use core::fmt;

use core::marker::PhantomData;

use core::pin::Pin;

use futures_core::future::Future;

use futures_core::ready;

use futures_core::stream::{FusedStream, Stream};

use futures_core::task::{Context, Poll};

use futures_sink::Sink;

use pin_project_lite::pin_project;

pin_project! {

    /// Sink for the [`with`](super::SinkExt::with) method.

    #[must_use = "sinks do nothing unless polled"]

    pub struct With<Si, Item, U, Fut, F> {

        #[pin]

        sink: Si,

        f: F,

        #[pin]

        state: Option<Fut>,

        _phantom: PhantomData<fn(U) -> Item>,

    }

}

impl<Si, Item, U, Fut, F> fmt::Debug for With<Si, Item, U, Fut, F>

where

    Si: fmt::Debug,

    Fut: fmt::Debug,

{

    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {

        f.debug_struct("With").field("sink", &self.sink).field("state", &self.state).finish()

    }

}

impl<Si, Item, U, Fut, F> With<Si, Item, U, Fut, F>

where

    Si: Sink<Item>,

    F: FnMut(U) -> Fut,

    Fut: Future,

{

    pub(super) fn new<E>(sink: Si, f: F) -> Self

    where

        Fut: Future<Output = Result<Item, E>>,

        E: From<Si::Error>,

    {

        Self { state: None, sink, f, _phantom: PhantomData }

    }

}

impl<Si, Item, U, Fut, F> Clone for With<Si, Item, U, Fut, F>

where

    Si: Clone,

    F: Clone,

    Fut: Clone,

{

    fn clone(&self) -> Self {

        Self {

            state: self.state.clone(),

            sink: self.sink.clone(),

            f: self.f.clone(),

            _phantom: PhantomData,

        }

    }

}

// Forwarding impl of Stream from the underlying sink

impl<S, Item, U, Fut, F> Stream for With<S, Item, U, Fut, F>

where

    S: Stream + Sink<Item>,

    F: FnMut(U) -> Fut,

    Fut: Future,

{

    type Item = S::Item;

    delegate_stream!(sink);

}

impl<S, Item, U, Fut, F> FusedStream for With<S, Item, U, Fut, F>

where

    S: FusedStream + Sink<Item>,

    F: FnMut(U) -> Fut,

    Fut: Future,

{

    fn is_terminated(&self) -> bool {

        self.sink.is_terminated()

    }

}

impl<Si, Item, U, Fut, F, E> With<Si, Item, U, Fut, F>

where

    Si: Sink<Item>,

    F: FnMut(U) -> Fut,

    Fut: Future<Output = Result<Item, E>>,

    E: From<Si::Error>,

{

    delegate_access_inner!(sink, Si, ());

    /// Completes the processing of previous item if any.

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<(), E>> {

        let mut this = self.project();

        let item = match this.state.as_mut().as_pin_mut() {

            None => return Poll::Ready(Ok(())),

            Some(fut) => ready!(fut.poll(cx))?,

        };

        this.state.set(None);

        this.sink.start_send(item)?;

        Poll::Ready(Ok(()))

    }

}

impl<Si, Item, U, Fut, F, E> Sink<U> for With<Si, Item, U, Fut, F>

where

    Si: Sink<Item>,

    F: FnMut(U) -> Fut,

    Fut: Future<Output = Result<Item, E>>,

    E: From<Si::Error>,

{

    type Error = E;

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

        ready!(self.as_mut().poll(cx))?;

        ready!(self.project().sink.poll_ready(cx)?);

        Poll::Ready(Ok(()))

    }

    fn start_send(self: Pin<&mut Self>, item: U) -> Result<(), Self::Error> {

        let mut this = self.project();

        assert!(this.state.is_none());

        this.state.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().poll(cx))?;

        ready!(self.project().sink.poll_flush(cx)?);

        Poll::Ready(Ok(()))

    }

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

        ready!(self.as_mut().poll(cx))?;

        ready!(self.project().sink.poll_close(cx)?);

        Poll::Ready(Ok(()))

    }

}