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    #![doc(html_root_url = "https://docs.rs/want/0.3.0")]
    #![deny(warnings)]
    #![deny(missing_docs)]
    #![deny(missing_debug_implementations)]

    //! A Futures channel-like utility to signal when a value is wanted.
    //!
    //! Futures are supposed to be lazy, and only starting work if `Future::poll`
    //! is called. The same is true of `Stream`s, but when using a channel as
    //! a `Stream`, it can be hard to know if the receiver is ready for the next
    //! value.
    //!
    //! Put another way, given a `(tx, rx)` from `futures::sync::mpsc::channel()`,
    //! how can the sender (`tx`) know when the receiver (`rx`) actually wants more
    //! work to be produced? Just because there is room in the channel buffer
    //! doesn't mean the work would be used by the receiver.
    //!
    //! This is where something like `want` comes in. Added to a channel, you can
    //! make sure that the `tx` only creates the message and sends it when the `rx`
    //! has `poll()` for it, and the buffer was empty.
    //!
    //! # Example
    //!
    //! ```nightly
    //! # //#![feature(async_await)]
    //! extern crate want;
    //!
    //! # fn spawn<T>(_t: T) {}
    //! # fn we_still_want_message() -> bool { true }
    //! # fn mpsc_channel() -> (Tx, Rx) { (Tx, Rx) }
    //! # struct Tx;
    //! # impl Tx { fn send<T>(&mut self, _: T) {} }
    //! # struct Rx;
    //! # impl Rx { async fn recv(&mut self) -> Option<Expensive> { Some(Expensive) } }
    //!
    //! // Some message that is expensive to produce.
    //! struct Expensive;
    //!
    //! // Some futures-aware MPSC channel...
    //! let (mut tx, mut rx) = mpsc_channel();
    //!
    //! // And our `want` channel!
    //! let (mut gv, mut tk) = want::new();
    //!
    //!
    //! // Our receiving task...
    //! spawn(async move {
    //!     // Maybe something comes up that prevents us from ever
    //!     // using the expensive message.
    //!     //
    //!     // Without `want`, the "send" task may have started to
    //!     // produce the expensive message even though we wouldn't
    //!     // be able to use it.
    //!     if !we_still_want_message() {
    //!         return;
    //!     }
    //!
    //!     // But we can use it! So tell the `want` channel.
    //!     tk.want();
    //!
    //!     match rx.recv().await {
    //!         Some(_msg) => println!("got a message"),
    //!         None => println!("DONE"),
    //!     }
    //! });
    //!
    //! // Our sending task
    //! spawn(async move {
    //!     // It's expensive to create a new message, so we wait until the
    //!     // receiving end truly *wants* the message.
    //!     if let Err(_closed) = gv.want().await {
    //!         // Looks like they will never want it...
    //!         return;
    //!     }
    //!
    //!     // They want it, let's go!
    //!     tx.send(Expensive);
    //! });
    //!
    //! # fn main() {}
    //! ```

    #[macro_use]
    extern crate log;

    use std::fmt;
    use std::future::Future;
    use std::mem;
    use std::pin::Pin;
    use std::sync::Arc;
    use std::sync::atomic::AtomicUsize;
    // SeqCst is the only ordering used to ensure accessing the state and
    // TryLock are never re-ordered.
    use std::sync::atomic::Ordering::SeqCst;
    use std::task::{self, Poll, Waker};

    use try_lock::TryLock;

    /// Create a new `want` channel.
    pub fn new() -> (Giver, Taker) {
        let inner = Arc::new(Inner {
            state: AtomicUsize::new(State::Idle.into()),
            task: TryLock::new(None),
        });
        let inner2 = inner.clone();
        (
            Giver {
                inner: inner,
            },
            Taker {
                inner: inner2,
            },
        )
    }

    /// An entity that gives a value when wanted.
    pub struct Giver {
        inner: Arc<Inner>,
    }

    /// An entity that wants a value.
    pub struct Taker {
        inner: Arc<Inner>,
    }

