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

use effectful::{
    bound::{Bool, Dynamic, IsSend, IsSync},
    effective::Effective,
    environment::{DynBind, EnvConfig, Environment, NativeForm},
    SendSync,
};

use crate::{
    any::TypeName,
    hkt::Marker,
    protocol::{
        walker::hint::{HasProtocol, HintMeta, Meta},
        DynVisitor,
    },
};

use super::VisitResult;

/// Trait object for the [`Value`] protocol.
///
/// Types implementing the [`Value`] protocol will implement this trait.
pub trait Value<'ctx, T: ?Sized + TypeName::MemberType<E>, E: Environment>: DynBind<E> {
    /// 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>(
        &'a mut self,
        value: TypeName::T<'a, 'ctx, T, E>,
    ) -> NativeForm<'a, VisitResult<Dynamic<TypeName::T<'a, 'ctx, T, E>>>, E>
    where
        Dynamic<TypeName::T<'a, 'ctx, T, E>>: DynBind<E>,
        TypeName::T<'a, 'ctx, T, E>: Sized,
        'ctx: 'a;
}

#[derive(SendSync)]
pub struct ValueProto<T: ?Sized + TypeName::MemberType<E>, E: Environment>(Marker<(*const T, E)>);

impl<'a, 'ctx, T: ?Sized, E> TypeName::MemberTypeForLt<'a, 'ctx, E, &'a &'ctx ()>
    for ValueProto<T, E>
where
    E: Environment,
    T: TypeName::MemberType<E>,
{
    type T = dyn Value<'ctx, T, E> + 'a;
}

impl<'a, 'ctx, T: ?Sized, E> TypeName::LowerTypeWithBound<'a, 'ctx, E, &'a &'ctx ()>
    for dyn Value<'ctx, T, E> + 'a
where
    E: Environment,
    T: TypeName::MemberType<E>,
{
    type Higher = ValueProto<T, E>;
}

#[derive(Copy, Clone, PartialEq, Debug, SendSync)]
pub struct ValueKnown<'a, T: ?Sized> {
    /// A preview of the value.
    ///
    /// This can be used to inspect the value before committing to a visit.
    pub preview: Option<Dynamic<&'a T>>,
}

#[derive(Copy, Clone, Debug, SendSync)]
pub struct ValueKnownHrt<T: ?Sized>(Marker<T>);

impl<'a, 'ctx, E: EnvConfig, T> Meta::MemberTypeForLt<'a, 'ctx, E, &'a &'ctx ()>
    for ValueKnownHrt<T>
where
    T: ?Sized + TypeName::MemberTypeForLt<'a, 'ctx, E, &'a &'ctx ()>,
    Dynamic<&'a TypeName::T<'a, 'ctx, T, E>>: DynBind<E>,
{
    type T = ValueKnown<'a, TypeName::T<'a, 'ctx, T, E>>;
}

impl<'a, 'ctx, E: EnvConfig, T: ?Sized> Meta::LowerTypeWithBound<'a, 'ctx, E, &'a &'ctx ()>
    for ValueKnown<'a, T>
where
    T: TypeName::LowerType<'a, 'ctx, E>,
    Dynamic<&'a T>: DynBind<E>,
{
    type Higher = ValueKnownHrt<TypeName::HigherRanked<'a, 'ctx, T, E>>;
}

// This enrolls the Value protocol into the walker hint system.
impl<T: TypeName::MemberType<E>, E: Environment> HintMeta for ValueProto<T, E>
where
    for<'a, 'ctx> Dynamic<&'a TypeName::T<'a, 'ctx, T, E>>: DynBind<E>,
{
    type Known = ValueKnownHrt<T>;

    type Hint = ();

    type Effect = E;
}

pub fn visit_value<
    'ctx: 'visitor,
    'visitor: 'e,
    'e,
    T: TypeName::LowerType<'e, 'ctx, E>,
    E: Environment,
>(
    visitor: DynVisitor<'visitor, 'ctx, E>,
    value: T,
) -> NativeForm<'e, VisitResult<Dynamic<T>>, E>
where
    TypeName::HigherRanked<'e, 'ctx, T, E>: TypeName::MemberType<E>,
    Dynamic<T>: DynBind<E>,
{
    if let Some(object) = visitor
        .0
        .upcast_mut::<ValueProto<TypeName::HigherRanked<'e, 'ctx, T, E>, E>>()
    {
        // Allow the visitor to give a hint if it wants.
        object.visit(value)
    } else {
        // If the visitor doesn't support request hint then we continue.
        E::value(VisitResult::Skipped(Dynamic(value))).cast()
    }
}

impl<'ctx, T, U: TypeName::MemberType<E>, E: Environment> HasProtocol<ValueProto<U, E>> for T where
    T: Value<'ctx, U, E>
{
}