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use std::mem::ManuallyDrop;
use std::ptr;
use std::sync::atomic::Ordering::{Acquire, Relaxed, Release};

use epoch::{self, Atomic, Owned};

/// Treiber's lock-free stack.
///
/// Usable with any number of producers and consumers.
#[derive(Debug)]
pub struct TreiberStack<T> {
    head: Atomic<Node<T>>,
}

#[derive(Debug)]
struct Node<T> {
    data: ManuallyDrop<T>,
    next: Atomic<Node<T>>,
}

impl<T> TreiberStack<T> {
    /// Create a new, empty stack.
    pub fn new() -> TreiberStack<T> {
        TreiberStack {
            head: Atomic::null(),
        }
    }

    /// Push `t` on top of the stack.
    pub fn push(&self, t: T) {
        let mut n = Owned::new(Node {
            data: ManuallyDrop::new(t),
            next: Atomic::null(),
        });
        let guard = epoch::pin();
        loop {
            let head = self.head.load(Relaxed, &guard);
            n.next.store(head, Relaxed);
            match self.head.compare_and_set(head, n, Release, &guard) {
                Ok(_) => break,
                Err(e) => n = e.new,
            }
        }
    }

    /// Attempt to pop the top element of the stack.
    ///
    /// Returns `None` if the stack is observed to be empty.
    pub fn try_pop(&self) -> Option<T> {
        let guard = epoch::pin();
        loop {
            let head_shared = self.head.load(Acquire, &guard);
            match unsafe { head_shared.as_ref() } {
                Some(head) => {
                    let next = head.next.load(Relaxed, &guard);
                    if self
                        .head
                        .compare_and_set(head_shared, next, Release, &guard)
                        .is_ok()
                    {
                        unsafe {
                            guard.defer_destroy(head_shared);
                            return Some(ManuallyDrop::into_inner(ptr::read(&(*head).data)));
                        }
                    }
                }
                None => return None,
            }
        }
    }

    /// Check if this queue is empty.
    pub fn is_empty(&self) -> bool {
        let guard = epoch::pin();
        self.head.load(Acquire, &guard).is_null()
    }
}

impl<T> Drop for TreiberStack<T> {
    fn drop(&mut self) {
        while self.try_pop().is_some() {}
    }
}

impl<T> Default for TreiberStack<T> {
    fn default() -> Self {
        TreiberStack::new()
    }
}

#[cfg(test)]
mod test {
    use super::*;
    use std::sync::atomic::{AtomicUsize, Ordering};
    use {epoch, thread};

    #[test]
    fn is_empty() {
        let q: TreiberStack<i64> = TreiberStack::new();
        assert!(q.is_empty());
        q.push(20);
        q.push(20);
        assert!(!q.is_empty());
        assert!(!q.is_empty());
        assert!(q.try_pop().is_some());
        assert!(q.try_pop().is_some());
        assert!(q.is_empty());
        q.push(25);
        assert!(!q.is_empty());
    }

    #[test]
    fn no_double_drop() {
        static DROP_COUNT: AtomicUsize = AtomicUsize::new(0);

        struct Dropper;

        impl Drop for Dropper {
            fn drop(&mut self) {
                DROP_COUNT.fetch_add(1, Ordering::SeqCst);
            }
        }

        const N_THREADS: usize = 8;
        thread::scope(|s| {
            for _ in 0..N_THREADS {
                s.spawn(|_| {
                    let s: TreiberStack<Dropper> = TreiberStack::new();
                    for _ in 0..4 {
                        s.push(Dropper);
                    }
                    drop(s);
                    epoch::pin().flush();
                });
            }
        }).unwrap();

        assert!(DROP_COUNT.load(Ordering::SeqCst) == N_THREADS * 4);
    }
}