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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.

    //! Generic support for building blocking abstractions.

    use thread::{self, Thread};
    use sync::atomic::{AtomicBool, Ordering};
    use sync::Arc;
    use marker::{Sync, Send};
    use mem;
    use clone::Clone;

    struct Inner {
        thread: Thread,
        woken: AtomicBool,
    }

    unsafe impl Send for Inner {}
    unsafe impl Sync for Inner {}

    #[derive(Clone)]
    pub struct SignalToken {
        inner: Arc<Inner>,
    }

    pub struct WaitToken {
        inner: Arc<Inner>,
    }

    impl !Send for WaitToken {}

    impl !Sync for WaitToken {}

    pub fn tokens() -> (WaitToken, SignalToken) {
        let inner = Arc::new(Inner {
            thread: thread::current(),
            woken: AtomicBool::new(false),
        });
        let wait_token = WaitToken {
            inner: inner.clone(),
        };
        let signal_token = SignalToken {
            inner: inner
        };
        (wait_token, signal_token)
    }

    impl SignalToken {
        pub fn signal(&self) -> bool {
            let wake = !self.inner.woken.compare_and_swap(false, true, Ordering::SeqCst);
            if wake {
                self.inner.thread.unpark();
            }
            wake
        }

        /// Convert to an unsafe usize value. Useful for storing in a pipe's state
        /// flag.
        #[inline]
        pub unsafe fn cast_to_usize(self) -> usize {
            mem::transmute(self.inner)
        }

        /// Convert from an unsafe usize value. Useful for retrieving a pipe's state
        /// flag.
        #[inline]
        pub unsafe fn cast_from_usize(signal_ptr: usize) -> SignalToken {
            SignalToken { inner: mem::transmute(signal_ptr) }
        }

    }

    impl WaitToken {
        pub fn wait(self) {
            while !self.inner.woken.load(Ordering::SeqCst) {
                thread::park()
            }
        }
    }

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