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    use core::fmt::{Debug, Formatter, Result as FmtResult};
    use core::mem::replace;

    use {Async, AsyncSink, Poll, Sink, StartSend};

    /// Sink that clones incoming items and forwards them to two sinks at the same time.
    ///
    /// Backpressure from any downstream sink propagates up, which means that this sink
    /// can only process items as fast as its _slowest_ downstream sink.
    pub struct Fanout<A: Sink, B: Sink> {
        left: Downstream<A>,
        right: Downstream<B>
    }

    impl<A: Sink, B: Sink> Fanout<A, B> {
        /// Consumes this combinator, returning the underlying sinks.
        ///
        /// 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) -> (A, B) {
            (self.left.sink, self.right.sink)
        }
    }

    impl<A: Sink + Debug, B: Sink + Debug> Debug for Fanout<A, B>
        where A::SinkItem: Debug,
              B::SinkItem: Debug
    {
        fn fmt(&self, f: &mut Formatter) -> FmtResult {
            f.debug_struct("Fanout")
                .field("left", &self.left)
                .field("right", &self.right)
                .finish()
        }
    }

    pub fn new<A: Sink, B: Sink>(left: A, right: B) -> Fanout<A, B> {
        Fanout {
            left: Downstream::new(left),
            right: Downstream::new(right)
        }
    }

    impl<A, B> Sink for Fanout<A, B>
        where A: Sink,
              A::SinkItem: Clone,
              B: Sink<SinkItem=A::SinkItem, SinkError=A::SinkError>
    {
        type SinkItem = A::SinkItem;
        type SinkError = A::SinkError;

        fn start_send(
            &mut self,
            item: Self::SinkItem
        ) -> StartSend<Self::SinkItem, Self::SinkError> {
            // Attempt to complete processing any outstanding requests.
            self.left.keep_flushing()?;
            self.right.keep_flushing()?;
            // Only if both downstream sinks are ready, start sending the next item.
            if self.left.is_ready() && self.right.is_ready() {
                self.left.state = self.left.sink.start_send(item.clone())?;
                self.right.state = self.right.sink.start_send(item)?;
                Ok(AsyncSink::Ready)
            } else {
                Ok(AsyncSink::NotReady(item))
            }
        }

        fn poll_complete(&mut self) -> Poll<(), Self::SinkError> {
            let left_async = self.left.poll_complete()?;
            let right_async = self.right.poll_complete()?;
            // Only if both downstream sinks are ready, signal readiness.
            if left_async.is_ready() && right_async.is_ready() {
                Ok(Async::Ready(()))
            } else {
                Ok(Async::NotReady)
            }
        }

        fn close(&mut self) -> Poll<(), Self::SinkError> {
            let left_async = self.left.close()?;
            let right_async = self.right.close()?;
            // Only if both downstream sinks are ready, signal readiness.
            if left_async.is_ready() && right_async.is_ready() {
                Ok(Async::Ready(()))
            } else {
                Ok(Async::NotReady)
            }
        }
    }

    #[derive(Debug)]
    struct Downstream<S: Sink> {
        sink: S,
        state: AsyncSink<S::SinkItem>
    }

    impl<S: Sink> Downstream<S> {
        fn new(sink: S) -> Self {
            Downstream { sink: sink, state: AsyncSink::Ready }
        }

        fn is_ready(&self) -> bool {
            self.state.is_ready()
        }

        fn keep_flushing(&mut self) -> Result<(), S::SinkError> {
            if let AsyncSink::NotReady(item) = replace(&mut self.state, AsyncSink::Ready) {
                self.state = self.sink.start_send(item)?;
            }
            Ok(())
        }

        fn poll_complete(&mut self) -> Poll<(), S::SinkError> {
            self.keep_flushing()?;
            let async = self.sink.poll_complete()?;
            // Only if all values have been sent _and_ the underlying
            // sink is completely flushed, signal readiness.
            if self.state.is_ready() && async.is_ready() {
                Ok(Async::Ready(()))
            } else {
                Ok(Async::NotReady)
            }
        }

        fn close(&mut self) -> Poll<(), S::SinkError> {
            self.keep_flushing()?;
            // If all items have been flushed, initiate close.
            if self.state.is_ready() {
                self.sink.close()
            } else {
                Ok(Async::NotReady)
            }
        }
    }

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