Java SE 7徹底理解　第3回 Fork/Join FrameworkからProject Lambdaへ

private final static int INSERTIONSORT_THRESHOLD = 7;

static class MergeSortTask extends RecursiveAction {
    final Object[] src;
    final Object[] dest;
    int low;
    int high;

    public MergeSortTask(Object[] src, Object[] dest, int low, int high) {
        this.src = src;
        this.dest = dest;
        this.low = low;
        this.high = high;
    }
 
    private static void swap(Object[] x, int a, int b) {
        Object t = x[a];
        x[a] = x[b];
        x[b] = t;
    }
 
    protected void compute() {
        int length = high - low;

        if (length <= INSERTIONSORT_THRESHOLD) {
            for (int i = low; i < high; i++) {
                for (int j = i; j > low && ((Comparable) dest[j - 1]).compareTo(dest[j]) > 0; j--) {
                    swap(dest, j, j - 1);
                }
            }
            return;
        }
 
        int destLow = low;
        int destHigh = high;
        int mid = (low + high) >>> 1;
 
        MergeSortTask left = new MergeSortTask(dest, src, low, mid);
        left.fork();

        MergeSortTask right = new MergeSortTask(dest, src, mid, high);
        right.compute(); 
        left.join();
     
        if (((Comparable) src[mid - 1]).compareTo(src[mid]) <= 0) {
            System.arraycopy(src, low, dest, destLow, length);
            return;
        }
 
        for (int i = destLow, p = low, q = mid; i < destHigh; i++) {
            if (q >= high || p < mid && ((Comparable) src[p]).compareTo(src[q]) <= 0) {
                dest[i] = src[p++];
            } else {
                dest[i] = src[q++];
            }
        }
    }
}