use crate::*;
use core::mem;
use core::pin::Pin;
use core::task::{Context, Poll};

pin_project_lite::pin_project! {
    /// A stream for the [`join`](fn.join.html) function.
    #[derive(Debug)]
    pub struct Join<A, B>
    where
        A: OrderedStream,
        B: OrderedStream<Data = A::Data, Ordering=A::Ordering>,
    {
        #[pin]
        stream_a: A,
        #[pin]
        stream_b: B,
        state: JoinState<A::Data, B::Data, A::Ordering>,
    }
}

/// Join two streams while preserving the overall ordering of elements.
///
/// You can think of this as implementing the "merge" step of a merge sort on the two streams,
/// producing a single stream that is sorted given two sorted streams.  If the streams return
/// [`PollResult::NoneBefore`] as intended, then the joined stream will be able to produce items
/// when only one of the sources has unblocked.
pub fn join<A, B>(stream_a: A, stream_b: B) -> Join<A, B>
where
    A: OrderedStream,
    B: OrderedStream<Data = A::Data, Ordering = A::Ordering>,
{
    Join {
        stream_a,
        stream_b,
        state: JoinState::None,
    }
}

#[derive(Debug)]
enum JoinState<A, B, T> {
    None,
    A(A, T),
    B(B, T),
    OnlyPollA,
    OnlyPollB,
    Terminated,
}

impl<A, B, T> JoinState<A, B, T> {
    fn take_split(&mut self) -> (PollState<A, T>, PollState<B, T>) {
        match mem::replace(self, JoinState::None) {
            JoinState::None => (PollState::Pending, PollState::Pending),
            JoinState::A(a, t) => (PollState::Item(a, t), PollState::Pending),
            JoinState::B(b, t) => (PollState::Pending, PollState::Item(b, t)),
            JoinState::OnlyPollA => (PollState::Pending, PollState::Terminated),
            JoinState::OnlyPollB => (PollState::Terminated, PollState::Pending),
            JoinState::Terminated => (PollState::Terminated, PollState::Terminated),
        }
    }
}

/// A helper equivalent to Poll<PollResult<T, I>> but easier to match
pub(crate) enum PollState<I, T> {
    Item(I, T),
    Pending,
    NoneBefore,
    Terminated,
}

impl<I, T: Ord> PollState<I, T> {
    fn ordering(&self) -> Option<&T> {
        match self {
            Self::Item(_, t) => Some(t),
            _ => None,
        }
    }

    fn update(
        &mut self,
        before: Option<&T>,
        other_token: Option<&T>,
        retry: bool,
        run: impl FnOnce(Option<&T>) -> Poll<PollResult<T, I>>,
    ) -> bool {
        match self {
            // Do not re-poll if we have an item already or if we are terminated
            Self::Item { .. } | Self::Terminated => return false,

            // No need to re-poll if we already declared no items <= before
            Self::NoneBefore if retry => return false,

            _ => {}
        }

        // Run the poll with the earlier of the two tokens to avoid transitioning to Pending (which
        // will stall the Join) when we could have transitioned to NoneBefore.
        let ordering = match (before, other_token) {
            (Some(u), Some(o)) => {
                if *u > *o {
                    // The other ordering is earlier - so a retry might let us upgrade a Pending to a
                    // NoneBefore
                    Some(o)
                } else if retry {
                    // A retry will not improve matters, so don't bother
                    return false;
                } else {
                    Some(u)
                }
            }
            (Some(t), None) | (None, Some(t)) => Some(t),
            (None, None) => None,
        };

        *self = run(ordering).into();
        matches!(self, Self::Item { .. })
    }
}

impl<I, T> From<PollState<I, T>> for Poll<PollResult<T, I>> {
    fn from(poll: PollState<I, T>) -> Self {
        match poll {
            PollState::Item(data, ordering) => Poll::Ready(PollResult::Item { data, ordering }),
            PollState::Pending => Poll::Pending,
            PollState::NoneBefore => Poll::Ready(PollResult::NoneBefore),
            PollState::Terminated => Poll::Ready(PollResult::Terminated),
        }
    }
}

impl<I, T> From<Poll<PollResult<T, I>>> for PollState<I, T> {
    fn from(poll: Poll<PollResult<T, I>>) -> Self {
        match poll {
            Poll::Ready(PollResult::Item { data, ordering }) => Self::Item(data, ordering),
            Poll::Ready(PollResult::NoneBefore) => Self::NoneBefore,
            Poll::Ready(PollResult::Terminated) => Self::Terminated,
            Poll::Pending => Self::Pending,
        }
    }
}

