for (int N = 2; N <= length; N <<= 1)
        {
            for (int i = 0; i < length; i += N)
            {
                for (int k = 0; k < N / 2; k++)
                {
                    int evenIndex = i + k;
                    int oddIndex = i + k + (N / 2);
                    var even = output[evenIndex];
                    var odd = output[oddIndex];
                    output[evenIndex] = even + odd * twiddles[k * (length / N)];
                    output[oddIndex] = even + odd * twiddles[(k + (N / 2)) * (length / N)];
                }
            }
        }