import java.io.IOException;
import java.util.Arrays;
import java.util.Random;
import java.util.concurrent.RecursiveAction;
import java.util.concurrent.RecursiveTask;

/**
 * 分割 合并 線程池
 * 類似于遞歸的歸并排序，這個是多線程的遞歸排序
 * jdk1.7之后加的
 */
public class ForkJoinPool {

  static int[] nums = new int[1000000];
  static final int MAX_NUM = 50000;
  static Random random = new Random();

  static {
    for(int i=0; i<nums.length; i++) {
      nums[i] = random.nextInt(100);
    }
    System.out.println("總和1:" + Arrays.stream(nums).sum());
  }

  /**
   * ForkJoinPool里面必須是ForkJoinTask，
   * 直接繼承ForkJoinTask類寫起來比較麻煩，
   * 可以繼承RecursiveAction(無返回值)和RecursiveTask(遞歸任務，有返回值)
   */
//  static class AddTask extends RecursiveAction {
//
//    int start, end;
//
//    public AddTask(int start, int end) {
//      this.start = start;
//      this.end = end;
//    }
//
//    @Override
//    protected void compute() {
//      if(end-start <= MAX_NUM) {
//        long sum = 0L;
//        for(int i=start; i<end; i++) sum += nums[i];
//        System.out.println("from:" + start + " to:" + end + " =" + sum);
//      }else{
//        int middle = start + (end - start) / 2;
//
//        AddTask subTask1 = new AddTask(start, middle);
//        AddTask subTask2 = new AddTask(middle, end);
//        subTask1.fork();
//        subTask2.fork();
//      }
//    }
//  }

  /**
   * 有返回值
   */
  static class AddTask extends RecursiveTask<Long> {

    int start, end;

    public AddTask(int start, int end) {
      this.start = start;
      this.end = end;
    }

    @Override
    protected Long compute() {
      if(end-start <= MAX_NUM) {
        long sum = 0L;
        for(int i=start; i<end; i++) sum += nums[i];
        System.out.println("from:" + start + " to:" + end + " =" + sum);
        return sum;
      }

      int middle = start + (end - start) / 2;
      AddTask subTask1 = new AddTask(start, middle);
      AddTask subTask2 = new AddTask(middle, end);
      subTask1.fork();
      subTask2.fork();
      return subTask1.join() + subTask2.join();
    }
  }

  public static void main(String[] args) {

    java.util.concurrent.ForkJoinPool pool = new java.util.concurrent.ForkJoinPool();
    AddTask task = new AddTask(0, nums.length);
    pool.execute(task); //精靈線程
    System.out.println("總和2:" + task.join());
    try {
      System.in.read(); //阻塞主線程
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

}