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use core::mem::transmute_copy;

/// The compact representation of `T`. Only one pointer wide.
///
/// Under the hood, it is simply a transparent wrapper of `*const u8`. The
/// inner data field is set private to ensure safety. For any case that you
/// need to access the inner data, just [`transmute`] it.
///
/// [`transmute`]: `std::mem::transmute`
#[repr(transparent)]
pub struct Compact<T>
where
    T: From<Compact<T>>,
    Compact<T>: From<T>,
{
    _data: *const u8,
    phantom: core::marker::PhantomData<T>,
}

impl<T> Compact<T>
where
    T: From<Compact<T>>,
    Compact<T>: From<T>,
{
    unsafe fn decompact_copy(&self) -> T {
        let this: Self = unsafe { transmute_copy(self) };
        T::from(this)
    }

    /// # Examples
    ///
    /// ```
    /// # use enum_ptr::{Compact, EnumPtr};
    /// #
    /// # #[derive(EnumPtr, Debug)]
    /// # #[repr(C, usize)]
    /// # enum Foo<'a, 'b> {
    /// #     A(&'a i32),
    /// #     B(&'b i32),
    /// # }
    /// #
    /// let mut foo: Compact<_> = Foo::A(&1).into();
    /// foo.map_ref(|f: &Foo| println!("{f:?}"));
    /// ```
    pub fn map_ref<U>(&self, func: impl FnOnce(&T) -> U) -> U {
        func(&unsafe { self.decompact_copy() })
    }

    /// # Examples
    ///
    /// ```
    /// # use enum_ptr::{Compact, EnumPtr};
    /// #
    /// # #[derive(EnumPtr, Debug)]
    /// # #[repr(C, usize)]
    /// # enum Foo<'a, 'b> {
    /// #     A(&'a i32),
    /// #     B(&'b i32),
    /// # }
    /// #
    /// let mut foo: Compact<_> = Foo::A(&1).into();
    /// foo.map_ref_mut(|f: &mut Foo| println!("{f:?}"));
    /// ```
    pub fn map_ref_mut<U>(&mut self, func: impl FnOnce(&mut T) -> U) -> U {
        func(&mut unsafe { self.decompact_copy() })
    }
}

impl<T> Drop for Compact<T>
where
    T: From<Compact<T>>,
    Compact<T>: From<T>,
{
    fn drop(&mut self) {
        let _ = unsafe { self.decompact_copy() };
    }
}

impl<T: core::fmt::Debug> core::fmt::Debug for Compact<T>
where
    T: From<Compact<T>>,
    Compact<T>: From<T>,
{
    fn fmt(&self, f: &mut core::fmt::Formatter<'_>) -> core::fmt::Result {
        self.map_ref(|this| this.fmt(f))
    }
}

impl<T: Clone> Clone for Compact<T>
where
    T: From<Compact<T>>,
    Compact<T>: From<T>,
{
    fn clone(&self) -> Self {
        self.map_ref(|this| this.clone().into())
    }
}

impl<T: PartialEq> PartialEq for Compact<T>
where
    T: From<Compact<T>>,
    Compact<T>: From<T>,
{
    fn eq(&self, other: &Self) -> bool {
        self.map_ref(|this| other.map_ref(|that| this.eq(that)))
    }
}

impl<T: Eq> Eq for Compact<T>
where
    T: From<Compact<T>>,
    Compact<T>: From<T>,
{
}

impl<T: PartialOrd> PartialOrd for Compact<T>
where
    T: From<Compact<T>>,
    Compact<T>: From<T>,
{
    fn partial_cmp(&self, other: &Self) -> Option<core::cmp::Ordering> {
        self.map_ref(|this| other.map_ref(|that| this.partial_cmp(that)))
    }
}

impl<T: Ord> Ord for Compact<T>
where
    T: From<Compact<T>>,
    Compact<T>: From<T>,
{
    fn cmp(&self, other: &Self) -> core::cmp::Ordering {
        self.map_ref(|this| other.map_ref(|that| this.cmp(that)))
    }
}

impl<T: core::hash::Hash> core::hash::Hash for Compact<T>
where
    T: From<Compact<T>>,
    Compact<T>: From<T>,
{
    fn hash<H: core::hash::Hasher>(&self, state: &mut H) {
        self.map_ref(|this| this.hash(state))
    }
}