public class AtomicArray<E> {
    private final AtomicReferenceArray<E> array;
    private volatile List<E> nonNullList;
    public AtomicArray(int size) {
        array = new AtomicReferenceArray<>(size);
    }
    /**
     * The size of the expected results, including potential null values.
     */
    public int length() {
        return array.length();
    }
    /**
     * Sets the element at position {@code i} to the given value.
     *
     * @param i     the index
     * @param value the new value
     */
    public void set(int i, E value) {
        array.set(i, value);
        if (nonNullList != null) { // read first, lighter, and most times it will be null...
            nonNullList = null;
        }
    }
    public final void setOnce(int i, E value) {
        if (array.compareAndSet(i, null, value) == false) {
            throw new IllegalStateException("index [" + i + "] has already been set");
        }
        if (nonNullList != null) { // read first, lighter, and most times it will be null...
            nonNullList = null;
        }
    }
    /**
     * Gets the current value at position {@code i}.
     *
     * @param i the index
     * @return the current value
     */
    public E get(int i) {
        return array.get(i);
    }
    /**
     * Returns the it as a non null list.
     */
    public List<E> asList() {
        if (nonNullList == null) {
            if (array == null || array.length() == 0) {
                nonNullList = Collections.emptyList();
            } else {
                List<E> list = new ArrayList<>(array.length());
                for (int i = 0; i < array.length(); i++) {
                    E e = array.get(i);
                    if (e != null) {
                        list.add(e);
                    }
                }
                nonNullList = list;
            }
        }
        return nonNullList;
    }
    /**
     * Copies the content of the underlying atomic array to a normal one.
     */
    public E[] toArray(E[] a) {
        if (a.length != array.length()) {
            throw new ElasticsearchGenerationException("AtomicArrays can only be copied to arrays of the same size");
        }
        for (int i = 0; i < array.length(); i++) {
            a[i] = array.get(i);
        }
        return a;
    }
}

public final class GroupedActionListener<T> implements ActionListener<T> {
    private final CountDown countDown;
    private final AtomicInteger pos = new AtomicInteger();
    private final AtomicArray<T> results;
    private final ActionListener<Collection<T>> delegate;
    private final Collection<T> defaults;
    private final AtomicReference<Exception> failure = new AtomicReference<>();
    /**
     * Creates a new listener
     * @param delegate the delegate listener
     * @param groupSize the group size
     */
    public GroupedActionListener(ActionListener<Collection<T>> delegate, int groupSize,
                                 Collection<T> defaults) {
        results = new AtomicArray<>(groupSize);
        countDown = new CountDown(groupSize);
        this.delegate = delegate;
        this.defaults = defaults;
    }
    @Override
    public void onResponse(T element) {
        results.setOnce(pos.incrementAndGet() - 1, element);
        if (countDown.countDown()) {
            if (failure.get() != null) {
                delegate.onFailure(failure.get());
            } else {
                List<T> collect = this.results.asList();
                collect.addAll(defaults);
                delegate.onResponse(Collections.unmodifiableList(collect));
            }
        }
    }
    @Override
    public void onFailure(Exception e) {
        if (failure.compareAndSet(null, e) == false) {
            failure.accumulateAndGet(e, (previous, current) -> {
                previous.addSuppressed(current);
                return previous;
            });
        }
        if (countDown.countDown()) {
            delegate.onFailure(failure.get());
        }
    }
}

public final class CountDown {
    private final AtomicInteger countDown;
    private final int originalCount;
    public CountDown(int count) {
        if (count < 0) {
            throw new IllegalArgumentException("count must be greater or equal to 0 but was: " + count);
        }
        this.originalCount = count;
        this.countDown = new AtomicInteger(count);
    }
    /**
     * Decrements the count-down and returns <code>true</code> iff this call
     * reached zero otherwise <code>false</code>
     */
    public boolean countDown() {
        assert originalCount > 0;
        for (;;) {
            final int current = countDown.get();
            assert current >= 0;
            if (current == 0) {
                return false;
            }
            if (countDown.compareAndSet(current, current - 1)) {
                return current == 1;
            }
        }
    }
    /**
     * Fast forwards the count-down to zero and returns <code>true</code> iff
     * the count down reached zero with this fast forward call otherwise
     * <code>false</code>
     */
    public boolean fastForward() {
        assert originalCount > 0;
        assert countDown.get() >= 0;
        return countDown.getAndSet(0) > 0;
    }
    /**
     * Returns <code>true</code> iff the count-down has reached zero. Otherwise <code>false</code>
     */
    public boolean isCountedDown() {
        assert countDown.get() >= 0;
        return countDown.get() == 0;
    }
}