//! [`Protocol`] for giving a visitor an owned value.
//!
//! In some sense, this is the most basic protocol.

use crate::{
    any::{TypeName, TypeNameable},
    effect::{Effect, Future},
    higher_ranked_type,
    hkt::AnySend,
    nameable,
    never::Never,
    protocol::{walker::hint::HintMeta, Visitor},
    Flow,
};

use super::Status;

/// Trait object for the [`Value`] protocol.
///
/// Types implementing the [`Value`] protocol will implement this trait.
pub trait Value<'a, 'ctx: 'a, T, E: Effect<'ctx>> {
    /// Visit a value of type `T`.
    ///
    /// Use this to give a value to a visitor. Its expected that a walker
    /// only calls this once per usage of the trait object, but that is not
    /// forced.
    ///
    /// If a [`ControlFlow::Break`] is returned then the walker
    /// should stop walking as soon as possible as there has likely been
    /// and error.
    fn visit(&'a mut self, value: T) -> Future<'a, 'ctx, Flow, E>;
}

pub type DynValue<'a, 'ctx, T, E> = dyn Value<'a, 'ctx, T, E> + Send + 'a;

nameable! {
    pub struct Name['a, 'ctx, T, E];

    impl [T::Name, E] for DynValue<'a, 'ctx, T, E> where {
        T: TypeNameable<'a, 'ctx> + ?Sized,
        E: Effect<'ctx>,
        'ctx: 'a,
    }

    impl [T, E] where DynValue<'a, 'ctx, T::Nameable, E> {
        T: TypeName<'a, 'ctx> + ?Sized,
        E: Effect<'ctx>,
        'ctx: 'a,
    }
}

higher_ranked_type! {
    pub type Known['ctx]: (AnySend) = for<'lt> ()
}

// This enrolls the Value protocol into the walker hint system.
impl<'a, 'ctx: 'a, T, E: Effect<'ctx>> HintMeta<'ctx> for DynValue<'a, 'ctx, T, E> {
    type Known = Known<'ctx>;

    type Hint = ();
}

pub fn visit_value<'a, 'ctx, T: TypeNameable<'a, 'ctx>, E: Effect<'ctx>>(
    visitor: Visitor<'a, 'ctx>,
    value: T,
) -> Future<'a, 'ctx, Status, E> {
    if let Some(object) = visitor.upcast_mut::<DynValue<'_, 'ctx, T, E>>() {
        // Allow the visitor to give a hint if it wants.
        E::map(object.visit(value), |flow| match flow {
            Flow::Continue => {
                // The visitor wants the walker to continue to it's normal
                // walking.
                Status::Continue
            }
            Flow::Break | Flow::Done => {
                // The visitor is done (either because of an error or because
                // it already used a hint).
                Status::Break
            }
        })
    } else {
        // If the visitor doesn't support request hint then we continue.
        E::ready(Status::Skipped)
    }
}

#[cfg(test)]
mod test {
    use core::{marker::PhantomData, ops::ControlFlow};

    use crate::{
        any::{
            static_wrapper::{BorrowedMutStatic, BorrowedStatic, OwnedStatic},
            AnyTrait,
        },
        any_trait,
        effect::{BlockOn, Blocking, Spin},
    };

    use super::*;

    #[test]
    fn visit() {
        struct Visitor<E>(Option<i32>, PhantomData<fn() -> E>);

        impl<'a, 'ctx: 'a, E> Value<'a, 'ctx, OwnedStatic<i32>, E> for Visitor<E>
        where
            E: Effect<'ctx>,
        {
            fn visit(
                &'a mut self,
                OwnedStatic(value): OwnedStatic<i32>,
            ) -> Future<'a, 'ctx, Flow, E> {
                E::wrap(async move {
                    self.0 = Some(value);
                    Flow::Continue
                })
            }
        }