    /// A cloneable `Giver`.
    ///
    /// It differs from `Giver` in that you cannot poll for `want`. It's only
    /// usable as a cancellation watcher.
    #[derive(Clone)]
    pub struct SharedGiver {
        inner: Arc<Inner>,
    }

    /// The `Taker` has canceled its interest in a value.
    pub struct Closed {
        _inner: (),
    }

    #[derive(Clone, Copy, Debug)]
    enum State {
        Idle,
        Want,
        Give,
        Closed,
    }

    impl From<State> for usize {
        fn from(s: State) -> usize {
            match s {
                State::Idle => 0,
                State::Want => 1,
                State::Give => 2,
                State::Closed => 3,
            }
        }
    }

    impl From<usize> for State {
        fn from(num: usize) -> State {
            match num {
                0 => State::Idle,
                1 => State::Want,
                2 => State::Give,
                3 => State::Closed,
                _ => unreachable!("unknown state: {}", num),
            }
        }
    }

    struct Inner {
        state: AtomicUsize,
        task: TryLock<Option<Waker>>,
    }

    // ===== impl Giver ======

    impl Giver {
        /// Returns a `Future` that fulfills when the `Taker` has done some action.
        pub fn want<'a>(&'a mut self) -> impl Future<Output = Result<(), Closed>> + 'a {
            Want(self)
        }

        /// Poll whether the `Taker` has registered interest in another value.
        ///
        /// - If the `Taker` has called `want()`, this returns `Async::Ready(())`.
        /// - If the `Taker` has not called `want()` since last poll, this
        ///   returns `Async::NotReady`, and parks the current task to be notified
        ///   when the `Taker` does call `want()`.
        /// - If the `Taker` has canceled (or dropped), this returns `Closed`.
        ///
        /// After knowing that the Taker is wanting, the state can be reset by
        /// calling [`give`](Giver::give).
        pub fn poll_want(&mut self, cx: &mut task::Context<'_>) -> Poll<Result<(), Closed>> {
            loop {
                let state = self.inner.state.load(SeqCst).into();
                match state {
                    State::Want => {
                        trace!("poll_want: taker wants!");
                        return Poll::Ready(Ok(()));
                    },
                    State::Closed => {
                        trace!("poll_want: closed");
                        return Poll::Ready(Err(Closed { _inner: () }));
                    },
                    State::Idle | State::Give => {
                        // Taker doesn't want anything yet, so park.
                        if let Some(mut locked) = self.inner.task.try_lock_order(SeqCst, SeqCst) {

                            // While we have the lock, try to set to GIVE.
                            let old = self.inner.state.compare_and_swap(
                                state.into(),
                                State::Give.into(),
                                SeqCst,
                            );
                            // If it's still the first state (Idle or Give), park current task.
                            if old == state.into() {
                                let park = locked.as_ref()
                                    .map(|w| !w.will_wake(cx.waker()))
                                    .unwrap_or(true);
                                if park {
                                    let old = mem::replace(&mut *locked, Some(cx.waker().clone()));
                                    drop(locked);
                                    old.map(|prev_task| {
                                        // there was an old task parked here.
                                        // it might be waiting to be notified,
                                        // so poke it before dropping.
                                        prev_task.wake();
                                    });
                                }
                                return Poll::Pending;
                            }
                            // Otherwise, something happened! Go around the loop again.
                        } else {
                            // if we couldn't take the lock, then a Taker has it.
                            // The *ONLY* reason is because it is in the process of notifying us
                            // of its want.
                            //
                            // We need to loop again to see what state it was changed to.
                        }
                    },
                }
            }
        }

        /// Mark the state as idle, if the Taker currently is wanting.
        ///
        /// Returns true if Taker was wanting, false otherwise.
        #[inline]
        pub fn give(&self) -> bool {
            // only set to IDLE if it is still Want
            self.inner.state.compare_and_swap(
                State::Want.into(),
                State::Idle.into(),
                SeqCst,
            ) == State::Want.into()
        }

