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use crate::stream::IntoStream;
use crate::utils;
use crate::Merge as MergeTrait;

use futures_core::Stream;
use std::pin::Pin;
use std::task::{Context, Poll};

impl<T, S0, S1> MergeTrait for (S0, S1)
where
    S0: IntoStream<Item = T>,
    S1: IntoStream<Item = T>,
{
    type Item = T;
    type Stream = Merge2<T, S0::IntoStream, S1::IntoStream>;

    fn merge(self) -> Self::Stream {
        Merge2::new((self.0.into_stream(), self.1.into_stream()))
    }
}

/// A stream that merges multiple streams into a single stream.
///
/// This `struct` is created by the [`merge`] method on [`Stream`]. See its
/// documentation for more.
///
/// [`merge`]: trait.Stream.html#method.merge
/// [`Stream`]: trait.Stream.html
#[derive(Debug)]
#[pin_project::pin_project]
pub struct Merge2<T, S0, S1>
where
    S0: Stream<Item = T>,
    S1: Stream<Item = T>,
{
    streams: (S0, S1),
}

impl<T, S0, S1> Merge2<T, S0, S1>
where
    S0: Stream<Item = T>,
    S1: Stream<Item = T>,
{
    pub(crate) fn new(streams: (S0, S1)) -> Self {
        Self { streams }
    }
}

impl<T, S0, S1> Stream for Merge2<T, S0, S1>
where
    S0: Stream<Item = T>,
    S1: Stream<Item = T>,
{
    type Item = T;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        let this = self.project();
        // SAFETY: we're manually projecting the tuple fields here.
        let s0 = unsafe { Pin::new_unchecked(&mut this.streams.0) };
        let s1 = unsafe { Pin::new_unchecked(&mut this.streams.1) };
        match utils::random(2) {
            0 => poll_next_in_order(s0, s1, cx),
            1 => poll_next_in_order(s1, s0, cx),
            _ => unreachable!(),
        }
    }
}
fn poll_next_in_order<S0, S1, T>(
    s0: Pin<&mut S0>,
    s1: Pin<&mut S1>,
    cx: &mut Context<'_>,
) -> Poll<Option<T>>
where
    S0: Stream<Item = T>,
    S1: Stream<Item = T>,
{
    match s0.poll_next(cx) {
        Poll::Ready(None) => s1.poll_next(cx),
        Poll::Ready(item) => Poll::Ready(item),
        Poll::Pending => match s1.poll_next(cx) {
            Poll::Ready(None) | Poll::Pending => Poll::Pending,
            Poll::Ready(item) => Poll::Ready(item),
        },
    }
}

// TODO: automate this!

impl<T, S0, S1, S2> MergeTrait for (S0, S1, S2)
where
    S0: IntoStream<Item = T>,
    S1: IntoStream<Item = T>,
    S2: IntoStream<Item = T>,
{
    type Item = T;
    type Stream = Merge3<T, S0::IntoStream, S1::IntoStream, S2::IntoStream>;

    fn merge(self) -> Self::Stream {
        Merge3::new((
            self.0.into_stream(),
            self.1.into_stream(),
            self.2.into_stream(),
        ))
    }
}

/// A stream that merges multiple streams into a single stream.
///
/// This `struct` is created by the [`merge`] method on [`Stream`]. See its
/// documentation for more.
///
/// [`merge`]: trait.Stream.html#method.merge
/// [`Stream`]: trait.Stream.html
#[derive(Debug)]
#[pin_project::pin_project]
pub struct Merge3<T, S0, S1, S2>
where
    S0: Stream<Item = T>,
    S1: Stream<Item = T>,
{
    streams: (S0, S1, S2),
}

impl<T, S0, S1, S2> Merge3<T, S0, S1, S2>
where
    S0: Stream<Item = T>,
    S1: Stream<Item = T>,
    S2: Stream<Item = T>,
{
    pub(crate) fn new(streams: (S0, S1, S2)) -> Self {
        Self { streams }
    }
}

impl<T, S0, S1, S2> Stream for Merge3<T, S0, S1, S2>
where
    S0: Stream<Item = T>,
    S1: Stream<Item = T>,
    S2: Stream<Item = T>,
{
    type Item = T;

    fn poll_next(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Option<Self::Item>> {
        fn poll_next_in_order<S0, S1, S2, T>(
            s0: Pin<&mut S0>,
            s1: Pin<&mut S1>,
            s2: Pin<&mut S2>,
            cx: &mut Context<'_>,
        ) -> Poll<Option<T>>
        where
            S0: Stream<Item = T>,
            S1: Stream<Item = T>,
            S2: Stream<Item = T>,
        {
            match s0.poll_next(cx) {
                Poll::Ready(None) => s1.poll_next(cx),
                Poll::Ready(item) => Poll::Ready(item),
                Poll::Pending => match s1.poll_next(cx) {
                    Poll::Ready(None) => s2.poll_next(cx),
                    Poll::Ready(item) => Poll::Ready(item),
                    Poll::Pending => match s2.poll_next(cx) {
                        Poll::Ready(None) | Poll::Pending => Poll::Pending,
                        Poll::Ready(item) => Poll::Ready(item),
                    },
                },
            }
        }
        let this = self.project();
        // SAFETY: we're manually projecting the tuple fields here.
        let s0 = unsafe { Pin::new_unchecked(&mut this.streams.0) };
        let s1 = unsafe { Pin::new_unchecked(&mut this.streams.1) };
        let s2 = unsafe { Pin::new_unchecked(&mut this.streams.2) };
        match utils::random(6) {
            0 => poll_next_in_order(s0, s1, s2, cx),
            1 => poll_next_in_order(s0, s2, s1, cx),
            2 => poll_next_in_order(s1, s0, s2, cx),
            3 => poll_next_in_order(s1, s2, s0, cx),
            4 => poll_next_in_order(s2, s0, s1, cx),
            5 => poll_next_in_order(s2, s1, s0, cx),
            _ => unreachable!(),
        }
    }
}