,
                has_unavailable = %stream.send_flow.has_unavailable()
            );

            if stream.send_flow.available() < stream.requested_send_capacity
                && stream.send_flow.has_unavailable()
            {
                // The stream requires additional capacity and the stream's
                // window has available capacity, but the connection window
                // does not.
                //
                // In this case, the stream needs to be queued up for when the
                // connection has more capacity.
                self.pending_capacity.push(stream);
            }

            // If data is buffered and the stream is send ready, then
            // schedule the stream for execution
            if stream.buffered_send_data > 0 && stream.is_send_ready() {
                // TODO: This assertion isn't *exactly* correct. There can still be
                // buffered send data while the stream's pending send queue is
                // empty. This can happen when a large data frame is in the process
                // of being **partially** sent. Once the window has been sent, the
                // data frame will be returned to the prioritization layer to be
                // re-scheduled.
                //
                // That said, it would be nice to figure out how to make this
                // assertion correctly.
                //
                // debug_assert!(!stream.pending_send.is_empty());

                self.pending_send.push(stream);
            }
        }

        pub fn poll_complete<T, B>(
            &mut self,
            cx: &mut Context,
            buffer: &mut Buffer<Frame<B>>,
            store: &mut Store,
            counts: &mut Counts,
            dst: &mut Codec<T, Prioritized<B>>,
        ) -> Poll<io::Result<()>>
        where
            T: AsyncWrite + Unpin,
            B: Buf,
        {
            // Ensure codec is ready
            ready!(dst.poll_ready(cx))?;

            // Reclaim any frame that has previously been written
            self.reclaim_frame(buffer, store, dst);

            // The max frame length
            let max_frame_len = dst.max_send_frame_size();

            tracing::trace!("poll_complete");

            loop {
                self.schedule_pending_open(store, counts);

                match self.pop_frame(buffer, store, max_frame_len, counts) {
                    Some(frame) => {
                        tracing::trace!(?frame, "writing");

                        debug_assert_eq!(self.in_flight_data_frame, InFlightData::Nothing);
                        if let Frame::Data(ref frame) = frame {
                            self.in_flight_data_frame = InFlightData::DataFrame(frame.payload().stream);
                        }
                        dst.buffer(frame).expect("invalid frame");

                        // Ensure the codec is ready to try the loop again.
                        ready!(dst.poll_ready(cx))?;

                        // Because, always try to reclaim...
                        self.reclaim_frame(buffer, store, dst);
                    }
                    None => {
                        // Try to flush the codec.
                        ready!(dst.flush(cx))?;

                        // This might release a data frame...
                        if !self.reclaim_frame(buffer, store, dst) {
                            return Poll::Ready(Ok(()));
                        }

                        // No need to poll ready as poll_complete() does this for
                        // us...
                    }
                }
            }
        }

        /// Tries to reclaim a pending data frame from the codec.
        ///
        /// Returns true if a frame was reclaimed.
        ///
        /// When a data frame is written to the codec, it may not be written in its
        /// entirety (large chunks are split up into potentially many data frames).
        /// In this case, the stream needs to be reprioritized.
        fn reclaim_frame<T, B>(
            &mut self,
            buffer: &mut Buffer<Frame<B>>,
            store: &mut Store,
            dst: &mut Codec<T, Prioritized<B>>,
        ) -> bool
        where
            B: Buf,
        {
            let span = tracing::trace_span!("try_reclaim_frame");
            let _e = span.enter();

            // First check if there are any data chunks to take back
            if let Some(frame) = dst.take_last_data_frame() {
                self.reclaim_frame_inner(buffer, store, frame)
            } else {
                false
            }
        }

        fn reclaim_frame_inner<B>(
            &mut self,
            buffer: &mut Buffer<Frame<B>>,
            store: &mut Store,
            frame: frame::Data<Prioritized<B>>,
        ) -> bool
        where
            B: Buf,
        {
            tracing::trace!(
                ?frame,
                sz = frame.payload().inner.get_ref().remaining(),
                "reclaimed"
            );

            let mut eos = false;
            let key = frame.payload().stream;

            match mem::replace(&mut self.in_flight_data_frame, InFlightData::Nothing) {
                InFlightData::Nothing => panic!("wasn't expecting a frame to reclaim"),
                InFlightData::Drop => {
                    tracing::trace!("not reclaiming frame for cancelled stream");
                    return false;
                }
                InFlightData::DataFrame(k) => {
                    debug_assert_eq!(k, key);
                }
            }

            let mut frame = frame.map(|prioritized| {
                // TODO: Ensure fully written
                eos = prioritized.end_of_stream;
                prioritized.inner.into_inner()
            });

            if frame.payload().has_remaining() {
                let mut stream = store.resolve(key);

                if eos {
                    frame.set_end_stream(true);
                }

                self.push_back_frame(frame.into(), buffer, &mut stream);

                return true;
            }

            false
        }

        /// Push the frame to the front of the stream's deque, scheduling the
        /// stream if needed.
        fn push_back_frame<B>(
            &mut self,
            frame: Frame<B>,
            buffer: &mut Buffer<Frame<B>>,
            stream: &mut store::Ptr,
        ) {
            // Push the frame to the front of the stream's deque
            stream.pending_send.push_front(buffer, frame);

