use core::fmt;
use core::marker::PhantomData;
use core::pin::Pin;
use futures_core::ready;
use futures_core::stream::{Stream, FusedStream};
use futures_core::task::{Context, Poll};
use futures_sink::Sink;
use pin_project_lite::pin_project;

pin_project! {
    /// Sink for the [`with_flat_map`](super::SinkExt::with_flat_map) method.
    #[must_use = "sinks do nothing unless polled"]
    pub struct WithFlatMap<Si, Item, U, St, F> {
        #[pin]
        sink: Si,
        f: F,
        #[pin]
        stream: Option<St>,
        buffer: Option<Item>,
        _marker: PhantomData<fn(U)>,
    }
}

impl<Si, Item, U, St, F> fmt::Debug for WithFlatMap<Si, Item, U, St, F>
where
    Si: fmt::Debug,
    St: fmt::Debug,
    Item: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("WithFlatMap")
            .field("sink", &self.sink)
            .field("stream", &self.stream)
            .field("buffer", &self.buffer)
            .finish()
    }
}

impl<Si, Item, U, St, F> WithFlatMap<Si, Item, U, St, F>
where
    Si: Sink<Item>,
    F: FnMut(U) -> St,
    St: Stream<Item = Result<Item, Si::Error>>,
{
    pub(super) fn new(sink: Si, f: F) -> Self {
        Self {
            sink,
            f,
            stream: None,
            buffer: None,
            _marker: PhantomData,
        }
    }

    delegate_access_inner!(sink, Si, ());

    fn try_empty_stream(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), Si::Error>> {
        let mut this = self.project();

        if this.buffer.is_some() {
            ready!(this.sink.as_mut().poll_ready(cx))?;
            let item = this.buffer.take().unwrap();
            this.sink.as_mut().start_send(item)?;
        }
        if let Some(mut some_stream) = this.stream.as_mut().as_pin_mut() {
            while let Some(item) = ready!(some_stream.as_mut().poll_next(cx)?) {
                match this.sink.as_mut().poll_ready(cx)? {
                    Poll::Ready(()) => this.sink.as_mut().start_send(item)?,
                    Poll::Pending => {
                        *this.buffer = Some(item);
                        return Poll::Pending;
                    }
                };
            }
        }
        this.stream.set(None);
        Poll::Ready(Ok(()))
    }
}

// Forwarding impl of Stream from the underlying sink
impl<S, Item, U, St, F> Stream for WithFlatMap<S, Item, U, St, F>
where
    S: Stream + Sink<Item>,
    F: FnMut(U) -> St,
    St: Stream<Item = Result<Item, S::Error>>,
{
    type Item = S::Item;

    delegate_stream!(sink);
}

impl<S, Item, U, St, F> FusedStream for WithFlatMap<S, Item, U, St, F>
where
    S: FusedStream + Sink<Item>,
    F: FnMut(U) -> St,
    St: Stream<Item = Result<Item, S::Error>>,
{
    fn is_terminated(&self) -> bool {
        self.sink.is_terminated()
    }
}

impl<Si, Item, U, St, F> Sink<U> for WithFlatMap<Si, Item, U, St, F>
where
    Si: Sink<Item>,
    F: FnMut(U) -> St,
    St: Stream<Item = Result<Item, Si::Error>>,
{
    type Error = Si::Error;

    fn poll_ready(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), Self::Error>> {
        self.try_empty_stream(cx)
    }

    fn start_send(
        self: Pin<&mut Self>,
        item: U,
    ) -> Result<(), Self::Error> {
        let mut this = self.project();

        assert!(this.stream.is_none());
        this.stream.set(Some((this.f)(item)));
        Ok(())
    }

    fn poll_flush(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), Self::Error>> {
        ready!(self.as_mut().try_empty_stream(cx)?);
        self.project().sink.poll_flush(cx)
    }

    fn poll_close(
        mut self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Result<(), Self::Error>> {
        ready!(self.as_mut().try_empty_stream(cx)?);
        self.project().sink.poll_close(cx)
    }
}