impl<A, B> OrderedStream for Join<A, B>
where
    A: OrderedStream,
    B: OrderedStream<Data = A::Data, Ordering = A::Ordering>,
{
    type Data = A::Data;
    type Ordering = A::Ordering;

    fn poll_next_before(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        before: Option<&Self::Ordering>,
    ) -> Poll<PollResult<Self::Ordering, Self::Data>> {
        let mut this = self.project();
        let (mut poll_a, mut poll_b) = this.state.take_split();

        poll_a.update(before, poll_b.ordering(), false, |ordering| {
            this.stream_a.as_mut().poll_next_before(cx, ordering)
        });
        if poll_b.update(before, poll_a.ordering(), false, |ordering| {
            this.stream_b.as_mut().poll_next_before(cx, ordering)
        }) {
            // If B just got an item, it's possible that A already knows that it won't have any
            // items before that item; we couldn't ask that question before.  Ask it now.
            poll_a.update(before, poll_b.ordering(), true, |ordering| {
                this.stream_a.as_mut().poll_next_before(cx, ordering)
            });
        }

        match (poll_a, poll_b) {
            // Both are ready - we can judge ordering directly (simplest case).  The first one is
            // returned while the other one is buffered for the next poll.
            (PollState::Item(a, ta), PollState::Item(b, tb)) => {
                if ta <= tb {
                    *this.state = JoinState::B(b, tb);
                    Poll::Ready(PollResult::Item {
                        data: a,
                        ordering: ta,
                    })
                } else {
                    *this.state = JoinState::A(a, ta);
                    Poll::Ready(PollResult::Item {
                        data: b,
                        ordering: tb,
                    })
                }
            }

            // If both sides are terminated, so are we.
            (PollState::Terminated, PollState::Terminated) => {
                *this.state = JoinState::Terminated;
                Poll::Ready(PollResult::Terminated)
            }

            // If one side is terminated, we can produce items directly from the other side.
            (a, PollState::Terminated) => {
                *this.state = JoinState::OnlyPollA;
                a.into()
            }
            (PollState::Terminated, b) => {
                *this.state = JoinState::OnlyPollB;
                b.into()
            }

            // If one side is pending, we can't return Ready until that gets resolved.  Because we
            // have already requested that our child streams wake us when it is possible to make
            // any kind of progress, we meet the requirements to return Poll::Pending.
            (PollState::Item(a, t), PollState::Pending) => {
                *this.state = JoinState::A(a, t);
                Poll::Pending
            }
            (PollState::Pending, PollState::Item(b, t)) => {
                *this.state = JoinState::B(b, t);
                Poll::Pending
            }
            (PollState::Pending, PollState::Pending) => Poll::Pending,
            (PollState::Pending, PollState::NoneBefore) => Poll::Pending,
            (PollState::NoneBefore, PollState::Pending) => Poll::Pending,

            // If both sides report NoneBefore, so can we.
            (PollState::NoneBefore, PollState::NoneBefore) => Poll::Ready(PollResult::NoneBefore),

            (PollState::Item(data, ordering), PollState::NoneBefore) => {
                // B was polled using either the Some value of (before) or using A's ordering.
                //
                // If before is set and is earlier than A's ordering, then B might later produce a
                // value with (bt >= before && bt < at), so we can't return A's item yet and must
                // buffer it.  However, we can return None because neither stream will produce
                // items before the ordering passed in before.
                //
                // If before is either None or after A's ordering, B's NoneBefore return represents a
                // promise to not produce an item before A's, so we can return A's item now.
                match before {
                    Some(before) if ordering > *before => {
                        *this.state = JoinState::A(data, ordering);
                        Poll::Ready(PollResult::NoneBefore)
                    }
                    _ => Poll::Ready(PollResult::Item { data, ordering }),
                }
            }

            (PollState::NoneBefore, PollState::Item(data, ordering)) => {
                // A was polled using either the Some value of (before) or using B's ordering.
                //
                // By a mirror of the above argument, this NoneBefore result gives us permission to
                // produce either B's item or NoneBefore.
                match before {
                    Some(before) if ordering > *before => {
                        *this.state = JoinState::B(data, ordering);
                        Poll::Ready(PollResult::NoneBefore)
                    }
                    _ => Poll::Ready(PollResult::Item { data, ordering }),
                }
            }
        }
    }
}

impl<A, B> FusedOrderedStream for Join<A, B>
where
    A: OrderedStream,
    B: OrderedStream<Data = A::Data, Ordering = A::Ordering>,
{
    fn is_terminated(&self) -> bool {
        matches!(self.state, JoinState::Terminated)
    }
}