use futures_core::ready;

use pin_project_lite::pin_project;

use std::{

    fmt,

    future::Future,

    pin::Pin,

    task::{Context, Poll},

};

use tower_service::Service;

pin_project! {

    /// A [`Future`] consuming a [`Service`] and request, waiting until the [`Service`]

    /// is ready, and then calling [`Service::call`] with the request, and

    /// waiting for that [`Future`].

    #[derive(Debug)]

    pub struct Oneshot<S: Service<Req>, Req> {

        #[pin]

        state: State<S, Req>,

    }

}

pin_project! {

    #[project = StateProj]

    enum State<S: Service<Req>, Req> {

        NotReady {

            svc: S,

            req: Option<Req>,

        },

        Called {

            #[pin]

            fut: S::Future,

        },

        Done,

    }

}

impl<S: Service<Req>, Req> State<S, Req> {

    fn not_ready(svc: S, req: Option<Req>) -> Self {

        Self::NotReady { svc, req }

    }

    fn called(fut: S::Future) -> Self {

        Self::Called { fut }

    }

}

impl<S, Req> fmt::Debug for State<S, Req>

where

    S: Service<Req> + fmt::Debug,

    Req: fmt::Debug,

{

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

        match self {

            State::NotReady {

                svc,

                req: Some(req),

            } => f

                .debug_tuple("State::NotReady")

                .field(svc)

                .field(req)

                .finish(),

            State::NotReady { req: None, .. } => unreachable!(),

            State::Called { .. } => f.debug_tuple("State::Called").field(&"S::Future").finish(),

            State::Done => f.debug_tuple("State::Done").finish(),

        }

    }

}

impl<S, Req> Oneshot<S, Req>

where

    S: Service<Req>,

{

    #[allow(missing_docs)]

    pub fn new(svc: S, req: Req) -> Self {

        Oneshot {

            state: State::not_ready(svc, Some(req)),

        }

    }

}

impl<S, Req> Future for Oneshot<S, Req>

where

    S: Service<Req>,

{

    type Output = Result<S::Response, S::Error>;

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

        let mut this = self.project();

        loop {

            match this.state.as_mut().project() {

                StateProj::NotReady { svc, req } => {

                    let _ = ready!(svc.poll_ready(cx))?;

                    let f = svc.call(req.take().expect("already called"));

                    this.state.set(State::called(f));

                }

                StateProj::Called { fut } => {

                    let res = ready!(fut.poll(cx))?;

                    this.state.set(State::Done);

                    return Poll::Ready(Ok(res));

                }

                StateProj::Done => panic!("polled after complete"),

            }

        }

    }

}