use pin_project_lite::pin_project;

use std::{

    future::Future,

    pin::Pin,

    task::{Context, Poll},

};

use tower_service::Service;

/// A policy which decides which requests can be cloned and sent to the B

/// service.

pub trait Policy<Request> {

    fn clone_request(&self, req: &Request) -> Option<Request>;

}

/// Select is a middleware which attempts to clone the request and sends the

/// original request to the A service and, if the request was able to be cloned,

/// the cloned request to the B service.  Both resulting futures will be polled

/// and whichever future completes first will be used as the result.

#[derive(Debug)]

pub struct Select<P, A, B> {

    policy: P,

    a: A,

    b: B,

}

pin_project! {

    #[derive(Debug)]

    pub struct ResponseFuture<AF, BF> {

        #[pin]

        a_fut: AF,

        #[pin]

        b_fut: Option<BF>,

    }

}

impl<P, A, B> Select<P, A, B> {

    pub const fn new<Request>(policy: P, a: A, b: B) -> Self

    where

        P: Policy<Request>,

        A: Service<Request>,

        A::Error: Into<crate::BoxError>,

        B: Service<Request, Response = A::Response>,

        B::Error: Into<crate::BoxError>,

    {

        Select { policy, a, b }

    }

}

impl<P, A, B, Request> Service<Request> for Select<P, A, B>

where

    P: Policy<Request>,

    A: Service<Request>,

    A::Error: Into<crate::BoxError>,

    B: Service<Request, Response = A::Response>,

    B::Error: Into<crate::BoxError>,

{

    type Response = A::Response;

    type Error = crate::BoxError;

    type Future = ResponseFuture<A::Future, B::Future>;

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

        match (self.a.poll_ready(cx), self.b.poll_ready(cx)) {

            (Poll::Ready(Ok(())), Poll::Ready(Ok(()))) => Poll::Ready(Ok(())),

            (Poll::Ready(Err(e)), _) => Poll::Ready(Err(e.into())),

            (_, Poll::Ready(Err(e))) => Poll::Ready(Err(e.into())),

            _ => Poll::Pending,

        }

    }

    fn call(&mut self, request: Request) -> Self::Future {

        let b_fut = if let Some(cloned_req) = self.policy.clone_request(&request) {

            Some(self.b.call(cloned_req))

        } else {

            None

        };

        ResponseFuture {

            a_fut: self.a.call(request),

            b_fut,

        }

    }

}

impl<AF, BF, T, AE, BE> Future for ResponseFuture<AF, BF>

where

    AF: Future<Output = Result<T, AE>>,

    AE: Into<crate::BoxError>,

    BF: Future<Output = Result<T, BE>>,

    BE: Into<crate::BoxError>,

{

    type Output = Result<T, crate::BoxError>;

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

        let this = self.project();

        if let Poll::Ready(r) = this.a_fut.poll(cx) {

            return Poll::Ready(Ok(r.map_err(Into::into)?));

        }

        if let Some(b_fut) = this.b_fut.as_pin_mut() {

            if let Poll::Ready(r) = b_fut.poll(cx) {

                return Poll::Ready(Ok(r.map_err(Into::into)?));

            }

        }

        Poll::Pending

    }

}