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    //! Definition of the Shared combinator, a future that is cloneable,
    //! and can be polled in multiple threads.
    //!
    //! # Examples
    //!
    //! ```
    //! use futures::future::*;
    //!
    //! let future = ok::<_, bool>(6);
    //! let shared1 = future.shared();
    //! let shared2 = shared1.clone();
    //! assert_eq!(6, *shared1.wait().unwrap());
    //! assert_eq!(6, *shared2.wait().unwrap());
    //! ```

    use {Future, Poll, Async};
    use task::{self, Task};
    use executor::{self, Notify, Spawn};

    use std::{error, fmt, mem, ops};
    use std::cell::UnsafeCell;
    use std::sync::{Arc, Mutex};
    use std::sync::atomic::AtomicUsize;
    use std::sync::atomic::Ordering::SeqCst;
    use std::collections::HashMap;

    /// A future that is cloneable and can be polled in multiple threads.
    /// Use `Future::shared()` method to convert any future into a `Shared` future.
    #[must_use = "futures do nothing unless polled"]
    pub struct Shared<F: Future> {
        inner: Arc<Inner<F>>,
        waiter: usize,
    }

    impl<F> fmt::Debug for Shared<F>
        where F: Future + fmt::Debug,
              F::Item: fmt::Debug,
              F::Error: fmt::Debug,
    {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("Shared")
                .field("inner", &self.inner)
                .field("waiter", &self.waiter)
                .finish()
        }
    }

    struct Inner<F: Future> {
        next_clone_id: AtomicUsize,
        future: UnsafeCell<Option<Spawn<F>>>,
        result: UnsafeCell<Option<Result<SharedItem<F::Item>, SharedError<F::Error>>>>,
        notifier: Arc<Notifier>,
    }

    struct Notifier {
        state: AtomicUsize,
        waiters: Mutex<HashMap<usize, Task>>,
    }

    const IDLE: usize = 0;
    const POLLING: usize = 1;
    const REPOLL: usize = 2;
    const COMPLETE: usize = 3;
    const POISONED: usize = 4;

    pub fn new<F: Future>(future: F) -> Shared<F> {
        Shared {
            inner: Arc::new(Inner {
                next_clone_id: AtomicUsize::new(1),
                notifier: Arc::new(Notifier {
                    state: AtomicUsize::new(IDLE),
                    waiters: Mutex::new(HashMap::new()),
                }),
                future: UnsafeCell::new(Some(executor::spawn(future))),
                result: UnsafeCell::new(None),
            }),
            waiter: 0,
        }
    }

    impl<F> Shared<F> where F: Future {
        // TODO: make this private
        #[deprecated(since = "0.1.12", note = "use `Future::shared` instead")]
        #[cfg(feature = "with-deprecated")]
        #[doc(hidden)]
        pub fn new(future: F) -> Self {
            new(future)
        }

        /// If any clone of this `Shared` has completed execution, returns its result immediately
        /// without blocking. Otherwise, returns None without triggering the work represented by
        /// this `Shared`.
        pub fn peek(&self) -> Option<Result<SharedItem<F::Item>, SharedError<F::Error>>> {
            match self.inner.notifier.state.load(SeqCst) {
                COMPLETE => {
                    Some(unsafe { self.clone_result() })
                }
                POISONED => panic!("inner future panicked during poll"),
                _ => None,
            }
        }

        fn set_waiter(&mut self) {
            let mut waiters = self.inner.notifier.waiters.lock().unwrap();
            waiters.insert(self.waiter, task::current());
        }

        unsafe fn clone_result(&self) -> Result<SharedItem<F::Item>, SharedError<F::Error>> {
            match *self.inner.result.get() {
                Some(Ok(ref item)) => Ok(SharedItem { item: item.item.clone() }),
                Some(Err(ref e)) => Err(SharedError { error: e.error.clone() }),
                _ => unreachable!(),
            }
        }

        fn complete(&self) {
            unsafe { *self.inner.future.get() = None };
            self.inner.notifier.state.store(COMPLETE, SeqCst);
            self.inner.notifier.notify(0);
        }
    }

    impl<F> Future for Shared<F>
        where F: Future
    {
        type Item = SharedItem<F::Item>;
        type Error = SharedError<F::Error>;

        fn poll(&mut self) -> Poll<Self::Item, Self::Error> {
            self.set_waiter();