        /// Check if the `Taker` has called `want()` without parking a task.
        ///
        /// This is safe to call outside of a futures task context, but other
        /// means of being notified is left to the user.
        #[inline]
        pub fn is_wanting(&self) -> bool {
            self.inner.state.load(SeqCst) == State::Want.into()
        }

        /// Check if the `Taker` has canceled interest without parking a task.
        #[inline]
        pub fn is_canceled(&self) -> bool {
            self.inner.state.load(SeqCst) == State::Closed.into()
        }

        /// Converts this into a `SharedGiver`.
        #[inline]
        pub fn shared(self) -> SharedGiver {
            SharedGiver {
                inner: self.inner,
            }
        }
    }

    impl fmt::Debug for Giver {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Giver")
                .field("state", &self.inner.state())
                .finish()
        }
    }

    // ===== impl SharedGiver ======

    impl SharedGiver {
        /// Check if the `Taker` has called `want()` without parking a task.
        ///
        /// This is safe to call outside of a futures task context, but other
        /// means of being notified is left to the user.
        #[inline]
        pub fn is_wanting(&self) -> bool {
            self.inner.state.load(SeqCst) == State::Want.into()
        }

        /// Check if the `Taker` has canceled interest without parking a task.
        #[inline]
        pub fn is_canceled(&self) -> bool {
            self.inner.state.load(SeqCst) == State::Closed.into()
        }
    }

    impl fmt::Debug for SharedGiver {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("SharedGiver")
                .field("state", &self.inner.state())
                .finish()
        }
    }

    // ===== impl Taker ======

    impl Taker {
        /// Signal to the `Giver` that the want is canceled.
        ///
        /// This is useful to tell that the channel is closed if you cannot
        /// drop the value yet.
        #[inline]
        pub fn cancel(&mut self) {
            trace!("signal: {:?}", State::Closed);
            self.signal(State::Closed)
        }

        /// Signal to the `Giver` that a value is wanted.
        #[inline]
        pub fn want(&mut self) {
            debug_assert!(
                self.inner.state.load(SeqCst) != State::Closed.into(),
                "want called after cancel"
            );
            trace!("signal: {:?}", State::Want);
            self.signal(State::Want)
        }

        #[inline]
        fn signal(&mut self, state: State) {
            let old_state = self.inner.state.swap(state.into(), SeqCst).into();
            match old_state {
                State::Idle | State::Want | State::Closed => (),
                State::Give => {
                    loop {
                        if let Some(mut locked) = self.inner.task.try_lock_order(SeqCst, SeqCst) {
                            if let Some(task) = locked.take() {
                                drop(locked);
                                trace!("signal found waiting giver, notifying");
                                task.wake();
                            }
                            return;
                        } else {
                            // if we couldn't take the lock, then a Giver has it.
                            // The *ONLY* reason is because it is in the process of parking.
                            //
                            // We need to loop and take the lock so we can notify this task.
                        }
                    }
                },
            }
        }
    }

    impl Drop for Taker {
        #[inline]
        fn drop(&mut self) {
            self.signal(State::Closed);
        }
    }

    impl fmt::Debug for Taker {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Taker")
                .field("state", &self.inner.state())
                .finish()
        }
    }

    // ===== impl Closed ======

    impl fmt::Debug for Closed {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.debug_struct("Closed")
                .finish()
        }
    }

    // ===== impl Inner ======

    impl Inner {
        #[inline]
        fn state(&self) -> State {
            self.state.load(SeqCst).into()
        }
    }

    // ===== impl PollFn ======

    struct Want<'a>(&'a mut Giver);

    impl Future for Want<'_> {
        type Output = Result<(), Closed>;

        fn poll(mut self: Pin<&mut Self>, cx: &mut task::Context<'_>) -> Poll<Self::Output> {
            self.0.poll_want(cx)
        }
    }

    #[cfg(test)]
    mod tests {
        use std::thread;
        use tokio_sync::oneshot;
        use super::*;

        fn block_on<F: Future>(f: F) -> F::Output {
            tokio_executor::enter()
                .expect("block_on enter")
                .block_on(f)
        }

