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use std::any::Any;
use std::mem;

pub trait Downcast<T: Any + Sized>: Any {
    fn is_type(&self) -> bool;
    
    unsafe fn unchecked_downcast_ref(&self) -> &T;

    fn downcast_ref(&self) -> Option<&T>;

    unsafe fn unchecked_downcast_mut(&mut self) -> &mut T;

    fn downcast_mut(&mut self) -> Option<&mut T>;  

    unsafe fn unchecked_downcast(self: Box<Self>) -> Box<T>;

    fn downcast(self: Box<Self>) -> Result<Box<T>, Box<Self>>;
}

#[doc(hidden)]
#[derive(Clone, Copy)]
pub struct TraitObject {
    pub data: *mut (),
    pub vtable: *mut (),
}

#[doc(hidden)]
#[inline(always)]
pub fn to_trait_object<B: ?Sized>(base: &B) -> TraitObject {
    assert_eq!(mem::size_of::<&B>(), mem::size_of::<TraitObject>());
    unsafe { *mem::transmute::<&&B, *const TraitObject>(&base) }
}

/// Implements `Downcast<T: $base>` for `$base`.
#[macro_export]
macro_rules! impl_downcast { 
    (@items $t:ty) => {
        #[inline(always)]
        fn is_type(&self) -> bool {
            unsafe { 
                let dummy: &Self = ::std::mem::uninitialized::<&$t>();
                ::downcast::to_trait_object(self).vtable == ::downcast::to_trait_object(dummy).vtable
            }
        }
        
        #[inline(always)]
        unsafe fn unchecked_downcast_ref(&self) -> &$t {
            ::std::mem::transmute(::downcast::to_trait_object(self).data)
        }

        #[inline(always)]
        fn downcast_ref(&self) -> Option<&$t> {
            if ::downcast::Downcast::<$t>::is_type(self) {
                Some(unsafe { self.unchecked_downcast_ref() })
            } else {
                None
            }
        }
        
        #[inline(always)]
        unsafe fn unchecked_downcast_mut(&mut self) -> &mut $t {
            ::std::mem::transmute(::downcast::to_trait_object(self).data)
        }
        
        #[inline(always)]
        fn downcast_mut(&mut self) -> Option<&mut $t> {
            if ::downcast::Downcast::<$t>::is_type(self) {
                Some(unsafe { self.unchecked_downcast_mut() })
            } else {
                None
            }
        }

        #[inline(always)]
        unsafe fn unchecked_downcast(self: Box<Self>) -> Box<$t> {
            let ret = ::std::mem::transmute::<_, Box<T>>(::downcast::to_trait_object(&*self).data);
            ::std::mem::forget(self);
            ret
        }

        #[inline(always)]
        fn downcast(self: Box<Self>) -> Result<Box<$t>, Box<Self>> {
            if ::downcast::Downcast::<$t>::is_type(&*self) {
                Ok(unsafe { self.unchecked_downcast() })
            } else {
                Err(self)            
            }
        }
    };
    ($base:ident) => {
        impl<T> ::downcast::Downcast<T> for $base
            where T: $base
        {
            impl_downcast!(@items T);
        }
    };
}

/// Use this macro if you want to provide `downcast`-methods on your type without 
/// requiring your users to import `Downcast`.
#[macro_export]
macro_rules! downcast_methods {
    (@items) => {
        #[allow(unused)]
        #[inline(always)]
        pub fn is<T>(&self) -> bool 
            where T: ::std::any::Any, Self: ::downcast::Downcast<T>
        {
            ::downcast::Downcast::<T>::is_type(self)
        }

        #[allow(unused)]
        #[inline(always)]
        pub unsafe fn unchecked_downcast_ref<T>(&self) -> &T
            where T: ::std::any::Any, Self: ::downcast::Downcast<T>
        {
            ::downcast::Downcast::<T>::unchecked_downcast_ref(self)
        }

        #[allow(unused)]
        #[inline(always)]
        pub fn downcast_ref<T>(&self) -> Option<&T> 
            where T: ::std::any::Any, Self: ::downcast::Downcast<T>
        {
            ::downcast::Downcast::<T>::downcast_ref(self)
        }

        #[allow(unused)]
        #[inline(always)]
        pub unsafe fn unchecked_downcast_mut<T>(&mut self) -> &mut T
            where T: ::std::any::Any, Self: ::downcast::Downcast<T>
        {
            ::downcast::Downcast::<T>::unchecked_downcast_mut(self)
        }

        #[allow(unused)]
        #[inline(always)]
        pub fn downcast_mut<T>(&mut self) -> Option<&mut T> 
            where T: ::std::any::Any, Self: ::downcast::Downcast<T>
        {
            ::downcast::Downcast::<T>::downcast_mut(self)
        }

        #[allow(unused)]
        #[inline(always)]
        pub unsafe fn unchecked_downcast<T>(self: Box<Self>) -> Box<T>
            where T: ::std::any::Any, Self: ::downcast::Downcast<T>
        {
            ::downcast::Downcast::<T>::unchecked_downcast(self)
        }

        #[allow(unused)]
        #[inline(always)]
        pub fn downcast<T>(self: Box<Self>) ->  Result<Box<T>, Box<Self>>
            where T: ::std::any::Any, Self: ::downcast::Downcast<T>
        {
            ::downcast::Downcast::<T>::downcast(self)
        }
    };
    ($ty:ty) => {
        impl $ty {
            downcast_methods!(@items);
        }
    };
}