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    //! Epoch-based memory management
    //!
    //! This module provides fast, easy to use memory management for lock free data
    //! structures. It's inspired by [Keir Fraser's *epoch-based
    //! reclamation*](https://www.cl.cam.ac.uk/techreports/UCAM-CL-TR-579.pdf).
    //!
    //! The basic problem this is solving is the fact that when one thread has
    //! removed a node from a data structure, other threads may still have pointers
    //! to that node (in the form of snapshots that will be validated through things
    //! like compare-and-swap), so the memory cannot be immediately freed. Put differently:
    //!
    //! 1. There are two sources of reachability at play -- the data structure, and
    //! the snapshots in threads accessing it. Before we delete a node, we need to know
    //! that it cannot be reached in either of these ways.
    //!
    //! 2. Once a node has been unliked from the data structure, no *new* snapshots
    //! reaching it will be created.
    //!
    //! Using the epoch scheme is fairly straightforward, and does not require
    //! understanding any of the implementation details:
    //!
    //! - When operating on a shared data structure, a thread must "pin the current
    //! epoch", which is done by calling `pin()`. This function returns a `Guard`
    //! which unpins the epoch when destroyed.
    //!
    //! - When the thread subsequently reads from a lock-free data structure, the
    //! pointers it extracts act like references with lifetime tied to the
    //! `Guard`. This allows threads to safely read from snapshotted data, being
    //! guaranteed that the data will remain allocated until they exit the epoch.
    //!
    //! To put the `Guard` to use, Crossbeam provides a set of three pointer types meant to work together:
    //!
    //! - `Owned<T>`, akin to `Box<T>`, which points to uniquely-owned data that has
    //!   not yet been published in a concurrent data structure.
    //!
    //! - `Shared<'a, T>`, akin to `&'a T`, which points to shared data that may or may
    //!   not be reachable from a data structure, but it guaranteed not to be freed
    //!   during lifetime `'a`.
    //!
    //! - `Atomic<T>`, akin to `std::sync::atomic::AtomicPtr`, which provides atomic
    //!   updates to a pointer using the `Owned` and `Shared` types, and connects them
    //!   to a `Guard`.
    //!
    //! Each of these types provides further documentation on usage.
    //!
    //! # Example
    //!
    //! ```
    //! use std::sync::atomic::Ordering::{Acquire, Release, Relaxed};
    //! use std::ptr;
    //!
    //! use crossbeam::mem::epoch::{self, Atomic, Owned};
    //!
    //! struct TreiberStack<T> {
    //!     head: Atomic<Node<T>>,
    //! }
    //!
    //! struct Node<T> {
    //!     data: T,
    //!     next: Atomic<Node<T>>,
    //! }
    //!
    //! impl<T> TreiberStack<T> {
    //!     fn new() -> TreiberStack<T> {
    //!         TreiberStack {
    //!             head: Atomic::null()
    //!         }
    //!     }
    //!
    //!     fn push(&self, t: T) {
    //!         // allocate the node via Owned
    //!         let mut n = Owned::new(Node {
    //!             data: t,
    //!             next: Atomic::null(),
    //!         });
    //!
    //!         // become active
    //!         let guard = epoch::pin();
    //!
    //!         loop {
    //!             // snapshot current head
    //!             let head = self.head.load(Relaxed, &guard);
    //!
    //!             // update `next` pointer with snapshot
    //!             n.next.store_shared(head, Relaxed);
    //!
    //!             // if snapshot is still good, link in the new node
    //!             match self.head.cas_and_ref(head, n, Release, &guard) {
    //!                 Ok(_) => return,
    //!                 Err(owned) => n = owned,
    //!             }
    //!         }
    //!     }
    //!
    //!     fn pop(&self) -> Option<T> {
    //!         // become active
    //!         let guard = epoch::pin();
    //!
    //!         loop {
    //!             // take a snapshot
    //!             match self.head.load(Acquire, &guard) {
    //!                 // the stack is non-empty
    //!                 Some(head) => {
    //!                     // read through the snapshot, *safely*!
    //!                     let next = head.next.load(Relaxed, &guard);
    //!
    //!                     // if snapshot is still good, update from `head` to `next`
    //!                     if self.head.cas_shared(Some(head), next, Release) {
    //!                         unsafe {
    //!                             // mark the node as unlinked
    //!                             guard.unlinked(head);
    //!
    //!                             // extract out the data from the now-unlinked node
    //!                             return Some(ptr::read(&(*head).data))
    //!                         }
    //!                     }
    //!                 }
    //!
    //!                 // we observed the stack empty
    //!                 None => return None
    //!             }
    //!         }
    //!     }
    //! }
    //! ```

