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//! This module implements the trait object used to check const generics.

use crate::{
    abi_stability::{
        check_layout_compatibility,
        extra_checks::{ExtraChecksError, TypeCheckerMut},
    },
    erased_types::{
        c_functions::{adapt_std_fmt, debug_impl, partial_eq_impl},
        FormattingMode,
    },
    marker_type::ErasedObject,
    prefix_type::{PrefixTypeTrait, WithMetadata},
    sabi_types::RRef,
    std_types::{RErr, ROk, RResult, RString},
    type_layout::TypeLayout,
    StableAbi,
};

use std::{
    cmp::{Eq, PartialEq},
    fmt::{self, Debug},
    marker::PhantomData,
};

///////////////////////////////////////////////////////////////////////////////

/// A trait object used to check equality between const generic parameters.
#[repr(C)]
#[derive(Copy, Clone, StableAbi)]
pub struct ConstGeneric {
    ptr: RRef<'static, ErasedObject>,
    vtable: ConstGenericVTable_Ref,
}

unsafe impl Send for ConstGeneric {}
unsafe impl Sync for ConstGeneric {}

impl ConstGeneric {
    /// Constructs a ConstGeneric from a reference and a vtable.
    ///
    /// To construct the `vtable_for` parameter use `ConstGenericVTableFor::NEW`.
    pub const fn new<T>(this: &'static T, vtable_for: ConstGenericVTableFor<T>) -> Self {
        Self {
            ptr: unsafe { RRef::from_raw(this as *const T as *const ErasedObject) },
            vtable: vtable_for.vtable,
        }
    }

    /// Constructs a ConstGeneric from an erased reference and a vtable.
    ///
    /// # Safety
    ///
    /// `this` must point to an object that lives for the `'static` lifetime,
    /// and `vtable` must be a `ConstGenericVTableFor::<T>::NEW`
    /// (where `T` is the unerased type of `this`)
    pub const unsafe fn from_erased(this: *const (), vtable: ConstGenericVTable_Ref) -> Self {
        Self {
            ptr: RRef::from_raw(this as *const ErasedObject),
            vtable,
        }
    }

    /// Compares this to another `ConstGeneric` for equality,
    /// returning an error if the type layout of `self` and `other` is not compatible.
    pub fn is_equal(
        &self,
        other: &Self,
        mut checker: TypeCheckerMut<'_>,
    ) -> Result<bool, ExtraChecksError> {
        match checker.check_compatibility(self.vtable.layout(), other.vtable.layout()) {
            ROk(_) => unsafe { Ok(self.vtable.partial_eq()(self.ptr, other.ptr)) },
            RErr(e) => Err(e),
        }
    }
}

impl Debug for ConstGeneric {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        unsafe { adapt_std_fmt::<ErasedObject>(self.ptr, self.vtable.debug(), f) }
    }
}

// Make sure that this isn't called within `check_layout_compatibility` itself,
// since it would cause infinite recursion.
impl PartialEq for ConstGeneric {
    fn eq(&self, other: &Self) -> bool {
        if check_layout_compatibility(self.vtable.layout(), other.vtable.layout()).is_err() {
            false
        } else {
            unsafe { self.vtable.partial_eq()(self.ptr, other.ptr) }
        }
    }
}

impl Eq for ConstGeneric {}

///////////////////////////////////////////////////////////////////////////////

/// The vtable of `ConstGeneric`,
/// only constructible with `ConstGenericVTableFor::<T>::new.erased()`
#[repr(C)]
#[derive(StableAbi)]
#[sabi(kind(Prefix))]
#[sabi(missing_field(panic))]
pub struct ConstGenericVTable {
    layout: &'static TypeLayout,
    partial_eq: unsafe extern "C" fn(RRef<'_, ErasedObject>, RRef<'_, ErasedObject>) -> bool,
    #[sabi(last_prefix_field)]
    debug: unsafe extern "C" fn(
        RRef<'_, ErasedObject>,
        FormattingMode,
        &mut RString,
    ) -> RResult<(), ()>,
}

/// A type that contains the vtable stored in the `ConstGeneric` constructed from a `T`.
/// This is used as a workaround for `const fn` not allowing trait bounds.
pub struct ConstGenericVTableFor<T> {
    vtable: ConstGenericVTable_Ref,
    _marker: PhantomData<T>,
}

impl<T> ConstGenericVTableFor<T> {
    /// Allows inferring `T` by passing a reference to it.
    pub const fn infer_type(&self, _: &T) {}

    /// Extracts the vtable for the erased type.
    pub const fn erased(&self) -> ConstGenericVTable_Ref {
        self.vtable
    }
}

impl<T> ConstGenericVTableFor<T>
where
    T: StableAbi + Eq + PartialEq + Debug + Send + Sync + 'static,
{
    const _VTABLE_STATIC: WithMetadata<ConstGenericVTable> = {
        WithMetadata::new(
            PrefixTypeTrait::METADATA,
            ConstGenericVTable {
                layout: <T as StableAbi>::LAYOUT,
                partial_eq: partial_eq_impl::<T>,
                debug: debug_impl::<T>,
            },
        )
    };

    /// Constructs a `ConstGenericVTableFor`
    pub const NEW: ConstGenericVTableFor<T> = ConstGenericVTableFor {
        vtable: ConstGenericVTable_Ref(Self::_VTABLE_STATIC.static_as_prefix()),
        _marker: PhantomData,
    };
}

#[doc(hidden)]
pub struct ConstGenericErasureHack<T: ?Sized> {
    pub vtable: ConstGenericVTable_Ref,
    pub value: T,
}

#[doc(hidden)]
impl<T> ConstGenericErasureHack<T> {
    pub const fn new(vtable: ConstGenericVTableFor<T>, value: T) -> Self {
        Self {
            vtable: vtable.erased(),
            value,
        }
    }
}