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    use super::assert_stream;
    use crate::stream::{StreamExt, Fuse};
    use core::pin::Pin;
    use futures_core::stream::{FusedStream, Stream};
    use futures_core::task::{Context, Poll};
    use pin_project_lite::pin_project;

    pin_project! {
        /// Stream for the [`select()`] function.
        #[derive(Debug)]
        #[must_use = "streams do nothing unless polled"]
        pub struct Select<St1, St2> {
            #[pin]
            stream1: Fuse<St1>,
            #[pin]
            stream2: Fuse<St2>,
            flag: bool,
        }
    }

    /// This function will attempt to pull items from both streams. Each
    /// stream will be polled in a round-robin fashion, and whenever a stream is
    /// ready to yield an item that item is yielded.
    ///
    /// After one of the two input stream completes, the remaining one will be
    /// polled exclusively. The returned stream completes when both input
    /// streams have completed.
    ///
    /// Note that this function consumes both streams and returns a wrapped
    /// version of them.
    pub fn select<St1, St2>(stream1: St1, stream2: St2) -> Select<St1, St2>
        where St1: Stream,
              St2: Stream<Item = St1::Item>
    {
        assert_stream::<St1::Item, _>(Select {
            stream1: stream1.fuse(),
            stream2: stream2.fuse(),
            flag: false,
        })
    }

    impl<St1, St2> Select<St1, St2> {
        /// Acquires a reference to the underlying streams that this combinator is
        /// pulling from.
        pub fn get_ref(&self) -> (&St1, &St2) {
            (self.stream1.get_ref(), self.stream2.get_ref())
        }

        /// Acquires a mutable reference to the underlying streams 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 St1, &mut St2) {
            (self.stream1.get_mut(), self.stream2.get_mut())
        }

        /// Acquires a pinned mutable reference to the underlying streams 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_pin_mut(self: Pin<&mut Self>) -> (Pin<&mut St1>, Pin<&mut St2>) {
            let this = self.project();
            (this.stream1.get_pin_mut(), this.stream2.get_pin_mut())
        }

        /// Consumes this combinator, returning the underlying streams.
        ///
        /// 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) -> (St1, St2) {
            (self.stream1.into_inner(), self.stream2.into_inner())
        }
    }

    impl<St1, St2> FusedStream for Select<St1, St2>
        where St1: Stream,
              St2: Stream<Item = St1::Item>
    {
        fn is_terminated(&self) -> bool {
            self.stream1.is_terminated() && self.stream2.is_terminated()
        }
    }

    impl<St1, St2> Stream for Select<St1, St2>
        where St1: Stream,
              St2: Stream<Item = St1::Item>
    {
        type Item = St1::Item;

        fn poll_next(
            self: Pin<&mut Self>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<St1::Item>> {
            let this = self.project();
            if !*this.flag {
                poll_inner(this.flag, this.stream1, this.stream2, cx)
            } else {
                poll_inner(this.flag, this.stream2, this.stream1, cx)
            }
        }
    }

    fn poll_inner<St1, St2>(
        flag: &mut bool,
        a: Pin<&mut St1>,
        b: Pin<&mut St2>,
        cx: &mut Context<'_>
    ) -> Poll<Option<St1::Item>>
        where St1: Stream, St2: Stream<Item = St1::Item>
    {
        let a_done = match a.poll_next(cx) {
            Poll::Ready(Some(item)) => {
                // give the other stream a chance to go first next time
                *flag = !*flag;
                return Poll::Ready(Some(item))
            },
            Poll::Ready(None) => true,
            Poll::Pending => false,
        };

        match b.poll_next(cx) {
            Poll::Ready(Some(item)) => {
                Poll::Ready(Some(item))
            }
            Poll::Ready(None) if a_done => Poll::Ready(None),
            Poll::Ready(None) | Poll::Pending => Poll::Pending,
        }
    }