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    // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    use sync::{Mutex, Condvar};

    /// A barrier enables multiple threads to synchronize the beginning
    /// of some computation.
    ///
    /// ```
    /// use std::sync::{Arc, Barrier};
    /// use std::thread;
    ///
    /// 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.
    ///     thread::spawn(move|| {
    ///         println!("before wait");
    ///         c.wait();
    ///         println!("after wait");
    ///     });
    /// }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Barrier {
        lock: Mutex<BarrierState>,
        cvar: Condvar,
        num_threads: usize,
    }

    // The inner state of a double barrier
    struct BarrierState {
        count: usize,
        generation_id: usize,
    }

    /// A result returned from wait.
    ///
    /// Currently this opaque structure only has one method, `.is_leader()`. Only
    /// one thread will receive a result that will return `true` from this function.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct BarrierWaitResult(bool);

    impl Barrier {
        /// Creates a new barrier that can block a given number of threads.
        ///
        /// A barrier will block `n`-1 threads which call `wait` and then wake up
        /// all threads at once when the `n`th thread calls `wait`.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new(n: usize) -> Barrier {
            Barrier {
                lock: Mutex::new(BarrierState {
                    count: 0,
                    generation_id: 0,
                }),
                cvar: Condvar::new(),
                num_threads: n,
            }
        }

        /// Blocks the current thread until all threads has rendezvoused here.
        ///
        /// Barriers are re-usable after all threads have rendezvoused once, and can
        /// be used continuously.
        ///
        /// A single (arbitrary) thread will receive a `BarrierWaitResult` that
        /// returns `true` from `is_leader` when returning from this function, and
        /// all other threads will receive a result that will return `false` from
        /// `is_leader`
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn wait(&self) -> BarrierWaitResult {
            let mut lock = self.lock.lock().unwrap();
            let local_gen = lock.generation_id;
            lock.count += 1;
            if lock.count < self.num_threads {
                // We need a while loop to guard against spurious wakeups.
                // http://en.wikipedia.org/wiki/Spurious_wakeup
                while local_gen == lock.generation_id &&
                      lock.count < self.num_threads {
                    lock = self.cvar.wait(lock).unwrap();
                }
                BarrierWaitResult(false)
            } else {
                lock.count = 0;
                lock.generation_id += 1;
                self.cvar.notify_all();
                BarrierWaitResult(true)
            }
        }
    }

    impl BarrierWaitResult {
        /// Returns whether 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.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn is_leader(&self) -> bool { self.0 }
    }

    #[cfg(test)]
    mod tests {
        use prelude::v1::*;

        use sync::{Arc, Barrier};
        use sync::mpsc::{channel, TryRecvError};
        use thread;

        #[test]
        fn test_barrier() {
            const N: usize = 10;

            let barrier = Arc::new(Barrier::new(N));
            let (tx, rx) = channel();

            for _ in 0..N - 1 {
                let c = barrier.clone();
                let tx = tx.clone();
                thread::spawn(move|| {
                    tx.send(c.wait().is_leader()).unwrap();
                });
            }

            // At this point, all spawned threads should be blocked,
            // so we shouldn't get anything from the port
            assert!(match rx.try_recv() {
                Err(TryRecvError::Empty) => true,
                _ => false,
            });

            let mut leader_found = barrier.wait().is_leader();

            // Now, the barrier is cleared and we should get data.
            for _ in 0..N - 1 {
                if rx.recv().unwrap() {
                    assert!(!leader_found);
                    leader_found = true;
                }
            }
            assert!(leader_found);
        }
    }

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