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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.

    #![unstable(feature = "semaphore",
                reason = "the interaction between semaphores and the acquisition/release \
                          of resources is currently unclear",
                issue = "27798")]
    #![rustc_deprecated(since = "1.7.0",
                        reason = "easily confused with system sempahores and not \
                                  used enough to pull its weight")]
    #![allow(deprecated)]

    use ops::Drop;
    use sync::{Mutex, Condvar};

    /// A counting, blocking, semaphore.
    ///
    /// Semaphores are a form of atomic counter where access is only granted if the
    /// counter is a positive value. Each acquisition will block the calling thread
    /// until the counter is positive, and each release will increment the counter
    /// and unblock any threads if necessary.
    ///
    /// # Examples
    ///
    /// ```
    /// #![feature(semaphore)]
    ///
    /// use std::sync::Semaphore;
    ///
    /// // Create a semaphore that represents 5 resources
    /// let sem = Semaphore::new(5);
    ///
    /// // Acquire one of the resources
    /// sem.acquire();
    ///
    /// // Acquire one of the resources for a limited period of time
    /// {
    ///     let _guard = sem.access();
    ///     // ...
    /// } // resources is released here
    ///
    /// // Release our initially acquired resource
    /// sem.release();
    /// ```
    pub struct Semaphore {
        lock: Mutex<isize>,
        cvar: Condvar,
    }

    /// An RAII guard which will release a resource acquired from a semaphore when
    /// dropped.
    pub struct SemaphoreGuard<'a> {
        sem: &'a Semaphore,
    }

    impl Semaphore {
        /// Creates a new semaphore with the initial count specified.
        ///
        /// The count specified can be thought of as a number of resources, and a
        /// call to `acquire` or `access` will block until at least one resource is
        /// available. It is valid to initialize a semaphore with a negative count.
        pub fn new(count: isize) -> Semaphore {
            Semaphore {
                lock: Mutex::new(count),
                cvar: Condvar::new(),
            }
        }

        /// Acquires a resource of this semaphore, blocking the current thread until
        /// it can do so.
        ///
        /// This method will block until the internal count of the semaphore is at
        /// least 1.
        pub fn acquire(&self) {
            let mut count = self.lock.lock().unwrap();
            while *count <= 0 {
                count = self.cvar.wait(count).unwrap();
            }
            *count -= 1;
        }

        /// Release a resource from this semaphore.
        ///
        /// This will increment the number of resources in this semaphore by 1 and
        /// will notify any pending waiters in `acquire` or `access` if necessary.
        pub fn release(&self) {
            *self.lock.lock().unwrap() += 1;
            self.cvar.notify_one();
        }

        /// Acquires a resource of this semaphore, returning an RAII guard to
        /// release the semaphore when dropped.
        ///
        /// This function is semantically equivalent to an `acquire` followed by a
        /// `release` when the guard returned is dropped.
        pub fn access(&self) -> SemaphoreGuard {
            self.acquire();
            SemaphoreGuard { sem: self }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<'a> Drop for SemaphoreGuard<'a> {
        fn drop(&mut self) {
            self.sem.release();
        }
    }

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

        use sync::Arc;
        use super::Semaphore;
        use sync::mpsc::channel;
        use thread;

        #[test]
        fn test_sem_acquire_release() {
            let s = Semaphore::new(1);
            s.acquire();
            s.release();
            s.acquire();
        }

        #[test]
        fn test_sem_basic() {
            let s = Semaphore::new(1);
            let _g = s.access();
        }

        #[test]
        fn test_sem_as_mutex() {
            let s = Arc::new(Semaphore::new(1));
            let s2 = s.clone();
            let _t = thread::spawn(move|| {
                let _g = s2.access();
            });
            let _g = s.access();
        }

        #[test]
        fn test_sem_as_cvar() {
            /* Child waits and parent signals */
            let (tx, rx) = channel();
            let s = Arc::new(Semaphore::new(0));
            let s2 = s.clone();
            let _t = thread::spawn(move|| {
                s2.acquire();
                tx.send(()).unwrap();
            });
            s.release();
            let _ = rx.recv();

            /* Parent waits and child signals */
            let (tx, rx) = channel();
            let s = Arc::new(Semaphore::new(0));
            let s2 = s.clone();
            let _t = thread::spawn(move|| {
                s2.release();
                let _ = rx.recv();
            });
            s.acquire();
            tx.send(()).unwrap();
        }

        #[test]
        fn test_sem_multi_resource() {
            // Parent and child both get in the critical section at the same
            // time, and shake hands.
            let s = Arc::new(Semaphore::new(2));
            let s2 = s.clone();
            let (tx1, rx1) = channel();
            let (tx2, rx2) = channel();
            let _t = thread::spawn(move|| {
                let _g = s2.access();
                let _ = rx2.recv();
                tx1.send(()).unwrap();
            });
            let _g = s.access();
            tx2.send(()).unwrap();
            rx1.recv().unwrap();
        }

        #[test]
        fn test_sem_runtime_friendly_blocking() {
            let s = Arc::new(Semaphore::new(1));
            let s2 = s.clone();
            let (tx, rx) = channel();
            {
                let _g = s.access();
                thread::spawn(move|| {
                    tx.send(()).unwrap();
                    drop(s2.access());
                    tx.send(()).unwrap();
                });
                rx.recv().unwrap(); // wait for child to come alive
            }
            rx.recv().unwrap(); // wait for child to be done
        }
    }

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