            // If needed, schedule the sender
            if stream.send_flow.available() > 0 {
                debug_assert!(!stream.pending_send.is_empty());
                self.pending_send.push(stream);
            }
        }

        pub fn clear_queue<B>(&mut self, buffer: &mut Buffer<Frame<B>>, stream: &mut store::Ptr) {
            let span = tracing::trace_span!("clear_queue", ?stream.id);
            let _e = span.enter();

            // TODO: make this more efficient?
            while let Some(frame) = stream.pending_send.pop_front(buffer) {
                tracing::trace!(?frame, "dropping");
            }

            stream.buffered_send_data = 0;
            stream.requested_send_capacity = 0;
            if let InFlightData::DataFrame(key) = self.in_flight_data_frame {
                if stream.key() == key {
                    // This stream could get cleaned up now - don't allow the buffered frame to get reclaimed.
                    self.in_flight_data_frame = InFlightData::Drop;
                }
            }
        }

        pub fn clear_pending_send(&mut self, store: &mut Store, counts: &mut Counts) {
            while let Some(stream) = self.pending_send.pop(store) {
                let is_pending_reset = stream.is_pending_reset_expiration();
                counts.transition_after(stream, is_pending_reset);
            }
        }

        pub fn clear_pending_open(&mut self, store: &mut Store, counts: &mut Counts) {
            while let Some(stream) = self.pending_open.pop(store) {
                let is_pending_reset = stream.is_pending_reset_expiration();
                counts.transition_after(stream, is_pending_reset);
            }
        }

        fn pop_frame<B>(
            &mut self,
            buffer: &mut Buffer<Frame<B>>,
            store: &mut Store,
            max_len: usize,
            counts: &mut Counts,
        ) -> Option<Frame<Prioritized<B>>>
        where
            B: Buf,
        {
            let span = tracing::trace_span!("pop_frame");
            let _e = span.enter();

            loop {
                match self.pending_send.pop(store) {
                    Some(mut stream) => {
                        let span = tracing::trace_span!("popped", ?stream.id, ?stream.state);
                        let _e = span.enter();

                        // It's possible that this stream, besides having data to send,
                        // is also queued to send a reset, and thus is already in the queue
                        // to wait for "some time" after a reset.
                        //
                        // To be safe, we just always ask the stream.
                        let is_pending_reset = stream.is_pending_reset_expiration();

                        tracing::trace!(is_pending_reset);

                        let frame = match stream.pending_send.pop_front(buffer) {
                            Some(Frame::Data(mut frame)) => {
                                // Get the amount of capacity remaining for stream's
                                // window.
                                let stream_capacity = stream.send_flow.available();
                                let sz = frame.payload().remaining();

                                tracing::trace!(
                                    sz,
                                    eos = frame.is_end_stream(),
                                    window = %stream_capacity,
                                    available = %stream.send_flow.available(),
                                    requested = stream.requested_send_capacity,
                                    buffered = stream.buffered_send_data,
                                    "data frame"
                                );

                                // Zero length data frames always have capacity to
                                // be sent.
                                if sz > 0 && stream_capacity == 0 {
                                    tracing::trace!("stream capacity is 0");

                                    // Ensure that the stream is waiting for
                                    // connection level capacity
                                    //
                                    // TODO: uncomment
                                    // debug_assert!(stream.is_pending_send_capacity);

                                    // The stream has no more capacity, this can
                                    // happen if the remote reduced the stream
                                    // window. In this case, we need to buffer the
                                    // frame and wait for a window update...
                                    stream.pending_send.push_front(buffer, frame.into());

                                    continue;
                                }

                                // Only send up to the max frame length
                                let len = cmp::min(sz, max_len);

                                // Only send up to the stream's window capacity
                                let len =
                                    cmp::min(len, stream_capacity.as_size() as usize) as WindowSize;

                                // There *must* be be enough connection level
                                // capacity at this point.
                                debug_assert!(len <= self.flow.window_size());

                                tracing::trace!(len, "sending data frame");

                                // Update the flow control
                                tracing::trace_span!("updating stream flow").in_scope(|| {
                                    stream.send_flow.send_data(len);

                                    // Decrement the stream's buffered data counter
                                    debug_assert!(stream.buffered_send_data >= len);
                                    stream.buffered_send_data -= len;
                                    stream.requested_send_capacity -= len;

                                    // Assign the capacity back to the connection that
                                    // was just consumed from the stream in the previous
                                    // line.
                                    self.flow.assign_capacity(len);
                                });

                                let (eos, len) = tracing::trace_span!("updating connection flow")
                                    .in_scope(|| {
                                        self.flow.send_data(len);

                                        // Wrap the frame's data payload to ensure that the
                                        // correct amount of data gets written.

