void digitrev_index_short(unsigned short *index, int n2, int radix)
{
    int i,j,k;
    index[0] = 0;
    for ( i = 1, j = n2/radix + 1; i < n2 - 1; i++)
    {
     index[i] = j - 1;
     for (k = n2/radix; k*(radix-1) < j; k /= radix)
     j -= k*(radix-1);
     j += k;
    }
    index[n2 - 1] = n2 - 1;
}