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use core::pin::Pin;
use futures_core::ready;
use futures_core::stream::{FusedStream, Stream};
use futures_core::task::{Context, Poll};
use pin_utils::unsafe_pinned;

impl<T: ?Sized> StreamExt for T where T: Stream {}

/// An extension trait for Streams that provides a variety of convenient combinator functions.
pub trait StreamExt: Stream {
    /// Flattens a stream of iterators into one continuous stream.
    fn flatten_iters(self) -> FlattenIters<Self>
    where
        Self::Item: IntoIterator,
        Self: Sized,
    {
        FlattenIters::new(self)
    }
}

/// Stream for the [`flatten_iters`](StreamExt::flatten_iters) method.
// #[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct FlattenIters<St>
where
    St: Stream,
    St::Item: IntoIterator,
{
    stream: St,
    next: Option<<St::Item as IntoIterator>::IntoIter>,
}

impl<St> Unpin for FlattenIters<St>
where
    St: Stream + Unpin,
    St::Item: IntoIterator,
    <St::Item as IntoIterator>::IntoIter: Unpin,
{
}

impl<St> FlattenIters<St>
where
    St: Stream,
    St::Item: IntoIterator,
{
    unsafe_pinned!(stream: St);
    unsafe_pinned!(next: Option<<St::Item as IntoIterator>::IntoIter>);
}

impl<St> FlattenIters<St>
where
    St: Stream,
    St::Item: IntoIterator,
{
    pub(crate) fn new(stream: St) -> Self {
        Self { stream, next: None }
    }

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

    /// 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 St {
        &mut self.stream
    }

    /// Acquires a pinned 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_pin_mut(self: Pin<&mut Self>) -> Pin<&mut St> {
        self.stream()
    }

    /// 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) -> St {
        self.stream
    }
}

impl<St> FusedStream for FlattenIters<St>
where
    St: FusedStream,
    St::Item: IntoIterator,
    <St::Item as IntoIterator>::IntoIter: Unpin,
{
    fn is_terminated(&self) -> bool {
        self.next.is_none() && self.stream.is_terminated()
    }
}

impl<St> Stream for FlattenIters<St>
where
    St: Stream,
    St::Item: IntoIterator,
    <St::Item as IntoIterator>::IntoIter: Unpin,
{
    type Item = <St::Item as IntoIterator>::Item;

    fn poll_next(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        loop {
            if self.next.is_none() {
                match ready!(self.as_mut().stream().poll_next(cx)) {
                    Some(e) => self.as_mut().next().set(Some(e.into_iter())),
                    None => return Poll::Ready(None),
                }
            }

            if let Some(item) = Option::as_mut(&mut self.as_mut().next()).unwrap().next() {
                return Poll::Ready(Some(item));
            } else {
                self.as_mut().next().set(None);
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use super::StreamExt as _;
    use futures::stream::{iter, StreamExt};

    #[tokio::test]
    async fn test_basic() {
        let mut stream =
            iter(vec![vec![0_usize, 1, 2], vec![3, 4], vec![], vec![5, 6, 7]]).flatten_iters();

        assert_eq!(stream.next().await, Some(0));
        assert_eq!(stream.next().await, Some(1));
        assert_eq!(stream.next().await, Some(2));
        assert_eq!(stream.next().await, Some(3));
        assert_eq!(stream.next().await, Some(4));
        assert_eq!(stream.next().await, Some(5));
        assert_eq!(stream.next().await, Some(6));
        assert_eq!(stream.next().await, Some(7));
        assert_eq!(stream.next().await, None);
    }

    #[tokio::test]
    async fn test_empty() {
        let mut stream = iter(Vec::<Vec<String>>::new()).flatten_iters();

        assert_eq!(stream.next().await, None);
    }
}