/* Using Java's Fork Join Framework to implement
 * the sum over an array
 */
import java.util.*;
import java.util.concurrent.*;
import java.util.stream.*;

public class ForkJoinRecursiveSum {
	public static final int SEQUENTIAL_THRESHOLD = 10;

    static class Problem extends RecursiveTask<Long> {
        private int lo, hi;
        private long[] arr;

        Problem(long[] arr, int lo, int hi) {
            this.lo = lo;
            this.hi = hi;
            this.arr = arr;
        }

        @Override
        public Long compute() {
            if (hi - lo <= SEQUENTIAL_THRESHOLD) {
                return Arrays.stream(arr, lo, hi).sum();
            } else {
                int mid = (lo + hi) / 2;
                var leftHalf = new Problem(arr, lo, mid).fork();
                var rightHalf = new Problem(arr, mid, hi).fork();
                // tasks now run in parallel
                return leftHalf.join() + rightHalf.join();
            }
        }
    }

	public static void main(String[] args) throws InterruptedException {
		long[] arr = LongStream.rangeClosed(1, 100).toArray();
        ForkJoinPool fjPool = new ForkJoinPool();
		long sum = fjPool.invoke(new Problem(arr, 0, arr.length));

		System.out.println("Sum: " + sum);
	}
}