/* Copyright (c) 2010-2011 Dmitry Vyukov. All rights reserved.
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions are met:
 *
 *    1. Redistributions of source code must retain the above copyright notice,
 *       this list of conditions and the following disclaimer.
 *
 *    2. Redistributions in binary form must reproduce the above copyright
 *       notice, this list of conditions and the following disclaimer in the
 *       documentation and/or other materials provided with the distribution.
 *
 * THIS SOFTWARE IS PROVIDED BY DMITRY VYUKOV "AS IS" AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
 * SHALL DMITRY VYUKOV OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
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 */

// http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue
// This queue is originally from the Rust stdlib

use std::sync::Arc;
use std::cell::UnsafeCell;

use std::sync::atomic::AtomicUsize;
use std::sync::atomic::Ordering::{Relaxed, Release, Acquire};

struct Node<T> {
    sequence: AtomicUsize,
    value: Option<T>,
}

// TODO: find a way to get the size of a cacheline instead of hardcoding 64?
type CachelinePadding = [u8; 64];

struct State<T> {
    _pad0: CachelinePadding,
    buffer: Vec<UnsafeCell<Node<T>>>,
    mask: usize,
    _pad1: CachelinePadding,
    enqueue_pos: AtomicUsize,
    _pad2: CachelinePadding,
    dequeue_pos: AtomicUsize,
    _pad3: CachelinePadding,
}

unsafe impl<T: Send + 'static> Send for State<T> {}
unsafe impl<T: Sync> Sync for State<T> {}

/// A bounded, array-based queue, with compare-and-swap based operations.
pub struct ArrayQueue<T> {
    state: Arc<State<T>>,
}

impl<T: Send + 'static> ArrayQueue<T> {
    /// Constructs a new, empty `ArrayQueue<T>` with the specified capacity.
    pub fn with_capacity(capacity: usize) -> ArrayQueue<T> {
        ArrayQueue{
            state: Arc::new(State::with_capacity(capacity))
        }
    }

    /// Adds the element `e` to the queue if possible.
    ///
    /// # Errors
    ///
    /// A call to `push` will fail if the queue is full; the provided element
    /// `value` is returned in the `Err` variant.
    pub fn push(&self, value: T) -> Result<(), T> {
        self.state.push(value)
    }

    /// Takes from the queue if there is an element available.
    pub fn pop(&self) -> Option<T> {
        self.state.pop()
    }
}

impl<T: Send + 'static> Clone for ArrayQueue<T> {
    fn clone(&self) -> ArrayQueue<T> {
        ArrayQueue { state: self.state.clone() }
    }
}

impl<T: Send + 'static> State<T> {
    fn with_capacity(capacity: usize) -> State<T> {
        let capacity = if capacity < 2 || (capacity & (capacity - 1)) != 0 {
            if capacity < 2 {
                2
            } else {
                // use next power of 2 as capacity
                capacity.next_power_of_two()
            }
        } else {
            capacity
        };
        let buffer = (0..capacity).map(|i| {
            UnsafeCell::new(Node { sequence:AtomicUsize::new(i), value: None })
        }).collect::<Vec<_>>();
        State{
            _pad0: [0; 64],
            buffer: buffer,
            mask: capacity-1,
            _pad1: [0; 64],
            enqueue_pos: AtomicUsize::new(0),
            _pad2: [0; 64],
            dequeue_pos: AtomicUsize::new(0),
            _pad3: [0; 64],
        }
    }

    fn push(&self, value: T) -> Result<(), T> {
        let mask = self.mask;
        let mut pos = self.enqueue_pos.load(Relaxed);
        loop {
            let node = &self.buffer[pos & mask];
            let seq = unsafe { (*node.get()).sequence.load(Acquire) };
            let diff: isize = seq as isize - pos as isize;

            if diff == 0 {
                let enqueue_pos = self.enqueue_pos.compare_and_swap(pos, pos+1, Relaxed);
                if enqueue_pos == pos {
                    unsafe {
                        (*node.get()).value = Some(value);
                        (*node.get()).sequence.store(pos+1, Release);
                    }
                    break
                } else {
                    pos = enqueue_pos;
                }
            } else if diff < 0 {
                return Err(value);
            } else {
                pos = self.enqueue_pos.load(Relaxed);
            }
        }
        Ok(())
    }

    fn pop(&self) -> Option<T> {
        let mask = self.mask;
        let mut pos = self.dequeue_pos.load(Relaxed);
        loop {
            let node = &self.buffer[pos & mask];
            let seq = unsafe { (*node.get()).sequence.load(Acquire) };
            let diff: isize = seq as isize - (pos + 1) as isize;
            if diff == 0 {
                let dequeue_pos = self.dequeue_pos.compare_and_swap(pos, pos+1, Relaxed);
                if dequeue_pos == pos {
                    unsafe {
                        let value = (*node.get()).value.take();
                        (*node.get()).sequence.store(pos + mask + 1, Release);
                        return value
                    }
                } else {
                    pos = dequeue_pos;
                }
            } else if diff < 0 {
                return None
            } else {
                pos = self.dequeue_pos.load(Relaxed);
            }
        }
    }
}

#[cfg(test)]
mod tests {
    use std::thread;
    use std::sync::mpsc::channel;
    use super::ArrayQueue;

    #[test]
    fn test() {
        let nthreads = 8;
        let nmsgs = 1000;
        let q = ArrayQueue::with_capacity(nthreads*nmsgs);
        assert_eq!(None, q.pop());
        let (tx, rx) = channel();

        for _ in 0..nthreads {
            let q = q.clone();
            let tx = tx.clone();
            thread::spawn(move || {
                let q = q;
                for i in 0..nmsgs {
                    assert!(q.push(i).is_ok());
                }
                tx.send(()).unwrap();
            });
        }

        let mut completion_rxs = vec![];
        for _ in 0..nthreads {
            let (tx, rx) = channel();
            completion_rxs.push(rx);
            let q = q.clone();
            thread::spawn(move || {
                let q = q;
                let mut i = 0;
                loop {
                    match q.pop() {
                        None => {},
                        Some(_) => {
                            i += 1;
                            if i == nmsgs { break }
                        }
                    }
                }
                tx.send(i).unwrap();
            });
        }

        for rx in completion_rxs.iter_mut() {
            assert_eq!(nmsgs, rx.recv().unwrap());
        }
        for _ in 0..nthreads {
            rx.recv().unwrap();
        }
    }
}