            match self.inner.notifier.state.compare_and_swap(IDLE, POLLING, SeqCst) {
                IDLE => {
                    // Lock acquired, fall through
                }
                POLLING | REPOLL => {
                    // Another task is currently polling, at this point we just want
                    // to ensure that our task handle is currently registered

                    return Ok(Async::NotReady);
                }
                COMPLETE => {
                    return unsafe { self.clone_result().map(Async::Ready) };
                }
                POISONED => panic!("inner future panicked during poll"),
                _ => unreachable!(),
            }

            loop {
                struct Reset<'a>(&'a AtomicUsize);

                impl<'a> Drop for Reset<'a> {
                    fn drop(&mut self) {
                        use std::thread;

                        if thread::panicking() {
                            self.0.store(POISONED, SeqCst);
                        }
                    }
                }

                let _reset = Reset(&self.inner.notifier.state);

                // Poll the future
                let res = unsafe {
                    (*self.inner.future.get()).as_mut().unwrap()
                        .poll_future_notify(&self.inner.notifier, 0)
                };
                match res {
                    Ok(Async::NotReady) => {
                        // Not ready, try to release the handle
                        match self.inner.notifier.state.compare_and_swap(POLLING, IDLE, SeqCst) {
                            POLLING => {
                                // Success
                                return Ok(Async::NotReady);
                            }
                            REPOLL => {
                                // Gotta poll again!
                                let prev = self.inner.notifier.state.swap(POLLING, SeqCst);
                                assert_eq!(prev, REPOLL);
                            }
                            _ => unreachable!(),
                        }

                    }
                    Ok(Async::Ready(i)) => {
                        unsafe {
                            (*self.inner.result.get()) = Some(Ok(SharedItem { item: Arc::new(i) }));
                        }

                        break;
                    }
                    Err(e) => {
                        unsafe {
                            (*self.inner.result.get()) = Some(Err(SharedError { error: Arc::new(e) }));
                        }

                        break;
                    }
                }
            }

            self.complete();
            unsafe { self.clone_result().map(Async::Ready) }
        }
    }

    impl<F> Clone for Shared<F> where F: Future {
        fn clone(&self) -> Self {
            let next_clone_id = self.inner.next_clone_id.fetch_add(1, SeqCst);

            Shared {
                inner: self.inner.clone(),
                waiter: next_clone_id,
            }
        }
    }

    impl<F> Drop for Shared<F> where F: Future {
        fn drop(&mut self) {
            let mut waiters = self.inner.notifier.waiters.lock().unwrap();
            waiters.remove(&self.waiter);
        }
    }

    impl Notify for Notifier {
        fn notify(&self, _id: usize) {
            self.state.compare_and_swap(POLLING, REPOLL, SeqCst);

            let waiters = mem::replace(&mut *self.waiters.lock().unwrap(), HashMap::new());

            for (_, waiter) in waiters {
                waiter.notify();
            }
        }
    }

    unsafe impl<F: Future> Sync for Inner<F> {}
    unsafe impl<F: Future> Send for Inner<F> {}

    impl<F> fmt::Debug for Inner<F>
        where F: Future + fmt::Debug,
              F::Item: fmt::Debug,
              F::Error: fmt::Debug,
    {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("Inner")
                .finish()
        }
    }

    /// A wrapped item of the original future that is cloneable and implements Deref
    /// for ease of use.
    #[derive(Clone, Debug)]
    pub struct SharedItem<T> {
        item: Arc<T>,
    }

    impl<T> ops::Deref for SharedItem<T> {
        type Target = T;

        fn deref(&self) -> &T {
            &self.item.as_ref()
        }
    }

    /// A wrapped error of the original future that is cloneable and implements Deref
    /// for ease of use.
    #[derive(Clone, Debug)]
    pub struct SharedError<E> {
        error: Arc<E>,
    }

    impl<E> ops::Deref for SharedError<E> {
        type Target = E;

        fn deref(&self) -> &E {
            &self.error.as_ref()
        }
    }

    impl<E> fmt::Display for SharedError<E>
        where E: fmt::Display,
    {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            self.error.fmt(f)
        }
    }

    impl<E> error::Error for SharedError<E>
        where E: error::Error,
    {
        fn description(&self) -> &str {
            self.error.description()
        }

        fn cause(&self) -> Option<&error::Error> {
            self.error.cause()
        }
    }

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