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#![feature(get_type_id)]

use std::any::{Any, TypeId};

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

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

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

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

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


#[doc(hidden)]
#[inline]
pub fn get_type_id<T: ?Sized + Any>(obj: &T) -> TypeId {
    obj.get_type_id()
}

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

/// Use this macro if `impl_downcast!` can't generate the full statement for your
/// type (due to assoc-types or type-params).  
#[macro_export]
macro_rules! impl_downcast_items {
    ($t_param:ty) => {
        fn is_type(&self) -> bool {
            use std::any::TypeId;

            $crate::get_type_id(self) == TypeId::of::<$t_param>()
        }
        
        unsafe fn unsafe_downcast_ref(&self) -> &$t_param {
            use $crate::TraitObject;
            use std::mem;
            
            let obj: TraitObject = mem::transmute(self);
            mem::transmute(obj.data)
        }

        fn downcast_ref(&self) -> Option<&$t_param> {
            use $crate::Downcast;

            if Downcast::<$t_param>::is_type(self) {
                Some(unsafe { self.unsafe_downcast_ref() })
            } else {
                None
            }
        }
        
        unsafe fn unsafe_downcast_mut(&mut self) -> &mut $t_param {
            use $crate::TraitObject;
            use std::mem;
            
            let obj: TraitObject = mem::transmute(self);
            mem::transmute(obj.data)
        }
        
        fn downcast_mut(&mut self) -> Option<&mut $t_param> {
            use $crate::Downcast;

            if Downcast::<$t_param>::is_type(self) {
                Some(unsafe { self.unsafe_downcast_mut() })
            } else {
                None
            }
        }

        fn downcast_boxed(self: Box<Self>) -> Result<Box<$t_param>, Box<Self>> {
            use $crate::{Downcast, TraitObject};
            use std::mem;

            if Downcast::<$t_param>::is_type(&*self) {
                unsafe {
                    let obj: TraitObject = mem::transmute(self);
                    Ok(mem::transmute(obj.data))
                }
            } else {
                Err(self)            
            }
        }
    };
}

#[macro_export]
macro_rules! impl_downcast {
    ($base:ty) => {
        impl<__T> $crate::Downcast<__T> for $base 
            where __T: ::std::any::Any
        {
            impl_downcast_items!(__T);
        }
    };
}

/// Use this macro if `downcast_methods!` can't generate the full statement for your
/// type (due to assoc-types or type-params).  
#[macro_export]
macro_rules! downcast_method_items {
    () => {
        pub fn is<__T>(&self) -> bool 
            where __T: ::std::any::Any, Self: $crate::Downcast<__T>
        {
            $crate::Downcast::<__T>::is_type(self)
        }

        pub unsafe fn unsafe_downcast_ref<__T>(&self) -> &__T
            where __T: ::std::any::Any, Self: $crate::Downcast<__T>
        {
            $crate::Downcast::<__T>::unsafe_downcast_ref(self)
        }

        pub fn downcast_ref<__T>(&self) -> Option<&__T> 
            where __T: ::std::any::Any, Self: $crate::Downcast<__T>
        {
            $crate::Downcast::<__T>::downcast_ref(self)
        }

        pub unsafe fn unsafe_downcast_mut<__T>(&mut self) -> &mut __T
            where __T: ::std::any::Any, Self: $crate::Downcast<__T>
        {
            $crate::Downcast::<__T>::unsafe_downcast_mut(self)
        }

        pub fn downcast_mut<__T>(&mut self) -> Option<&mut __T> 
            where __T: ::std::any::Any, Self: $crate::Downcast<__T>
        {
            $crate::Downcast::<__T>::downcast_mut(self)
        }

        pub fn downcast_boxed<__T>(self: Box<Self>) ->  Result<Box<__T>, Box<Self>>
            where __T: ::std::any::Any, Self: $crate::Downcast<__T>
        {
            $crate::Downcast::<__T>::downcast_boxed(self)
        }
    };
}


/// 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 {
    ($ty:ty) => {
        impl $ty {
            downcast_method_items!();
        }
    };
}