import java.util.concurrent.*;
import java.util.*;

public class SumArray extends RecursiveTask<Integer>
{
	static final ForkJoinPool fjPool = new ForkJoinPool();
	int[] array;
	int lo, hi;
	
	static final int SEQUENTIAL_CUTOFF = 1000;
	public SumArray (int[] arr, int l, int h)
	{
		array = arr;
		lo = l;
		hi = h;
	}
	
	public Integer compute()
	{
		if (hi-lo <= SEQUENTIAL_CUTOFF)
		{
		    int ans = 0;
		    for(int i = lo; i < hi; i++) 
			ans += array[i];
		    return ans;
		}
		else
		{
			SumArray left = new SumArray(array, lo, (hi+lo)/2);
			SumArray right = new SumArray(array, (hi+lo)/2, hi);
			left.fork();
			int rightAns = right.compute();
			int leftAns = left.join();
			return rightAns + leftAns;
		}
	}
	
	public static void main(String[] args)
	{
		System.out.println("Creating the list of numbers...");
		Random r = new Random();
		int num = 100000000;
		int[] a = new int[num];
		for(int i = 0; i < a.length; i++) 
		    a[i] = r.nextInt(num);
		
		System.out.println("Finding the sum...");
		int sum = fjPool.invoke(new SumArray(a, 0, a.length));
		System.out.println("The sum is: " + sum);
	}
}