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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

    use std::marker::PhantomData;
    use std::marker;
    use std::mem;
    use std::ops::{Deref, DerefMut};
    use std::ptr;
    use std::sync::atomic::{self, Ordering};

    mod participant;
    mod participants;
    mod global;
    mod local;
    mod garbage;

    /// 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 _
        }
    }

    /// Like `std::sync::atomic::AtomicPtr`.
    ///
    /// Provides atomic access to a (nullable) pointer of type `T`, interfacing with
    /// the `Owned` and `Shared` types.
    pub struct Atomic<T> {
        ptr: atomic::AtomicPtr<T>,
        _marker: PhantomData<*const ()>,
    }

    unsafe impl<T: Sync> Send for Atomic<T> {}
    unsafe impl<T: Sync> Sync for Atomic<T> {}

    fn opt_shared_into_raw<T>(val: Option<Shared<T>>) -> *mut T {
        val.map(|p| p.as_raw()).unwrap_or(ptr::null_mut())
    }

    fn opt_owned_as_raw<T>(val: &Option<Owned<T>>) -> *mut T {
        val.as_ref().map(Owned::as_raw).unwrap_or(ptr::null_mut())
    }

    fn opt_owned_into_raw<T>(val: Option<Owned<T>>) -> *mut T {
        let ptr = val.as_ref().map(Owned::as_raw).unwrap_or(ptr::null_mut());
        mem::forget(val);
        ptr
    }

    impl<T> Atomic<T> {
        /// Create a new, null atomic pointer.
        #[cfg(feature = "nightly")]
        pub const fn null() -> Atomic<T> {
            Atomic {
                ptr: atomic::AtomicPtr::new(0 as *mut _),
                _marker: PhantomData
            }
        }

        #[cfg(not(feature = "nightly"))]
        pub fn null() -> Atomic<T> {
            Atomic {
                ptr: atomic::AtomicPtr::new(0 as *mut _),
                _marker: PhantomData
            }
        }

        /// Create a new atomic pointer
        pub fn new(data: T) -> Atomic<T> {
            Atomic {
                ptr: atomic::AtomicPtr::new(Box::into_raw(Box::new(data))),
                _marker: PhantomData
            }
        }

        /// Do an atomic load with the given memory ordering.
        ///
        /// In order to perform the load, we must pass in a borrow of a
        /// `Guard`. This is a way of guaranteeing that the thread has pinned the
        /// epoch for the entire lifetime `'a`. In return, you get an optional
        /// `Shared` pointer back (`None` if the `Atomic` is currently null), with
        /// lifetime tied to the guard.
        ///
        /// # Panics
        ///
        /// Panics if `ord` is `Release` or `AcqRel`.
        pub fn load<'a>(&self, ord: Ordering, _: &'a Guard) -> Option<Shared<'a, T>> {
            unsafe { Shared::from_raw(self.ptr.load(ord)) }
        }

        /// Do an atomic store with the given memory ordering.
        ///
        /// Transfers ownership of the given `Owned` pointer, if any. Since no
        /// lifetime information is acquired, no `Guard` value is needed.
        ///
        /// # Panics
        ///
        /// Panics if `ord` is `Acquire` or `AcqRel`.
        pub fn store(&self, val: Option<Owned<T>>, ord: Ordering) {
            self.ptr.store(opt_owned_into_raw(val), ord)
        }

        /// Do an atomic store with the given memory ordering, immediately yielding
        /// a shared reference to the pointer that was stored.
        ///
        /// Transfers ownership of the given `Owned` pointer, yielding a `Shared`
        /// reference to it. Since the reference is valid only for the curent epoch,
        /// it's lifetime is tied to a `Guard` value.
        ///
        /// # Panics
        ///
        /// Panics if `ord` is `Acquire` or `AcqRel`.
        pub fn store_and_ref<'a>(&self, val: Owned<T>, ord: Ordering, _: &'a Guard)
                                 -> Shared<'a, T>
        {
            unsafe {
                let shared = Shared::from_owned(val);
                self.store_shared(Some(shared), ord);
                shared
            }
        }

        /// Do an atomic store of a `Shared` pointer with the given memory ordering.
        ///
        /// This operation does not require a guard, because it does not yield any
        /// new information about the lifetime of a pointer.
        ///
        /// # Panics
        ///
        /// Panics if `ord` is `Acquire` or `AcqRel`.
        pub fn store_shared(&self, val: Option<Shared<T>>, ord: Ordering) {
            self.ptr.store(opt_shared_into_raw(val), ord)
        }

