CryChat/crychat/server/util/Queue.java

package server.util;

/*
    !!! WORK IN PROGRESS !!!

    trying to implement a wait-free queue
 */

import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicMarkableReference;
import java.util.concurrent.atomic.AtomicReferenceArray;

public class Queue<T> {

    private AtomicMarkableReference<Integer> head, tail;
    private final int capacity;
    private AtomicReferenceArray<T> items;

    public Queue(int capacity) {
        this.capacity = capacity;
        this.head = new AtomicMarkableReference(0, false);
        this.tail = new AtomicMarkableReference(0, false);
        items = new AtomicReferenceArray<T>(capacity);
    }

    public boolean enq(T x) {
        boolean[] mark = new boolean[1];
        if (tail.get(mark) - head.get(mark) == capacity) return false;
        int tmp;
        do {
            tmp = tail.get(mark);
            System.out.println("enq: " + x);
            System.out.println("val tmp: " + tmp);
            System.out.println("val tmp: " + next(tmp));
            items.compareAndSet(tmp, null, x);
        } while (tail.compareAndSet(tmp, next(tmp), true, false));
        System.out.println("head: " + head.getReference() + "\ntail: " + tail.getReference() + "\ndiff: " + (tail.getReference() - head.getReference()));
        return true;
    }

    public T deq() {
        boolean[] mark = new boolean[1];
        System.out.println("test: " + (tail.get(mark) - head.get(mark)) + "\ncap: " + capacity);
        if (tail.get(mark) - head.get(mark) == 0) return null;
        int tmp;
        T x;
        do {
            tmp = head.get(mark);
            System.out.println("head: " + next(tmp));
            x = items.getAndSet(tmp, null);
        } while (head.compareAndSet(tmp, next(tmp), false, true));
        return x;
    }

    private int next(int x) {
        return (x + 1) % capacity;
    }
}