//! Interface for interfaces.
//!
//! ## Design
//! The design of protocols is based on an idea found in the
//! [`gdbstub`](https://docs.rs/gdbstub/latest/gdbstub/target/ext/index.html) crate.
//! This idea is of so called inlinable dyn extension traits.
//! However, in the form given in `gdbstub` they can't be used for arbitrary interfaces.
//! The main trait still needs to know about all the possible protocols.
//! That is where this module comes in.
//!
//! This module implements a technique we name dynamic inlinable dyn extension traits (DIDETs).
//! DIDETs adds one more layer to IDETs. Instead of a trait that knows all the possible protocols,
//! we have a single trait [`Implementer`] that allows looking up an extension trait
//! using a type ID. This may seem like it defeats the purpose of IDETs, that being to
//! make them inlinable. However, it turns out LLVM (the optimizer) is able to see
//! through this style of runtime reflection. As such, we still gain the benefits of
//! IDETs but with more flexability.
//! Protocols can now be defined in *any* crate and used between arbitrary crates.
//!
//! A protocol is a special trait that can participate as a DIDET. The only thing needed
//! for a protocol is an associated trait object. Because we need to use the
//! [`TypeId`][core::any::TypeId] of a protocol to perform reflection, we can't just use
//! the trait object itself as the protocol type. Instead an uninhabited type is used
//! as a marker for the trait.
//!
//! We then "implement" a protocol for a type by using [`Implementation`]. This provides
//! a mapping from `T` to the protocol's trait object.
//! By itself, [`Implementation`] is not enough for DIDET. A type also needs to implement
//! [`Implementer`] which allows looking up a particular [`Implementation`] trait object
//! from a [`ProtocolId`].
//!
//! The implementation of DIDETs defined by this module allows [`Implementer`] to be object safe.
//! This is done via the help of the [`AnyImpl`] type. This is not required for the core
//! idea of DIDETs.

use crate::any::{IndirectLtAny, LtAny, LtTypeId, Mut, Ref, TypeNameable};

pub trait Implementer<'ctx> {
    fn interface(&self, id: LtTypeId<'ctx>) -> Option<IndirectLtAny<'_, 'ctx, Ref>>;
}

pub trait ImplementerMut<'ctx> {
    fn interface_mut<'a>(&'a mut self, id: LtTypeId<'ctx>) -> Option<IndirectLtAny<'a, 'ctx, Mut>> where 'ctx: 'a;
}

/// Extension trait for getting the implementation of a protocol.
pub trait ImplementerMutExt<'ctx>: ImplementerMut<'ctx> {
    /// Get an implementation given a protocol type.
    ///
    /// This wraps [`Implementer::interface`] and [`AnyImpl::downcast`].
    /// If [`Implementer::interface`] returns a [`AnyImpl`] for the wrong protocol then a panic is
    /// generated.
    fn interface_mut_for<'a, P: TypeNameable<'a, 'ctx>>(&'a mut self) -> Option<&'a mut P>
    where 
        'ctx: 'a;
}

impl<'ctx, T: ImplementerMut<'ctx> + ?Sized> ImplementerMutExt<'ctx> for T {
    fn interface_mut_for<'a, P: TypeNameable<'a, 'ctx>>(&'a mut self) -> Option<&'a mut P> 
    where 
        'ctx: 'a
    {
        match self.interface_mut(LtTypeId::of::<P>()) {
            Some(interface) => match interface.downcast::<P>() {
                Ok(implementation) => Some(implementation),
                Err(interface) => panic!(
                    "unexpected protocol implementation: `{:?}`, expected: `{:?}`",
                    interface.id(),
                    LtTypeId::of::<P>()
                ),
            },
            None => None,
        }
    }
}

/// Implement [`Implementer`] and [`Implementation`] for a set of protocols.
#[doc(hidden)]
#[macro_export]
macro_rules! implementer {
    {
        impl[$a:lifetime, $ctx:lifetime $($generic:tt)*] $name:ty = [$($protocol:ty),* $(,)?];
    } => {
        impl<$ctx $($generic)*> $crate::protocol::ImplementerMut<$ctx> for $name {
            #[inline]
            fn interface_mut<$a>(
                &$a mut self,
                id: $crate::any::LtTypeId<$ctx>
            ) -> ::core::option::Option<$crate::any::IndirectLtAny<$a, $ctx, $crate::any::Mut>>
            where 
                $ctx: $a 
            {   
                match id {
                    $(id if id == $crate::any::LtTypeId::of::<$protocol>()
                        => Some($crate::any::IndirectLtAny::<$a, $ctx, _>::new::<$protocol>(self as _)),)*
                    _ => None
                }
            }
        }
    };
}
#[doc(inline)]
pub use implementer;

#[cfg(test)]
mod test {
    use crate::nameable;

    use super::*;

    #[test]
    fn implementer_macro() {
        trait Z {}

        nameable! {
            ['a, 'ctx]
            dyn Z + 'a where {'ctx: 'a}
        }

        struct X<T>(T);

        impl<T: Clone> Z for X<T> {}

        implementer! {
            impl['a, 'ctx, T: Clone] X<T> = [
                dyn Z + 'a
            ];
        }
    }
}