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    use crate::codec::UserError;
    use crate::codec::UserError::*;
    use crate::frame::{self, Frame, FrameSize};
    use crate::hpack;

    use bytes::{Buf, BufMut, BytesMut};
    use std::pin::Pin;
    use std::task::{Context, Poll};
    use tokio::io::{AsyncRead, AsyncWrite, ReadBuf};

    use std::io::{self, Cursor, IoSlice};

    // A macro to get around a method needing to borrow &mut self
    macro_rules! limited_write_buf {
        ($self:expr) => {{
            let limit = $self.max_frame_size() + frame::HEADER_LEN;
            $self.buf.get_mut().limit(limit)
        }};
    }

    #[derive(Debug)]
    pub struct FramedWrite<T, B> {
        /// Upstream `AsyncWrite`
        inner: T,

        encoder: Encoder<B>,
    }

    #[derive(Debug)]
    struct Encoder<B> {
        /// HPACK encoder
        hpack: hpack::Encoder,

        /// Write buffer
        ///
        /// TODO: Should this be a ring buffer?
        buf: Cursor<BytesMut>,

        /// Next frame to encode
        next: Option<Next<B>>,

        /// Last data frame
        last_data_frame: Option<frame::Data<B>>,

        /// Max frame size, this is specified by the peer
        max_frame_size: FrameSize,

        /// Whether or not the wrapped `AsyncWrite` supports vectored IO.
        is_write_vectored: bool,
    }

    #[derive(Debug)]
    enum Next<B> {
        Data(frame::Data<B>),
        Continuation(frame::Continuation),
    }

    /// Initialize the connection with this amount of write buffer.
    ///
    /// The minimum MAX_FRAME_SIZE is 16kb, so always be able to send a HEADERS
    /// frame that big.
    const DEFAULT_BUFFER_CAPACITY: usize = 16 * 1_024;

    /// Min buffer required to attempt to write a frame
    const MIN_BUFFER_CAPACITY: usize = frame::HEADER_LEN + CHAIN_THRESHOLD;

    /// Chain payloads bigger than this. The remote will never advertise a max frame
    /// size less than this (well, the spec says the max frame size can't be less
    /// than 16kb, so not even close).
    const CHAIN_THRESHOLD: usize = 256;

    // TODO: Make generic
    impl<T, B> FramedWrite<T, B>
    where
        T: AsyncWrite + Unpin,
        B: Buf,
    {
        pub fn new(inner: T) -> FramedWrite<T, B> {
            let is_write_vectored = inner.is_write_vectored();
            FramedWrite {
                inner,
                encoder: Encoder {
                    hpack: hpack::Encoder::default(),
                    buf: Cursor::new(BytesMut::with_capacity(DEFAULT_BUFFER_CAPACITY)),
                    next: None,
                    last_data_frame: None,
                    max_frame_size: frame::DEFAULT_MAX_FRAME_SIZE,
                    is_write_vectored,
                },
            }
        }

        /// Returns `Ready` when `send` is able to accept a frame
        ///
        /// Calling this function may result in the current contents of the buffer
        /// to be flushed to `T`.
        pub fn poll_ready(&mut self, cx: &mut Context) -> Poll<io::Result<()>> {
            if !self.encoder.has_capacity() {
                // Try flushing
                ready!(self.flush(cx))?;

                if !self.encoder.has_capacity() {
                    return Poll::Pending;
                }
            }

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

        /// Buffer a frame.
        ///
        /// `poll_ready` must be called first to ensure that a frame may be
        /// accepted.
        pub fn buffer(&mut self, item: Frame<B>) -> Result<(), UserError> {
            self.encoder.buffer(item)
        }

