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// TODO: Eventually to be replaced with tower_util::Oneshot.

use pin_project::pin_project;
use tower_service::Service;

use crate::common::{task, Future, Pin, Poll};

pub(crate) fn oneshot<S, Req>(svc: S, req: Req) -> Oneshot<S, Req>
where
    S: Service<Req>,
{
    Oneshot {
        state: State::NotReady(svc, req),
    }
}

// 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`.
#[allow(missing_debug_implementations)]
#[pin_project]
pub struct Oneshot<S: Service<Req>, Req> {
    #[pin]
    state: State<S, Req>,
}

#[pin_project(project = StateProj, project_replace = StateProjOwn)]
enum State<S: Service<Req>, Req> {
    NotReady(S, Req),
    Called(#[pin] S::Future),
    Tmp,
}

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 task::Context<'_>) -> Poll<Self::Output> {
        let mut me = self.project();

        loop {
            match me.state.as_mut().project() {
                StateProj::NotReady(ref mut svc, _) => {
                    ready!(svc.poll_ready(cx))?;
                    // fallthrough out of the match's borrow
                }
                StateProj::Called(fut) => {
                    return fut.poll(cx);
                }
                StateProj::Tmp => unreachable!(),
            }

            match me.state.as_mut().project_replace(State::Tmp) {
                StateProjOwn::NotReady(mut svc, req) => {
                    me.state.set(State::Called(svc.call(req)));
                }
                _ => unreachable!(),
            }
        }
    }
}