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    use std::mem;
    use std::prelude::v1::*;

    use {Async, Poll};
    use stream::{Stream, Fuse};

    /// An adaptor that chunks up elements in a vector.
    ///
    /// This adaptor will buffer up a list of items in the stream and pass on the
    /// vector used for buffering when a specified capacity has been reached. This
    /// is created by the `Stream::chunks` method.
    #[derive(Debug)]
    #[must_use = "streams do nothing unless polled"]
    pub struct Chunks<S>
        where S: Stream
    {
        items: Vec<S::Item>,
        err: Option<S::Error>,
        stream: Fuse<S>
    }

    pub fn new<S>(s: S, capacity: usize) -> Chunks<S>
        where S: Stream
    {
        assert!(capacity > 0);

        Chunks {
            items: Vec::with_capacity(capacity),
            err: None,
            stream: super::fuse::new(s),
        }
    }

    // Forwarding impl of Sink from the underlying stream
    impl<S> ::sink::Sink for Chunks<S>
        where S: ::sink::Sink + Stream
    {
        type SinkItem = S::SinkItem;
        type SinkError = S::SinkError;

        fn start_send(&mut self, item: S::SinkItem) -> ::StartSend<S::SinkItem, S::SinkError> {
            self.stream.start_send(item)
        }

        fn poll_complete(&mut self) -> Poll<(), S::SinkError> {
            self.stream.poll_complete()
        }

        fn close(&mut self) -> Poll<(), S::SinkError> {
            self.stream.close()
        }
    }

    impl<S> Chunks<S> where S: Stream {
        fn take(&mut self) -> Vec<S::Item> {
            let cap = self.items.capacity();
            mem::replace(&mut self.items, Vec::with_capacity(cap))
        }

        /// Acquires a reference to the underlying stream that this combinator is
        /// pulling from.
        pub fn get_ref(&self) -> &S {
            self.stream.get_ref()
        }

        /// Acquires a mutable reference to the underlying stream that this
        /// combinator is pulling from.
        ///
        /// Note that care must be taken to avoid tampering with the state of the
        /// stream which may otherwise confuse this combinator.
        pub fn get_mut(&mut self) -> &mut S {
            self.stream.get_mut()
        }

        /// Consumes this combinator, returning the underlying stream.
        ///
        /// Note that this may discard intermediate state of this combinator, so
        /// care should be taken to avoid losing resources when this is called.
        pub fn into_inner(self) -> S {
            self.stream.into_inner()
        }
    }

    impl<S> Stream for Chunks<S>
        where S: Stream
    {
        type Item = Vec<<S as Stream>::Item>;
        type Error = <S as Stream>::Error;

        fn poll(&mut self) -> Poll<Option<Self::Item>, Self::Error> {
            if let Some(err) = self.err.take() {
                return Err(err)
            }

            let cap = self.items.capacity();
            loop {
                match self.stream.poll() {
                    Ok(Async::NotReady) => return Ok(Async::NotReady),

                    // Push the item into the buffer and check whether it is full.
                    // If so, replace our buffer with a new and empty one and return
                    // the full one.
                    Ok(Async::Ready(Some(item))) => {
                        self.items.push(item);
                        if self.items.len() >= cap {
                            return Ok(Some(self.take()).into())
                        }
                    }

                    // Since the underlying stream ran out of values, return what we
                    // have buffered, if we have anything.
                    Ok(Async::Ready(None)) => {
                        return if self.items.len() > 0 {
                            let full_buf = mem::replace(&mut self.items, Vec::new());
                            Ok(Some(full_buf).into())
                        } else {
                            Ok(Async::Ready(None))
                        }
                    }

                    // If we've got buffered items be sure to return them first,
                    // we'll defer our error for later.
                    Err(e) => {
                        if self.items.len() == 0 {
                            return Err(e)
                        } else {
                            self.err = Some(e);
                            return Ok(Some(self.take()).into())
                        }
                    }
                }
            }
        }
    }

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