        #[test]
        fn want_ready() {
            let (mut gv, mut tk) = new();

            tk.want();

            block_on(gv.want()).unwrap();
        }

        #[test]
        fn want_notify_0() {
            let (mut gv, mut tk) = new();
            let (tx, rx) = oneshot::channel();

            thread::spawn(move || {
                tk.want();
                // use a oneshot to keep this thread alive
                // until other thread was notified of want
                block_on(rx).expect("rx");
            });

            block_on(gv.want()).expect("want");

            assert!(gv.is_wanting(), "still wanting after poll_want success");
            assert!(gv.give(), "give is true when wanting");

            assert!(!gv.is_wanting(), "no longer wanting after give");
            assert!(!gv.is_canceled(), "give doesn't cancel");

            assert!(!gv.give(), "give is false if not wanting");

            tx.send(()).expect("tx");
        }

        /*
        /// This tests that if the Giver moves tasks after parking,
        /// it will still wake up the correct task.
        #[test]
        fn want_notify_moving_tasks() {
            use std::sync::Arc;
            use futures::executor::{spawn, Notify, NotifyHandle};

            struct WantNotify;

            impl Notify for WantNotify {
                fn notify(&self, _id: usize) {
                }
            }

            fn n() -> NotifyHandle {
                Arc::new(WantNotify).into()
            }

            let (mut gv, mut tk) = new();

            let mut s = spawn(poll_fn(move || {
                gv.poll_want()
            }));

            // Register with t1 as the task::current()
            let t1 = n();
            assert!(s.poll_future_notify(&t1, 1).unwrap().is_not_ready());

            thread::spawn(move || {
                thread::sleep(::std::time::Duration::from_millis(100));
                tk.want();
            });

            // And now, move to a ThreadNotify task.
            s.into_inner().wait().expect("poll_want");
        }
        */

        #[test]
        fn cancel() {
            // explicit
            let (mut gv, mut tk) = new();

            assert!(!gv.is_canceled());

            tk.cancel();

            assert!(gv.is_canceled());
            block_on(gv.want()).unwrap_err();

            // implicit
            let (mut gv, tk) = new();

            assert!(!gv.is_canceled());

            drop(tk);

            assert!(gv.is_canceled());
            block_on(gv.want()).unwrap_err();

            // notifies
            let (mut gv, tk) = new();

            thread::spawn(move || {
                let _tk = tk;
                // and dropped
            });

            block_on(gv.want()).unwrap_err();
        }

        /*
        #[test]
        fn stress() {
            let nthreads = 5;
            let nwants = 100;

            for _ in 0..nthreads {
                let (mut gv, mut tk) = new();
                let (mut tx, mut rx) = mpsc::channel(0);

                // rx thread
                thread::spawn(move || {
                    let mut cnt = 0;
                    poll_fn(move || {
                        while cnt < nwants {
                            let n = match rx.poll().expect("rx poll") {
                                Async::Ready(n) => n.expect("rx opt"),
                                Async::NotReady => {
                                    tk.want();
                                    return Ok(Async::NotReady);
                                },
                            };
                            assert_eq!(cnt, n);
                            cnt += 1;
                        }
                        Ok::<_, ()>(Async::Ready(()))
                    }).wait().expect("rx wait");
                });

                // tx thread
                thread::spawn(move || {
                    let mut cnt = 0;
                    let nsent = poll_fn(move || {
                        loop {
                            while let Ok(()) = tx.try_send(cnt) {
                                cnt += 1;
                            }
                            match gv.poll_want() {
                                Ok(Async::Ready(_)) => (),
                                Ok(Async::NotReady) => return Ok::<_, ()>(Async::NotReady),
                                Err(_) => return Ok(Async::Ready(cnt)),
                            }
                        }
                    }).wait().expect("tx wait");

                    assert_eq!(nsent, nwants);
                }).join().expect("thread join");
            }
        }
        */
    }