use crate::metrics::Observer;
use std::{
    future::Future,
    pin::Pin,
    task::{Context, Poll},
};

enum State {
    Passive,
    Polling,
}

pub struct ObservedFuture<Obs, Fut>
where
    Obs: Observer<Fut::Output>,
    Fut: Future,
{
    state: State,
    observer: Obs,
    inner: Fut,
}

impl<Obs, Fut> Future for ObservedFuture<Obs, Fut>
where
    Obs: Observer<Fut::Output>,
    Fut: Future,
{
    type Output = Fut::Output;

    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        // Safety: We're not moving both the inner future and the observer.
        let this = unsafe { self.get_unchecked_mut() };
        let inner = unsafe { Pin::new_unchecked(&mut this.inner) };
        if matches!(this.state, State::Passive) {
            // Interface allows us to call this code twice, but since the user
            // is required to make observer behavior unspecified but safe in
            // this cases, we're ok with just two states.
            this.state = State::Polling;
            this.observer.on_first_poll();
        }
        this.observer.on_poll();
        match inner.poll(cx) {
            Poll::Ready(output) => {
                this.state = State::Passive;
                this.observer.on_poll_ready(&output);
                this.observer.on_finish(Some(&output));
                Poll::Ready(output)
            }
            Poll::Pending => Poll::Pending,
        }
    }
}

impl<Obs, Fut> Drop for ObservedFuture<Obs, Fut>
where
    Obs: Observer<Fut::Output>,
    Fut: Future,
{
    fn drop(&mut self) {
        if matches!(self.state, State::Polling) {
            self.observer.on_finish(None);
        }
        self.observer.on_drop();
    }
}

pub trait ObservedFutureExt: Future + Sized {
    fn observe<Obs: Observer<Self::Output>>(self, observer: Obs) -> ObservedFuture<Obs, Self> {
        ObservedFuture {
            state: State::Passive,
            observer,
            inner: self,
        }
    }
}

impl<F: Future + Sized> ObservedFutureExt for F {}