use crate::stream::{Fuse, FuturesUnordered, StreamExt, IntoStream};
use crate::future::{IntoFuture, TryFutureExt};
use futures_core::future::TryFuture;
use futures_core::stream::{Stream, TryStream};
use futures_core::task::{Context, Poll};
#[cfg(feature = "sink")]
use futures_sink::Sink;
use pin_project::{pin_project, project};
use core::pin::Pin;

/// Stream for the
/// [`try_buffer_unordered`](super::TryStreamExt::try_buffer_unordered) method.
#[pin_project]
#[derive(Debug)]
#[must_use = "streams do nothing unless polled"]
pub struct TryBufferUnordered<St>
    where St: TryStream
{
    #[pin]
    stream: Fuse<IntoStream<St>>,
    in_progress_queue: FuturesUnordered<IntoFuture<St::Ok>>,
    max: usize,
}

impl<St> TryBufferUnordered<St>
    where St: TryStream,
          St::Ok: TryFuture,
{
    pub(super) fn new(stream: St, n: usize) -> Self {
        TryBufferUnordered {
            stream: IntoStream::new(stream).fuse(),
            in_progress_queue: FuturesUnordered::new(),
            max: n,
        }
    }

    delegate_access_inner!(stream, St, (. .));
}

impl<St> Stream for TryBufferUnordered<St>
    where St: TryStream,
          St::Ok: TryFuture<Error = St::Error>,
{
    type Item = Result<<St::Ok as TryFuture>::Ok, St::Error>;

    #[project]
    fn poll_next(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Self::Item>> {
        #[project]
        let TryBufferUnordered { mut stream, in_progress_queue, max } = self.project();

        // First up, try to spawn off as many futures as possible by filling up
        // our queue of futures. Propagate errors from the stream immediately.
        while in_progress_queue.len() < *max {
            match stream.as_mut().poll_next(cx)? {
                Poll::Ready(Some(fut)) => in_progress_queue.push(fut.into_future()),
                Poll::Ready(None) | Poll::Pending => break,
            }
        }

        // Attempt to pull the next value from the in_progress_queue
        match in_progress_queue.poll_next_unpin(cx) {
            x @ Poll::Pending | x @ Poll::Ready(Some(_)) => return x,
            Poll::Ready(None) => {}
        }

        // If more values are still coming from the stream, we're not done yet
        if stream.is_done() {
            Poll::Ready(None)
        } else {
            Poll::Pending
        }
    }
}

// Forwarding impl of Sink from the underlying stream
#[cfg(feature = "sink")]
impl<S, Item, E> Sink<Item> for TryBufferUnordered<S>
    where S: TryStream + Sink<Item, Error = E>,
          S::Ok: TryFuture<Error = E>,
{
    type Error = E;

    delegate_sink!(stream, Item);
}