        impl<'a, 'ctx: 'a, E> Value<'a, 'ctx, BorrowedStatic<'ctx, i32>, E> for Visitor<E>
        where
            E: Effect<'ctx>,
        {
            fn visit(
                &'a mut self,
                BorrowedStatic(value): BorrowedStatic<'ctx, i32>,
            ) -> Future<'a, 'ctx, Flow, E> {
                E::wrap(async {
                    self.0 = Some(*value);
                    Flow::Continue
                })
            }
        }

        any_trait! {
            impl['a, 'ctx, E] Visitor<E> = [
                DynValue<'a, 'ctx, OwnedStatic<i32>, E>,
                DynValue<'a, 'ctx, BorrowedStatic<'ctx, i32>, E>,
            ] where E: Effect<'ctx>,
        }

        let mut v = Visitor::<Blocking>(None, PhantomData);
        let object: &mut (dyn AnyTrait<'_> + Send) = &mut v;
        Spin::block_on(
            object
                .upcast_mut::<DynValue<'_, '_, OwnedStatic<i32>, Blocking>>()
                .unwrap()
                .visit(OwnedStatic(42)),
        );

        assert_eq!(v.0, Some(42));

        let object: &mut (dyn AnyTrait<'_> + Send) = &mut v;
        Spin::block_on(
            object
                .upcast_mut::<DynValue<'_, '_, BorrowedStatic<'_, i32>, Blocking>>()
                .unwrap()
                .visit(BorrowedStatic(&101)),
        );

        assert_eq!(v.0, Some(101));
    }

    #[test]
    fn visit_borrowed() {
        struct Visitor<'ctx, E>(Option<&'ctx mut String>, PhantomData<fn() -> E>);

        impl<'a, 'ctx: 'a, E> Value<'a, 'ctx, BorrowedMutStatic<'ctx, String>, E> for Visitor<'ctx, E>
        where
            E: Effect<'ctx>,
        {
            fn visit(
                &'a mut self,
                BorrowedMutStatic(value): BorrowedMutStatic<'ctx, String>,
            ) -> Future<'a, 'ctx, Flow, E> {
                E::wrap(async {
                    self.0 = Some(value);

                    Flow::Continue
                })
            }
        }

        any_trait! {
            impl['a, 'ctx, E] Visitor<'ctx, E> = [
                DynValue<'a, 'ctx, BorrowedMutStatic<'ctx, String>, E>,
            ] where E: Effect<'ctx>
        }

        let mut v = Visitor::<Blocking>(None, PhantomData);

        let mut y = String::from("abc");
        let object: &mut (dyn AnyTrait<'_> + Send) = &mut v;
        Spin::block_on(
            object
                .upcast_mut::<DynValue<'_, '_, _, Blocking>>()
                .unwrap()
                .visit(BorrowedMutStatic(&mut y)),
        );

        v.0.unwrap().push_str("def");
        assert_eq!(y, "abcdef");
    }

    #[test]
    fn visit_borrowed_unsized() {
        struct Visitor<'ctx, E>(Option<&'ctx str>, PhantomData<fn() -> E>);

        impl<'a, 'ctx: 'a, E> Value<'a, 'ctx, BorrowedStatic<'ctx, str>, E> for Visitor<'ctx, E>
        where
            E: Effect<'ctx>,
        {
            fn visit(
                &'a mut self,
                BorrowedStatic(value): BorrowedStatic<'ctx, str>,
            ) -> Future<'a, 'ctx, Flow, E> {
                E::wrap(async {
                    self.0 = Some(value);
                    Flow::Continue
                })
            }
        }

        any_trait! {
            impl['a, 'ctx, E] Visitor<'ctx, E> = [
                DynValue<'a, 'ctx, BorrowedStatic<'ctx, str>, E>,
            ] where E: Effect<'ctx>
        }

        let mut v = Visitor::<Blocking>(None, PhantomData);

        let y = String::from("abc");
        let object: &mut (dyn AnyTrait<'_> + Send) = &mut v;
        Spin::block_on(
            object
                .upcast_mut::<DynValue<'_, '_, BorrowedStatic<'_, str>, Blocking>>()
                .unwrap()
                .visit(BorrowedStatic(&y)),
        );

        assert_eq!(v.0, Some("abc"));
    }
}