Java stream reduce

Question

I have the following example data set that I want to transform / reduce using Java stream api based on direction\'s value

Direction    int[]
IN           1, 2
O         


        

Answer 1

How about this. First define a small helper method:

private static Tuple mergeTwo(Tuple left, Tuple right) {
    int[] leftArray = left.getData();
    int[] rightArray = right.getData();
    int[] result = new int[leftArray.length + rightArray.length];
    System.arraycopy(leftArray, 0, result, 0, leftArray.length);
    System.arraycopy(rightArray, 0, result, leftArray.length, rightArray.length);
    return new Tuple(left.getDirection(), result);
}

This is close to your concat/merge I guess, but a single one. Basically a way to merge two Tuple(s) together.

And a helper method to produce the needed Collector, you can put this into a utility so that it can be re-used:

private static Collector> mergedTuplesCollector() {
    class Acc {

        ArrayDeque deque = new ArrayDeque<>();

        void add(Tuple elem) {
            Tuple head = deque.peek();
            if (head == null || head.getDirection() != elem.getDirection()) {
                deque.offerFirst(elem);
            } else {
                deque.offerFirst(mergeTwo(deque.poll(), elem));
            }
        }

        Acc merge(Acc right) {

            Tuple lastLeft = deque.peekLast();
            Tuple firstRight = right.deque.peekFirst();

            if (lastLeft.getDirection() == firstRight.getDirection()) {
                deque.offerLast(mergeTwo(deque.pollLast(), right.deque.pollFirst()));
            } else {
                deque.addAll(right.deque);
            }

            return this;
        }

        public List finisher() {
            return new ArrayList<>(deque);
        }

    }
    return Collector.of(Acc::new, Acc::add, Acc::merge, Acc::finisher);
}

And usage would be, for example:

List merged = tuples.stream()
            .parallel()
            .collect(mergedTuplesCollector());