    // FIXME: document implementation details

    mod atomic;
    mod garbage;
    mod global;
    mod guard;
    mod local;
    mod participant;
    mod participants;

    pub use self::atomic::Atomic;
    pub use self::guard::{pin, Guard};

    use std::ops::{Deref, DerefMut};
    use std::ptr;
    use std::mem;

    /// Like `Box<T>`: an owned, heap-allocated data value of type `T`.
    pub struct Owned<T> {
        data: Box<T>,
    }

    impl<T> Owned<T> {
        /// Move `t` to a new heap allocation.
        pub fn new(t: T) -> Owned<T> {
            Owned { data: Box::new(t) }
        }

        fn as_raw(&self) -> *mut T {
            self.deref() as *const _ as *mut _
        }

        /// Move data out of the owned box, deallocating the box.
        pub fn into_inner(self) -> T {
            *self.data
        }
    }

    impl<T> Deref for Owned<T> {
        type Target = T;
        fn deref(&self) -> &T {
            &self.data
        }
    }

    impl<T> DerefMut for Owned<T> {
        fn deref_mut(&mut self) -> &mut T {
            &mut self.data
        }
    }

    #[derive(PartialEq, Eq)]
    /// Like `&'a T`: a shared reference valid for lifetime `'a`.
    pub struct Shared<'a, T: 'a> {
        data: &'a T,
    }

    impl<'a, T> Copy for Shared<'a, T> {}
    impl<'a, T> Clone for Shared<'a, T> {
        fn clone(&self) -> Shared<'a, T> {
            Shared { data: self.data }
        }
    }

    impl<'a, T> Deref for Shared<'a, T> {
        type Target = &'a T;
        fn deref(&self) -> &&'a T {
            &self.data
        }
    }

    impl<'a, T> Shared<'a, T> {
        unsafe fn from_raw(raw: *mut T) -> Option<Shared<'a, T>> {
            if raw == ptr::null_mut() { None }
            else {
                Some(Shared {
                    data: mem::transmute::<*mut T, &T>(raw)
                })
            }
        }

        unsafe fn from_ref(r: &T) -> Shared<'a, T> {
            Shared { data: mem::transmute(r) }
        }

        unsafe fn from_owned(owned: Owned<T>) -> Shared<'a, T> {
            let ret = Shared::from_ref(owned.deref());
            mem::forget(owned);
            ret
        }

        pub fn as_raw(&self) -> *mut T {
            self.data as *const _ as *mut _
        }
    }

    #[cfg(test)]
    mod test {
        use std::sync::atomic::Ordering;
        use super::*;
        use mem::epoch;

        #[test]
        fn test_no_drop() {
            static mut DROPS: i32 = 0;
            struct Test;
            impl Drop for Test {
                fn drop(&mut self) {
                    unsafe {
                        DROPS += 1;
                    }
                }
            }
            let g = pin();

            let x = Atomic::null();
            x.store(Some(Owned::new(Test)), Ordering::Relaxed);
            x.store_and_ref(Owned::new(Test), Ordering::Relaxed, &g);
            let y = x.load(Ordering::Relaxed, &g);
            let z = x.cas_and_ref(y, Owned::new(Test), Ordering::Relaxed, &g).ok();
            let _ = x.cas(z, Some(Owned::new(Test)), Ordering::Relaxed);
            x.swap(Some(Owned::new(Test)), Ordering::Relaxed, &g);

            unsafe {
                assert_eq!(DROPS, 0);
            }
        }

        #[test]
        fn test_new() {
            let guard = epoch::pin();
            let my_atomic = Atomic::new(42);

            assert_eq!(**my_atomic.load(Ordering::Relaxed, &guard).unwrap(), 42);
        }
    }

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