        /// Do a compare-and-set from a `Shared` to an `Owned` pointer with the
        /// given memory ordering.
        ///
        /// As with `store`, this operation does not require a guard; it produces no new
        /// lifetime information. The `Result` indicates whether the CAS succeeded; if
        /// not, ownership of the `new` pointer is returned to the caller.
        pub fn cas(&self, old: Option<Shared<T>>, new: Option<Owned<T>>, ord: Ordering)
                   -> Result<(), Option<Owned<T>>>
        {
            if self.ptr.compare_and_swap(opt_shared_into_raw(old),
                                         opt_owned_as_raw(&new),
                                         ord) == opt_shared_into_raw(old)
            {
                mem::forget(new);
                Ok(())
            } else {
                Err(new)
            }
        }

        /// Do a compare-and-set from a `Shared` to an `Owned` pointer with the
        /// given memory ordering, immediatley acquiring a new `Shared` reference to
        /// the previously-owned pointer if successful.
        ///
        /// This operation is analogous to `store_and_ref`.
        pub fn cas_and_ref<'a>(&self, old: Option<Shared<T>>, new: Owned<T>,
                               ord: Ordering, _: &'a Guard)
                               -> Result<Shared<'a, T>, Owned<T>>
        {
            if self.ptr.compare_and_swap(opt_shared_into_raw(old), new.as_raw(), ord)
                == opt_shared_into_raw(old)
            {
                Ok(unsafe { Shared::from_owned(new) })
            } else {
                Err(new)
            }
        }

        /// Do a compare-and-set from a `Shared` to another `Shared` pointer with
        /// the given memory ordering.
        ///
        /// The boolean return value is `true` when the CAS is successful.
        pub fn cas_shared(&self, old: Option<Shared<T>>, new: Option<Shared<T>>, ord: Ordering)
                          -> bool
        {
            self.ptr.compare_and_swap(opt_shared_into_raw(old),
                                      opt_shared_into_raw(new),
                                      ord) == opt_shared_into_raw(old)
        }

        /// Do an atomic swap with an `Owned` pointer with the given memory ordering.
        pub fn swap<'a>(&self, new: Option<Owned<T>>, ord: Ordering, _: &'a Guard)
                        -> Option<Shared<'a, T>> {
            unsafe { Shared::from_raw(self.ptr.swap(opt_owned_into_raw(new), ord)) }
        }

        /// Do an atomic swap with a `Shared` pointer with the given memory ordering.
        pub fn swap_shared<'a>(&self, new: Option<Shared<T>>, ord: Ordering, _: &'a Guard)
                               -> Option<Shared<'a, T>> {
            unsafe { Shared::from_raw(self.ptr.swap(opt_shared_into_raw(new), ord)) }
        }
    }

    /// An RAII-style guard for pinning the current epoch.
    ///
    /// A guard must be acquired before most operations on an `Atomic` pointer. On
    /// destruction, it unpins the epoch.
    #[must_use]
    pub struct Guard {
        _marker: marker::PhantomData<*mut ()>, // !Send and !Sync
    }

    /// Pin the current epoch.
    ///
    /// Threads generally pin before interacting with a lock-free data
    /// structure. Pinning requires a full memory barrier, so is somewhat
    /// expensive. It is rentrant -- you can safely acquire nested guards, and only
    /// the first guard requires a barrier. Thus, in cases where you expect to
    /// perform several lock-free operations in quick succession, you may consider
    /// pinning around the entire set of operations.
    pub fn pin() -> Guard {
        local::with_participant(|p| {
            p.enter();

            let g = Guard {
                _marker: marker::PhantomData,
            };

            if p.should_gc() {
                p.try_collect(&g);
            }

            g
        })
    }

    impl Guard {
        /// Assert that the value is no longer reachable from a lock-free data
        /// structure and should be collected when sufficient epochs have passed.
        pub unsafe fn unlinked<T>(&self, val: Shared<T>) {
            local::with_participant(|p| p.reclaim(val.as_raw()))
        }

        /// Move the thread-local garbage into the global set of garbage.
        pub fn migrate_garbage(&self) {
            local::with_participant(|p| p.migrate_garbage())
        }
    }

    impl Drop for Guard {
        fn drop(&mut self) {
            local::with_participant(|p| p.exit());
        }
    }

    #[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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