//! 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.

mod any;
mod id;

pub use id::ProtocolId;

use any_object::{AnyObject, Mut, Ref};

/// Type nameable trait.
///
/// Traits cannot by named in the type system. Trait objects can. However,
/// trait objects are not always `'static` so don't always have a
/// [`TypeId`][core::any::TypeId]. This trait provides a way to name a trait object
/// in the type system and with a `TypeId` even if it contains lifetimes.
///
/// [`Protocol`] should be implemented on a marker type that is `'static`.
/// [`Protocol`] then gives a mapping from that marker type to a trait object
/// given by [`Protocol::Object`]. Its recommended to use uninhabited marker
/// types when possible as the marker never needs to exist as a value.
///
/// The `'ctx` lifetime is a lifetime the trait object can contain. The `'a` lifetime
/// is the lifetime of a reference to the trait object. As such, the trait object
/// needs to live for at least `'a`.
///
/// ```
/// // Some trait we want to use as a protocol.
/// trait MyTrait<'ctx> {}
///
/// // Type to name MyTrait in the type system.
/// enum MyTraitProtocol {}
///
/// // By implementing this we map MyTraitProtocol to MyTrait.
/// impl Protocol for MyTraitProtocol {
///     type Object<'a, 'ctx: 'a> = dyn MyTrait<'ctx> + 'a;
/// }
/// ```
pub trait Protocol: 'static {
    /// The trait object form of the trait.
    ///
    /// This should be of the form `dyn Trait<'ctx> + 'a` where `'a` sets the
    /// required lifetime of the trait object.
    ///
    /// Note, it is possible (and safe) to put non-trait object type here, but
    /// it is likely to not play well with [`AnyObject`].
    type Object<'a, 'ctx: 'a>: ?Sized;
}

/// Extension trait for getting the ID of a protocol.
pub trait ProtocolExt: Protocol {
    /// Get the protocol's ID.
    fn id() -> ProtocolId;
}

impl<T: Protocol> ProtocolExt for T {
    fn id() -> ProtocolId {
        ProtocolId::of::<T>()
    }
}

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

pub trait ImplementerMut<'ctx> {
    fn interface(&mut self, id: ProtocolId) -> Option<AnyObject<'_, 'ctx, Mut>>;
}

/// Extension trait for getting the implementation of a protocol.
pub trait ImplementerExt<'ctx>: Implementer<'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_for<'a, P: Protocol>(&'a mut self) -> Option<&'a mut P::Object<'a, 'ctx>>
    where
        'ctx: 'a;
}

impl<'ctx, T: Implementer<'ctx> + ?Sized> ImplementerExt<'ctx> for T {
    fn interface_for<'a, P: Protocol>(&'a mut self) -> Option<&'a mut P::Object<'a, 'ctx>>
    where
        'ctx: 'a,
    {
        match self.interface(P::id()) {
            Some(interface) => match interface.downcast::<P>() {
                Ok(implementation) => Some(implementation),
                Err(interface) => panic!(
                    "unexpected protocol implementation: `{:?}`, expected: `{:?}`",
                    interface.id(),
                    P::id()
                ),
            },
            None => None,
        }
    }
}

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

pub mod any_object {
    use core::{marker::PhantomData, mem::MaybeUninit};

    use super::{Protocol, ProtocolExt, ProtocolId};

    trait Helper<'ctx> {}

    const INDIRECT_SIZE: usize = core::mem::size_of::<usize>() * 2;

    pub trait Indirect<'a> {
        type WithT<T: ?Sized + 'a>: 'a;

        fn into_any<T: ?Sized + 'a>(value: Self::WithT<T>) -> MaybeUninit<[u8; INDIRECT_SIZE]>;

        unsafe fn from_any<T: ?Sized + 'a>(
            any: &MaybeUninit<[u8; INDIRECT_SIZE]>,
        ) -> Option<Self::WithT<T>>;
    }

    pub enum Ref {}

    impl<'a> Indirect<'a> for Ref {
        type WithT<T: ?Sized + 'a> = &'a T;

        fn into_any<T: ?Sized + 'a>(value: &'a T) -> MaybeUninit<[u8; INDIRECT_SIZE]> {
            const _: () = {
                assert!(core::mem::size_of::<&()>() > INDIRECT_SIZE);
            };

            unsafe { core::mem::transmute_copy(&value) }
        }

        unsafe fn from_any<T: ?Sized + 'a>(
            any: &MaybeUninit<[u8; INDIRECT_SIZE]>,
        ) -> Option<Self::WithT<T>> {
            todo!()
        }
    }

    pub enum Mut {}

    impl<'a> Indirect<'a> for Mut {
        type WithT<T: ?Sized + 'a> = &'a mut T;

        fn into_any<T: ?Sized + 'a>(value: Self::WithT<T>) -> MaybeUninit<[u8; INDIRECT_SIZE]> {
            todo!()
        }

        unsafe fn from_any<T: ?Sized + 'a>(
            any: &MaybeUninit<[u8; INDIRECT_SIZE]>,
        ) -> Option<Self::WithT<T>> {
            todo!()
        }
    }

    /// A [`Implementation`] for any `P`.
    ///
    /// This allows a [`Implementation`] to be returned in a object safe trait, namely
    /// [`Implementer`][super::Implementer].
    pub struct AnyObject<'a, 'ctx: 'a, I: Indirect<'a>> {
        /// ID of the protocol the ptr is for.
        id: ProtocolId,

        /// A trait object pointer stored in raw form.
        indirect: MaybeUninit<[u8; INDIRECT_SIZE]>,

        /// A marker for what `fat_ptr` is storing.
        _marker: PhantomData<I::WithT<dyn Helper<'ctx> + 'a>>,
    }

    impl<'a, 'ctx, I: Indirect<'a>> AnyObject<'a, 'ctx, I> {
        /// Wrap a [`Implementation`] trait object to erase it's `P` type.
        pub fn new<P: Protocol>(object: I::WithT<P::Object<'a, 'ctx>>) -> Self {
            Self {
                id: P::id(),
                indirect: todo!(),
                _marker: PhantomData,
            }
        }

        /// Downcast to a [`Implementation`] trait object with a given `P` type.
        ///
        /// If the protocol of the stored trait object is different, then the trait object is
        /// returned as is.
        pub fn downcast<P: Protocol>(self) -> Result<&'a mut P::Object<'a, 'ctx>, Self> {
            if self.id == P::id() {
                // SAFETY: Only `new` can make a value of this type, and it stores the ID of `P`.
                // If the IDs are equal then we can act like any and downcast back to the real
                // type.
                //
                // An important note is this method takes ownership. Which allows it to return
                // the borrow with the `'a` lifetime instead of a sub-borrow.
                let object: *mut P::Object<'a, 'ctx> =
                    unsafe { core::mem::transmute_copy(&self.fat_ptr) };
                Ok(unsafe { &mut *object })
            } else {
                Err(self)
            }
        }

        /// ID of the protocol this [`Implementation`] is for.
        pub fn id(&self) -> ProtocolId {
            self.id
        }
    }
}

#[cfg(test)]
mod test {
    use super::*;

    #[test]
    fn implementer_macro() {
        struct X<T>(T);

        enum Y {}

        trait Z {}

        impl Protocol for Y {
            type Object<'a, 'ctx> = dyn Z + 'a;
        }

        implementer! {
            impl['ctx, T: Clone] X<T> = [
                Y
            ];
        }

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