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    use crate::sync::watch;

    use std::sync::Mutex;

    /// A barrier enables multiple threads to synchronize the beginning of some computation.
    ///
    /// ```
    /// # #[tokio::main]
    /// # async fn main() {
    /// use tokio::sync::Barrier;
    /// use std::sync::Arc;
    ///
    /// let mut handles = Vec::with_capacity(10);
    /// let barrier = Arc::new(Barrier::new(10));
    /// for _ in 0..10 {
    ///     let c = barrier.clone();
    ///     // The same messages will be printed together.
    ///     // You will NOT see any interleaving.
    ///     handles.push(tokio::spawn(async move {
    ///         println!("before wait");
    ///         let wait_result = c.wait().await;
    ///         println!("after wait");
    ///         wait_result
    ///     }));
    /// }
    ///
    /// // Will not resolve until all "after wait" messages have been printed
    /// let mut num_leaders = 0;
    /// for handle in handles {
    ///     let wait_result = handle.await.unwrap();
    ///     if wait_result.is_leader() {
    ///         num_leaders += 1;
    ///     }
    /// }
    ///
    /// // Exactly one barrier will resolve as the "leader"
    /// assert_eq!(num_leaders, 1);
    /// # }
    /// ```
    #[derive(Debug)]
    pub struct Barrier {
        state: Mutex<BarrierState>,
        wait: watch::Receiver<usize>,
        n: usize,
    }

    #[derive(Debug)]
    struct BarrierState {
        waker: watch::Sender<usize>,
        arrived: usize,
        generation: usize,
    }

    impl Barrier {
        /// Creates a new barrier that can block a given number of threads.
        ///
        /// A barrier will block `n`-1 threads which call [`Barrier::wait`] and then wake up all
        /// threads at once when the `n`th thread calls `wait`.
        pub fn new(mut n: usize) -> Barrier {
            let (waker, wait) = crate::sync::watch::channel(0);

            if n == 0 {
                // if n is 0, it's not clear what behavior the user wants.
                // in std::sync::Barrier, an n of 0 exhibits the same behavior as n == 1, where every
                // .wait() immediately unblocks, so we adopt that here as well.
                n = 1;
            }

            Barrier {
                state: Mutex::new(BarrierState {
                    waker,
                    arrived: 0,
                    generation: 1,
                }),
                n,
                wait,
            }
        }

        /// Does not resolve until all tasks have rendezvoused here.
        ///
        /// Barriers are re-usable after all threads have rendezvoused once, and can
        /// be used continuously.
        ///
        /// A single (arbitrary) future will receive a [`BarrierWaitResult`] that returns `true` from
        /// [`BarrierWaitResult::is_leader`] when returning from this function, and all other threads
        /// will receive a result that will return `false` from `is_leader`.
        pub async fn wait(&self) -> BarrierWaitResult {
            // NOTE: we are taking a _synchronous_ lock here.
            // It is okay to do so because the critical section is fast and never yields, so it cannot
            // deadlock even if another future is concurrently holding the lock.
            // It is _desireable_ to do so as synchronous Mutexes are, at least in theory, faster than
            // the asynchronous counter-parts, so we should use them where possible [citation needed].
            // NOTE: the extra scope here is so that the compiler doesn't think `state` is held across
            // a yield point, and thus marks the returned future as !Send.
            let generation = {
                let mut state = self.state.lock().unwrap();
                let generation = state.generation;
                state.arrived += 1;
                if state.arrived == self.n {
                    // we are the leader for this generation
                    // wake everyone, increment the generation, and return
                    state
                        .waker
                        .send(state.generation)
                        .expect("there is at least one receiver");
                    state.arrived = 0;
                    state.generation += 1;
                    return BarrierWaitResult(true);
                }

                generation
            };

            // we're going to have to wait for the last of the generation to arrive
            let mut wait = self.wait.clone();

            loop {
                let _ = wait.changed().await;

                // note that the first time through the loop, this _will_ yield a generation
                // immediately, since we cloned a receiver that has never seen any values.
                if *wait.borrow() >= generation {
                    break;
                }
            }

            BarrierWaitResult(false)
        }
    }

    /// A `BarrierWaitResult` is returned by `wait` when all threads in the `Barrier` have rendezvoused.
    #[derive(Debug, Clone)]
    pub struct BarrierWaitResult(bool);

    impl BarrierWaitResult {
        /// Returns `true` if this thread from wait is the "leader thread".
        ///
        /// Only one thread will have `true` returned from their result, all other threads will have
        /// `false` returned.
        pub fn is_leader(&self) -> bool {
            self.0
        }
    }