                                        let eos = frame.is_end_stream();
                                        let len = len as usize;

                                        if frame.payload().remaining() > len {
                                            frame.set_end_stream(false);
                                        }
                                        (eos, len)
                                    });

                                Frame::Data(frame.map(|buf| Prioritized {
                                    inner: buf.take(len),
                                    end_of_stream: eos,
                                    stream: stream.key(),
                                }))
                            }
                            Some(Frame::PushPromise(pp)) => {
                                let mut pushed =
                                    stream.store_mut().find_mut(&pp.promised_id()).unwrap();
                                pushed.is_pending_push = false;
                                // Transition stream from pending_push to pending_open
                                // if possible
                                if !pushed.pending_send.is_empty() {
                                    if counts.can_inc_num_send_streams() {
                                        counts.inc_num_send_streams(&mut pushed);
                                        self.pending_send.push(&mut pushed);
                                    } else {
                                        self.queue_open(&mut pushed);
                                    }
                                }
                                Frame::PushPromise(pp)
                            }
                            Some(frame) => frame.map(|_| {
                                unreachable!(
                                    "Frame::map closure will only be called \
                                     on DATA frames."
                                )
                            }),
                            None => {
                                if let Some(reason) = stream.state.get_scheduled_reset() {
                                    stream.state.set_reset(reason);

                                    let frame = frame::Reset::new(stream.id, reason);
                                    Frame::Reset(frame)
                                } else {
                                    // If the stream receives a RESET from the peer, it may have
                                    // had data buffered to be sent, but all the frames are cleared
                                    // in clear_queue(). Instead of doing O(N) traversal through queue
                                    // to remove, lets just ignore the stream here.
                                    tracing::trace!("removing dangling stream from pending_send");
                                    // Since this should only happen as a consequence of `clear_queue`,
                                    // we must be in a closed state of some kind.
                                    debug_assert!(stream.state.is_closed());
                                    counts.transition_after(stream, is_pending_reset);
                                    continue;
                                }
                            }
                        };

                        tracing::trace!("pop_frame; frame={:?}", frame);

                        if cfg!(debug_assertions) && stream.state.is_idle() {
                            debug_assert!(stream.id > self.last_opened_id);
                            self.last_opened_id = stream.id;
                        }

                        if !stream.pending_send.is_empty() || stream.state.is_scheduled_reset() {
                            // TODO: Only requeue the sender IF it is ready to send
                            // the next frame. i.e. don't requeue it if the next
                            // frame is a data frame and the stream does not have
                            // any more capacity.
                            self.pending_send.push(&mut stream);
                        }

                        counts.transition_after(stream, is_pending_reset);

                        return Some(frame);
                    }
                    None => return None,
                }
            }
        }

        fn schedule_pending_open(&mut self, store: &mut Store, counts: &mut Counts) {
            tracing::trace!("schedule_pending_open");
            // check for any pending open streams
            while counts.can_inc_num_send_streams() {
                if let Some(mut stream) = self.pending_open.pop(store) {
                    tracing::trace!("schedule_pending_open; stream={:?}", stream.id);

                    counts.inc_num_send_streams(&mut stream);
                    self.pending_send.push(&mut stream);
                    stream.notify_send();
                } else {
                    return;
                }
            }
        }
    }

    // ===== impl Prioritized =====

    impl<B> Buf for Prioritized<B>
    where
        B: Buf,
    {
        fn remaining(&self) -> usize {
            self.inner.remaining()
        }

        fn chunk(&self) -> &[u8] {
            self.inner.chunk()
        }

        fn chunks_vectored<'a>(&'a self, dst: &mut [std::io::IoSlice<'a>]) -> usize {
            self.inner.chunks_vectored(dst)
        }

        fn advance(&mut self, cnt: usize) {
            self.inner.advance(cnt)
        }
    }

    impl<B: Buf> fmt::Debug for Prioritized<B> {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("Prioritized")
                .field("remaining", &self.inner.get_ref().remaining())
                .field("end_of_stream", &self.end_of_stream)
                .field("stream", &self.stream)
                .finish()
        }
    }