private void cftx020(float[] a, int offa) {
        float x0r, x0i;
        x0r = a[offa] - a[offa + 2];
        x0i = -a[offa + 1] + a[offa + 3];
        a[offa] += a[offa + 2];
        a[offa + 1] += a[offa + 3];
        a[offa + 2] = x0r;
        a[offa + 3] = x0i;
    }