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//! Async [Monad] based on [`Pin<Box<dyn Future>>`] (see: [`Pin`], [`Box::pin`], [`Future`]).
//! This generally allows just using [`.await`] on wrapped instances.
//!
//! For sync, see [`solo`].
//!
//! For fallible futures, see [`tryfuture`].
//!
//! [`solo`]: super::solo
//! [`tryfuture`]: super::tryfuture
//! [`.await`]: https://doc.rust-lang.org/std/keyword.await.html

use std::{future::Future, pin::Pin};

use futures::{
    future::{join, select, Either, Shared},
    join, FutureExt,
};

use crate::func::class_prelude::*;

pub struct FutureInstance;

impl WeakFunctorAny for FutureInstance {
    type FAny<'a, A: 'a + Send> = Pin<Box<dyn 'a + Send + Future<Output = A>>>;
}

impl<'a> Functor<'a> for FutureInstance {
    fn fmap<A: 'a + Send, B: 'a + Send>(
        fa: Self::F<A>,
        f: impl 'a + Send + FnOnce(A) -> B,
    ) -> Self::F<B> {
        Box::pin(async { f(fa.await) })
    }

    fn replace<A: 'a + Send, B: 'a + Send>(fa: Self::F<A>, b: B) -> Self::F<B> {
        Box::pin(async {
            fa.await;
            b
        })
    }
}

impl<'a> Pure<'a> for FutureInstance {
    fn pure<A: 'a + Send>(a: A) -> Self::F<A> {
        Box::pin(async { a })
    }
}

impl<'a> ApplicativeSeq<'a> for FutureInstance {
    fn seq<A: 'a + Send, B: 'a + Send>(
        ff: Self::F<impl 'a + Send + FnOnce(A) -> B>,
        fa: Self::F<A>,
    ) -> Self::F<B> {
        Box::pin(async {
            let (f, a) = join!(ff, fa);
            f(a)
        })
    }
}

impl<'a> ApplicativeLA2<'a> for FutureInstance {
    fn la2<A: 'a + Send, B: 'a + Send, C: 'a + Send>(
        fa: Self::F<A>,
        fb: Self::F<B>,
        f: impl 'a + Send + FnOnce(A, B) -> C,
    ) -> Self::F<C> {
        Box::pin(async {
            let (a, b) = join!(fa, fb);
            f(a, b)
        })
    }
}

impl<'a> ApplicativeTuple<'a> for FutureInstance {
    fn tuple<A: 'a + Send, B: 'a + Send>((fa, fb): (Self::F<A>, Self::F<B>)) -> Self::F<(A, B)> {
        Box::pin(join(fa, fb))
    }
}

impl<'a> ApplicativeSelect<'a> for FutureInstance {
    fn select<A: 'a + Send, B: 'a + Send>(
        fa: Self::F<A>,
        fb: Self::F<B>,
    ) -> SelectedWrapped<'a, A, B, Self> {
        Box::pin(async {
            match select(fa, fb).await {
                Either::Left((a, fb)) => Selected::A(a, fb),
                Either::Right((b, fa)) => Selected::B(fa, b),
            }
        })
    }
}

impl<'a> Applicative<'a> for FutureInstance {
    fn discard_first<A: 'a + Send, B: 'a + Send>(fa: Self::F<A>, fb: Self::F<B>) -> Self::F<B> {
        Box::pin(async { join!(fa, fb).1 })
    }

    fn discard_second<A: 'a + Send, B: 'a + Send>(fa: Self::F<A>, fb: Self::F<B>) -> Self::F<A> {
        Box::pin(async { join!(fa, fb).0 })
    }
}

impl<'a> Monad<'a> for FutureInstance {
    fn bind<A: 'a + Send, B: 'a + Send>(
        fa: Self::F<A>,
        f: impl 'a + Send + FnOnce(A) -> Self::F<B>,
    ) -> Self::F<B> {
        Box::pin(async { f(fa.await).await })
    }

    fn iterate<B: 'a + Send>(mut f: impl Iterative<'a, T = Self, B = B>) -> Self::F<B> {
        Box::pin(async move {
            loop {
                match f.next().await {
                    ControlFlow::Continue(next_f) => f = next_f,
                    ControlFlow::Break(b) => return b,
                }
            }
        })
    }

    fn join<A: 'a + Send>(ffa: Self::F<Self::F<A>>) -> Self::F<A> {
        Box::pin(async { ffa.await.await })
    }
}

impl<'a> SharedFunctor<'a> for FutureInstance {
    type Shared<A: 'a + Send + Sync + Clone> = Shared<Pin<Box<dyn 'a + Send + Future<Output = A>>>>;

    fn share<A: 'a + Send + Sync + Clone>(fa: Self::F<A>) -> Self::Shared<A> {
        fa.shared()
    }

    fn unshare<A: 'a + Send + Sync + Clone>(sa: Self::Shared<A>) -> Self::F<A> {
        Box::pin(sa)
    }
}

#[cfg(test)]
mod future_tests {
    use std::sync::Arc;

    use crate::func::{applicative_select::Selected, ApplicativeSelect};

    use super::{test_suite, tests, FutureInstance};

    type T = FutureInstance;

    impl<'a> tests::ToEq<'a> for T {
        type Eq<A: 'a + Send + PartialEq + std::fmt::Debug> = A;

        fn to_eq<A: 'a + Send + PartialEq + std::fmt::Debug>(fa: Self::F<A>) -> Self::Eq<A> {
            futures::executor::block_on(fa)
        }
    }

    impl<'a> test_suite::FunctorTestSuite<'a> for T {
        fn sample<A: 'a + Send, F: FnMut(Arc<dyn test_suite::WrapFunction<'a, A, Self::F<A>>>)>(
            mut f: F,
        ) {
            f(Arc::new(|a| Box::pin(futures::future::ready(a))));
        }
    }

    #[test]
    fn monad_follows_laws() {
        test_suite::monad_follows_laws::<T>().unwrap();
    }

    #[test]
    fn select_second() {
        let res = futures::executor::block_on(T::select(
            Box::pin(futures::future::pending::<i32>()),
            Box::pin(futures::future::ready(2)),
        ));
        assert!(matches!(res, Selected::B(_, 2)));
    }

    #[test]
    fn shared_follows_laws() {
        test_suite::shared_follows_laws::<T>().unwrap();
    }
}