//! [`PollAll`]

use futures::FutureExt as _;

use smallvec::{SmallVec, smallvec};

use std::future::Future;

use std::pin::Pin;

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

/// The future type in a [`PollAll`].

type BoxedFut<'a, T> = Pin<Box<dyn Future<Output = T> + Send + 'a>>;

/// Helper for driving multiple futures in lockstep.

///

/// When `.await`ed, a [`PollAll`] will unconditionally poll *all* of its

/// underlying futures, in the order they were [`push`](PollAll::push)ed,

/// until one or more of them resolves.

/// Any remaining unresolved futures will be dropped.

/// An empty `PollAll` will resolve immediately, yielding an empty list.

///

/// `PollAll` resolves to an *ordered* list of results, obtained from polling

/// the futures in insertion order. Because some of the futures may not

/// get a chance to resolve, the number of results will always

/// be less than or equal to the number of inserted futures.

///

/// Because `PollAll` drives the futures in lockstep,

/// if one future becomes ready, all of the futures will get polled,

/// even if they didn't generate a wakeup notification.

///

/// ### Invariants

///

/// All of the futures inserted into this set **must** be cancellation safe.

#[derive(Default)]

pub(crate) struct PollAll<'a, const N: usize, T> {

    /// The futures to drive in lockstep.

    inner: SmallVec<[BoxedFut<'a, T>; N]>,

}

impl<'a, const N: usize, T> PollAll<'a, N, T> {

    /// Create an empty [`PollAll`].

    /// Add a future to this [`PollAll`].

}

impl<'a, const N: usize, T> Future for PollAll<'a, N, T> {

    type Output = SmallVec<[T; N]>;

            // Nothing to do.

            }

        }

}

#[cfg(test)]

mod test {

    // @@ begin test lint list maintained by maint/add_warning @@

    #![allow(clippy::bool_assert_comparison)]

    #![allow(clippy::clone_on_copy)]

    #![allow(clippy::dbg_macro)]

    #![allow(clippy::mixed_attributes_style)]

    #![allow(clippy::print_stderr)]

    #![allow(clippy::print_stdout)]

    #![allow(clippy::single_char_pattern)]

    #![allow(clippy::unwrap_used)]

    #![allow(clippy::unchecked_time_subtraction)]

    #![allow(clippy::useless_vec)]

    #![allow(clippy::needless_pass_by_value)]

    //! <!-- @@ end test lint list maintained by maint/add_warning @@ -->

    use super::*;

    use tor_rtmock::MockRuntime;

    use std::sync::Arc;

    use std::sync::atomic::{AtomicUsize, Ordering};

    /// Dummy smallvec capacity.

    const RES_COUNT: usize = 5;

    /// A wrapper over a future, that counts how many times it is polled.

    struct PollCounter<F> {

        /// The poll count, shared with the caller.

        count: Arc<AtomicUsize>,

        /// The underlying future.

        inner: F,

    }

    /// A future that resolves after a fixed number of calls to `poll()`.

    struct ResolveAfter {

        /// The number of poll() calls until this future resolves

        resolve_after: usize,

        /// The number of times poll() was called on this.

        poll_count: usize,

    }

    impl ResolveAfter {

        fn new(resolve_after: usize) -> Self {

            Self {

                resolve_after,

                poll_count: 0,

            }

        }

    }

    impl<F> PollCounter<F> {

        fn new(inner: F) -> (Self, Arc<AtomicUsize>) {

            let count = Arc::new(AtomicUsize::new(0));

            let poll_counter = Self {

                count: Arc::clone(&count),

                inner,

            };

            (poll_counter, count)

        }

    }

    impl<F: Future + Unpin> Future for PollCounter<F> {

        type Output = F::Output;

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

            let _ = self.count.fetch_add(1, Ordering::Relaxed);

            self.inner.poll_unpin(cx)

        }

    }

    impl Future for ResolveAfter {

        type Output = usize;

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

            self.poll_count += 1;

            // TODO MSRV 1.87: Remove this allow.

            #[allow(

                clippy::comparison_chain,

                reason = "This is more readable than a match, and the lint is

                moved to clippy::pedantic in 1.87."

            )]

            if self.poll_count == self.resolve_after {

                Poll::Ready(self.resolve_after)

            } else if self.poll_count > self.resolve_after {

                panic!("future polled after completion?!");

            } else {

                // Immediately wake the waker

                cx.waker().wake_by_ref();

                Poll::Pending

            }

        }

    }

    #[test]

    fn poll_none() {

        MockRuntime::test_with_various(|_| async move {

            assert!(PollAll::<RES_COUNT, ()>::new().await.is_empty());

        });

    }

    #[test]

    fn poll_multiple() {

        MockRuntime::test_with_various(|_| async move {

            let mut poll_all = PollAll::<RES_COUNT, usize>::new();

            let (never_fut, never_count) = PollCounter::new(futures::future::pending::<usize>());

            poll_all.push(never_fut);

            let (futures, counters): (Vec<_>, Vec<_>) = [

                PollCounter::new(ResolveAfter::new(5)),

                PollCounter::new(ResolveAfter::new(5)),

                // These won't get a chance to resolve

                PollCounter::new(ResolveAfter::new(8)),

                PollCounter::new(ResolveAfter::new(9)),

            ]

            .into_iter()

            .unzip();

            for fut in futures {

                poll_all.push(fut);

            }

            let res = poll_all.await;

            assert_eq!(&res[..], &[5, 5]);

            // All futures were polled 5 times.

            assert_eq!(never_count.load(Ordering::Relaxed), 5);

            for counter in counters {

                assert_eq!(counter.load(Ordering::Relaxed), 5);

            }

        });

    }

}