        /// Flush buffered data to the wire
        pub fn flush(&mut self, cx: &mut Context) -> Poll<io::Result<()>> {
            let span = tracing::trace_span!("FramedWrite::flush");
            let _e = span.enter();

            loop {
                while !self.encoder.is_empty() {
                    match self.encoder.next {
                        Some(Next::Data(ref mut frame)) => {
                            tracing::trace!(queued_data_frame = true);
                            let mut buf = (&mut self.encoder.buf).chain(frame.payload_mut());
                            ready!(write(
                                &mut self.inner,
                                self.encoder.is_write_vectored,
                                &mut buf,
                                cx,
                            ))?
                        }
                        _ => {
                            tracing::trace!(queued_data_frame = false);
                            ready!(write(
                                &mut self.inner,
                                self.encoder.is_write_vectored,
                                &mut self.encoder.buf,
                                cx,
                            ))?
                        }
                    }
                }

                match self.encoder.unset_frame() {
                    ControlFlow::Continue => (),
                    ControlFlow::Break => break,
                    ControlFlow::EndlessLoopHeaderTooBig => {
                        return Poll::Ready(Err(std::io::Error::new(
                            std::io::ErrorKind::InvalidInput,
                            UserError::HeaderTooBig,
                        )));
                    }
                }
            }

            tracing::trace!("flushing buffer");
            // Flush the upstream
            ready!(Pin::new(&mut self.inner).poll_flush(cx))?;

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

        /// Close the codec
        pub fn shutdown(&mut self, cx: &mut Context) -> Poll<io::Result<()>> {
            ready!(self.flush(cx))?;
            Pin::new(&mut self.inner).poll_shutdown(cx)
        }
    }

    fn write<T, B>(
        writer: &mut T,
        is_write_vectored: bool,
        buf: &mut B,
        cx: &mut Context<'_>,
    ) -> Poll<io::Result<()>>
    where
        T: AsyncWrite + Unpin,
        B: Buf,
    {
        // TODO(eliza): when tokio-util 0.5.1 is released, this
        // could just use `poll_write_buf`...
        const MAX_IOVS: usize = 64;
        let n = if is_write_vectored {
            let mut bufs = [IoSlice::new(&[]); MAX_IOVS];
            let cnt = buf.chunks_vectored(&mut bufs);
            ready!(Pin::new(writer).poll_write_vectored(cx, &bufs[..cnt]))?
        } else {
            ready!(Pin::new(writer).poll_write(cx, buf.chunk()))?
        };
        buf.advance(n);
        Ok(()).into()
    }

    #[must_use]
    enum ControlFlow {
        Continue,
        Break,
        EndlessLoopHeaderTooBig,
    }

    impl<B> Encoder<B>
    where
        B: Buf,
    {
        fn unset_frame(&mut self) -> ControlFlow {
            // Clear internal buffer
            self.buf.set_position(0);
            self.buf.get_mut().clear();

            // The data frame has been written, so unset it
            match self.next.take() {
                Some(Next::Data(frame)) => {
                    self.last_data_frame = Some(frame);
                    debug_assert!(self.is_empty());
                    ControlFlow::Break
                }
                Some(Next::Continuation(frame)) => {
                    // Buffer the continuation frame, then try to write again
                    let mut buf = limited_write_buf!(self);
                    if let Some(continuation) = frame.encode(&mut self.hpack, &mut buf) {
                        // We previously had a CONTINUATION, and after encoding
                        // it, we got *another* one? Let's just double check
                        // that at least some progress is being made...
                        if self.buf.get_ref().len() == frame::HEADER_LEN {
                            // If *only* the CONTINUATION frame header was
                            // written, and *no* header fields, we're stuck
                            // in a loop...
                            tracing::warn!(
                                "CONTINUATION frame write loop; header value too big to encode"
                            );
                            return ControlFlow::EndlessLoopHeaderTooBig;
                        }

                        self.next = Some(Next::Continuation(continuation));
                    }
                    ControlFlow::Continue
                }
                None => ControlFlow::Break,
            }
        }

        fn buffer(&mut self, item: Frame<B>) -> Result<(), UserError> {
            // Ensure that we have enough capacity to accept the write.
            assert!(self.has_capacity());
            let span = tracing::trace_span!("FramedWrite::buffer", frame = ?item);
            let _e = span.enter();

            tracing::debug!(frame = ?item, "send");

            match item {
                Frame::Data(mut v) => {
                    // Ensure that the payload is not greater than the max frame.
                    let len = v.payload().remaining();

                    if len > self.max_frame_size() {
                        return Err(PayloadTooBig);
                    }

                    if len >= CHAIN_THRESHOLD {
                        let head = v.head();

