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    use std::marker::PhantomData;
    use std::mem;
    use std::ptr;
    use std::sync::atomic::{self, Ordering};

    use super::{Owned, Shared, Guard};

    /// Like `std::sync::atomic::AtomicPtr`.
    ///
    /// Provides atomic access to a (nullable) pointer of type `T`, interfacing with
    /// the `Owned` and `Shared` types.
    #[derive(Debug)]
    pub struct Atomic<T> {
        ptr: atomic::AtomicPtr<T>,
        _marker: PhantomData<*const ()>,
    }

    unsafe impl<T: Sync> Send for Atomic<T> {}
    unsafe impl<T: Sync> Sync for Atomic<T> {}

    fn opt_shared_into_raw<T>(val: Option<Shared<T>>) -> *mut T {
        val.map(|p| p.as_raw()).unwrap_or(ptr::null_mut())
    }

    fn opt_owned_as_raw<T>(val: &Option<Owned<T>>) -> *mut T {
        val.as_ref().map(Owned::as_raw).unwrap_or(ptr::null_mut())
    }

    fn opt_owned_into_raw<T>(val: Option<Owned<T>>) -> *mut T {
        let ptr = val.as_ref().map(Owned::as_raw).unwrap_or(ptr::null_mut());
        mem::forget(val);
        ptr
    }

    impl<T> Atomic<T> {
        /// Create a new, null atomic pointer.
        #[cfg(feature = "nightly")]
        pub const fn null() -> Atomic<T> {
            Atomic {
                ptr: atomic::AtomicPtr::new(0 as *mut _),
                _marker: PhantomData
            }
        }

        #[cfg(not(feature = "nightly"))]
        pub fn null() -> Atomic<T> {
            Atomic {
                ptr: atomic::AtomicPtr::new(0 as *mut _),
                _marker: PhantomData
            }
        }

        /// Create a new atomic pointer
        pub fn new(data: T) -> Atomic<T> {
            Atomic {
                ptr: atomic::AtomicPtr::new(Box::into_raw(Box::new(data))),
                _marker: PhantomData
            }
        }

        /// Do an atomic load with the given memory ordering.
        ///
        /// In order to perform the load, we must pass in a borrow of a
        /// `Guard`. This is a way of guaranteeing that the thread has pinned the
        /// epoch for the entire lifetime `'a`. In return, you get an optional
        /// `Shared` pointer back (`None` if the `Atomic` is currently null), with
        /// lifetime tied to the guard.
        ///
        /// # Panics
        ///
        /// Panics if `ord` is `Release` or `AcqRel`.
        pub fn load<'a>(&self, ord: Ordering, _: &'a Guard) -> Option<Shared<'a, T>> {
            unsafe { Shared::from_raw(self.ptr.load(ord)) }
        }

        /// Do an atomic store with the given memory ordering.
        ///
        /// Transfers ownership of the given `Owned` pointer, if any. Since no
        /// lifetime information is acquired, no `Guard` value is needed.
        ///
        /// # Panics
        ///
        /// Panics if `ord` is `Acquire` or `AcqRel`.
        pub fn store(&self, val: Option<Owned<T>>, ord: Ordering) {
            self.ptr.store(opt_owned_into_raw(val), ord)
        }

        /// Do an atomic store with the given memory ordering, immediately yielding
        /// a shared reference to the pointer that was stored.
        ///
        /// Transfers ownership of the given `Owned` pointer, yielding a `Shared`
        /// reference to it. Since the reference is valid only for the curent epoch,
        /// it's lifetime is tied to a `Guard` value.
        ///
        /// # Panics
        ///
        /// Panics if `ord` is `Acquire` or `AcqRel`.
        pub fn store_and_ref<'a>(&self, val: Owned<T>, ord: Ordering, _: &'a Guard)
                                 -> Shared<'a, T>
        {
            unsafe {
                let shared = Shared::from_owned(val);
                self.store_shared(Some(shared), ord);
                shared
            }
        }

        /// Do an atomic store of a `Shared` pointer with the given memory ordering.
        ///
        /// This operation does not require a guard, because it does not yield any
        /// new information about the lifetime of a pointer.
        ///
        /// # Panics
        ///
        /// Panics if `ord` is `Acquire` or `AcqRel`.
        pub fn store_shared(&self, val: Option<Shared<T>>, ord: Ordering) {
            self.ptr.store(opt_shared_into_raw(val), ord)
        }

        /// Do a compare-and-set from a `Shared` to an `Owned` pointer with the
        /// given memory ordering.
        ///
        /// As with `store`, this operation does not require a guard; it produces no new
        /// lifetime information. The `Result` indicates whether the CAS succeeded; if
        /// not, ownership of the `new` pointer is returned to the caller.
        pub fn cas(&self, old: Option<Shared<T>>, new: Option<Owned<T>>, ord: Ordering)
                   -> Result<(), Option<Owned<T>>>
        {
            if self.ptr.compare_and_swap(opt_shared_into_raw(old),
                                         opt_owned_as_raw(&new),
                                         ord) == opt_shared_into_raw(old)
            {
                mem::forget(new);
                Ok(())
            } else {
                Err(new)
            }
        }

        /// Do a compare-and-set from a `Shared` to an `Owned` pointer with the
        /// given memory ordering, immediatley acquiring a new `Shared` reference to
        /// the previously-owned pointer if successful.
        ///
        /// This operation is analogous to `store_and_ref`.
        pub fn cas_and_ref<'a>(&self, old: Option<Shared<T>>, new: Owned<T>,
                               ord: Ordering, _: &'a Guard)
                               -> Result<Shared<'a, T>, Owned<T>>
        {
            if self.ptr.compare_and_swap(opt_shared_into_raw(old), new.as_raw(), ord)
                == opt_shared_into_raw(old)
            {
                Ok(unsafe { Shared::from_owned(new) })
            } else {
                Err(new)
            }
        }

        /// Do a compare-and-set from a `Shared` to another `Shared` pointer with
        /// the given memory ordering.
        ///
        /// The boolean return value is `true` when the CAS is successful.
        pub fn cas_shared(&self, old: Option<Shared<T>>, new: Option<Shared<T>>, ord: Ordering)
                          -> bool
        {
            self.ptr.compare_and_swap(opt_shared_into_raw(old),
                                      opt_shared_into_raw(new),
                                      ord) == opt_shared_into_raw(old)
        }

        /// Do an atomic swap with an `Owned` pointer with the given memory ordering.
        pub fn swap<'a>(&self, new: Option<Owned<T>>, ord: Ordering, _: &'a Guard)
                        -> Option<Shared<'a, T>> {
            unsafe { Shared::from_raw(self.ptr.swap(opt_owned_into_raw(new), ord)) }
        }

        /// Do an atomic swap with a `Shared` pointer with the given memory ordering.
        pub fn swap_shared<'a>(&self, new: Option<Shared<T>>, ord: Ordering, _: &'a Guard)
                               -> Option<Shared<'a, T>> {
            unsafe { Shared::from_raw(self.ptr.swap(opt_shared_into_raw(new), ord)) }
        }
    }

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