use core::fmt;
use core::pin::Pin;
use futures_core::future::{TryFuture};
use futures_core::stream::{Stream, TryStream, FusedStream};
use futures_core::task::{Context, Poll};
#[cfg(feature = "sink")]
use futures_sink::Sink;
use pin_project::pin_project;

/// Stream for the [`try_filter_map`](super::TryStreamExt::try_filter_map)
/// method.
#[pin_project]
#[must_use = "streams do nothing unless polled"]
pub struct TryFilterMap<St, Fut, F> {
    #[pin]
    stream: St,
    f: F,
    #[pin]
    pending: Option<Fut>,
}

impl<St, Fut, F> fmt::Debug for TryFilterMap<St, Fut, F>
where
    St: fmt::Debug,
    Fut: fmt::Debug,
{
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("TryFilterMap")
            .field("stream", &self.stream)
            .field("pending", &self.pending)
            .finish()
    }
}

impl<St, Fut, F> TryFilterMap<St, Fut, F> {
    pub(super) fn new(stream: St, f: F) -> Self {
        TryFilterMap { stream, f, pending: None }
    }

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

impl<St, Fut, F, T> FusedStream for TryFilterMap<St, Fut, F>
    where St: TryStream + FusedStream,
          Fut: TryFuture<Ok = Option<T>, Error = St::Error>,
          F: FnMut(St::Ok) -> Fut,
{
    fn is_terminated(&self) -> bool {
        self.pending.is_none() && self.stream.is_terminated()
    }
}

impl<St, Fut, F, T> Stream for TryFilterMap<St, Fut, F>
    where St: TryStream,
          Fut: TryFuture<Ok = Option<T>, Error = St::Error>,
          F: FnMut(St::Ok) -> Fut,
{
    type Item = Result<T, St::Error>;

    fn poll_next(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
    ) -> Poll<Option<Self::Item>> {
        let mut this = self.project();

        Poll::Ready(loop {
            if let Some(p) = this.pending.as_mut().as_pin_mut() {
                // We have an item in progress, poll that until it's done
                let item = ready!(p.try_poll(cx)?);
                this.pending.set(None);
                if item.is_some() {
                    break item.map(Ok);
                }
            } else if let Some(item) = ready!(this.stream.as_mut().try_poll_next(cx)?) {
                // No item in progress, but the stream is still going
                this.pending.set(Some((this.f)(item)));
            } else {
                // The stream is done
                break None;
            }
        })
    }

    fn size_hint(&self) -> (usize, Option<usize>) {
        let pending_len = if self.pending.is_some() { 1 } else { 0 };
        let (_, upper) = self.stream.size_hint();
        let upper = match upper {
            Some(x) => x.checked_add(pending_len),
            None => None,
        };
        (0, upper) // can't know a lower bound, due to the predicate
    }
}

// Forwarding impl of Sink from the underlying stream
#[cfg(feature = "sink")]
impl<S, Fut, F, Item> Sink<Item> for TryFilterMap<S, Fut, F>
    where S: Sink<Item>,
{
    type Error = S::Error;

    delegate_sink!(stream, Item);
}