                        // Encode the frame head to the buffer
                        head.encode(len, self.buf.get_mut());

                        // Save the data frame
                        self.next = Some(Next::Data(v));
                    } else {
                        v.encode_chunk(self.buf.get_mut());

                        // The chunk has been fully encoded, so there is no need to
                        // keep it around
                        assert_eq!(v.payload().remaining(), 0, "chunk not fully encoded");

                        // Save off the last frame...
                        self.last_data_frame = Some(v);
                    }
                }
                Frame::Headers(v) => {
                    let mut buf = limited_write_buf!(self);
                    if let Some(continuation) = v.encode(&mut self.hpack, &mut buf) {
                        self.next = Some(Next::Continuation(continuation));
                    }
                }
                Frame::PushPromise(v) => {
                    let mut buf = limited_write_buf!(self);
                    if let Some(continuation) = v.encode(&mut self.hpack, &mut buf) {
                        self.next = Some(Next::Continuation(continuation));
                    }
                }
                Frame::Settings(v) => {
                    v.encode(self.buf.get_mut());
                    tracing::trace!(rem = self.buf.remaining(), "encoded settings");
                }
                Frame::GoAway(v) => {
                    v.encode(self.buf.get_mut());
                    tracing::trace!(rem = self.buf.remaining(), "encoded go_away");
                }
                Frame::Ping(v) => {
                    v.encode(self.buf.get_mut());
                    tracing::trace!(rem = self.buf.remaining(), "encoded ping");
                }
                Frame::WindowUpdate(v) => {
                    v.encode(self.buf.get_mut());
                    tracing::trace!(rem = self.buf.remaining(), "encoded window_update");
                }

                Frame::Priority(_) => {
                    /*
                    v.encode(self.buf.get_mut());
                    tracing::trace!("encoded priority; rem={:?}", self.buf.remaining());
                    */
                    unimplemented!();
                }
                Frame::Reset(v) => {
                    v.encode(self.buf.get_mut());
                    tracing::trace!(rem = self.buf.remaining(), "encoded reset");
                }
            }

            Ok(())
        }

        fn has_capacity(&self) -> bool {
            self.next.is_none() && self.buf.get_ref().remaining_mut() >= MIN_BUFFER_CAPACITY
        }

        fn is_empty(&self) -> bool {
            match self.next {
                Some(Next::Data(ref frame)) => !frame.payload().has_remaining(),
                _ => !self.buf.has_remaining(),
            }
        }
    }

    impl<B> Encoder<B> {
        fn max_frame_size(&self) -> usize {
            self.max_frame_size as usize
        }
    }

    impl<T, B> FramedWrite<T, B> {
        /// Returns the max frame size that can be sent
        pub fn max_frame_size(&self) -> usize {
            self.encoder.max_frame_size()
        }

        /// Set the peer's max frame size.
        pub fn set_max_frame_size(&mut self, val: usize) {
            assert!(val <= frame::MAX_MAX_FRAME_SIZE as usize);
            self.encoder.max_frame_size = val as FrameSize;
        }

        /// Set the peer's header table size.
        pub fn set_header_table_size(&mut self, val: usize) {
            self.encoder.hpack.update_max_size(val);
        }

        /// Retrieve the last data frame that has been sent
        pub fn take_last_data_frame(&mut self) -> Option<frame::Data<B>> {
            self.encoder.last_data_frame.take()
        }

        pub fn get_mut(&mut self) -> &mut T {
            &mut self.inner
        }
    }

    impl<T: AsyncRead + Unpin, B> AsyncRead for FramedWrite<T, B> {
        fn poll_read(
            mut self: Pin<&mut Self>,
            cx: &mut Context<'_>,
            buf: &mut ReadBuf,
        ) -> Poll<io::Result<()>> {
            Pin::new(&mut self.inner).poll_read(cx, buf)
        }
    }

    // We never project the Pin to `B`.
    impl<T: Unpin, B> Unpin for FramedWrite<T, B> {}

    #[cfg(feature = "unstable")]
    mod unstable {
        use super::*;

        impl<T, B> FramedWrite<T, B> {
            pub fn get_ref(&self) -> &T {
                &self.inner
            }
        }
    }