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use std::{
	ptr, any::Any,
	os::raw::{ c_void, c_char }
};


/// A Rust-box implementation with a `Box<dyn Any + 'static>` as backing
mod rust_box_impl {
	use super::*;
	
	pub unsafe extern "C" fn dealloc(object: *mut *mut c_void) {
		// Dereference the outer pointer
		let object = (object as *mut *mut Box<dyn Any + 'static>).as_mut()
			.expect("Unexpected NULL pointer");
		
		// Deallocate the vec and set the outer pointer to NULL
		if !object.is_null() {
			let _opaque = Box::from_raw(*object);
			*object = ptr::null_mut();
		}
	}
	pub unsafe extern "C" fn type_hint() -> *const c_char {
		b"Rust::Box<dyn Any + 'static>\0".as_ptr() as *const c_char
	}
}


/// A FFI-compatible wrapper for opaque types
#[repr(C)]
pub struct Opaque {
	/// The deallocator if the object is owned
	pub dealloc: unsafe extern "C" fn(*mut *mut c_void),
	/// A pointer to a type hint (which is a NULL-terminated string)
	pub type_hint: unsafe extern "C" fn() -> *const c_char,
	/// The underlying object (implementation dependent)
	pub object: *mut c_void
}
impl Opaque {
	/// The type hint of the opaque object
	pub fn type_hint(&self) -> *const c_char {
		unsafe{ (self.type_hint)() }
	}
	/// A reference to a `Box<dyn Any + 'static>` if the opaque object contains such a box
	pub fn as_rust_box(&self) -> Option<&Box<dyn Any + 'static>> {
		match self.type_hint {
			_ if self.type_hint == rust_box_impl::type_hint =>
				unsafe{ (self.object as *const Box<dyn Any + 'static>).as_ref() }
					.expect("Unexpected NULL pointer").into(),
			_ => None
		}
	}
	/// A mutable reference to a `Box<dyn Any + 'static>` if the opaque object contains such a box
	pub fn as_rust_box_mut(&mut self) -> Option<&mut Box<dyn Any + 'static>> {
		match self.type_hint {
			_ if self.type_hint == rust_box_impl::type_hint =>
				unsafe{ (self.object as *mut Box<dyn Any + 'static>).as_mut() }
					.expect("Unexpected NULL pointer").into(),
			_ => None
		}
	}
}
impl From<Box<dyn Any + 'static>> for Opaque {
	fn from(boxed: Box<dyn Any + 'static>) -> Self {
		Self {
			dealloc: rust_box_impl::dealloc,
			type_hint: rust_box_impl::type_hint,
			object: Box::into_raw(Box::new(boxed)) as *mut c_void
		}
	}
}
impl Drop for Opaque {
	fn drop(&mut self) {
		unsafe{ (self.dealloc)(&mut self.object) }
	}
}