Accord.Math.FourierTransform.FFT C# (CSharp) Method

        public static void FFT(Complex[] data, Direction direction)
        {
            int n = data.Length;
            int m = Tools.Log2(n);

            // reorder data first
            ReorderData(data);

            // compute FFT
            int tn = 1, tm;

            for (int k = 1; k <= m; k++)
            {
                Complex[] rotation = FourierTransform.GetComplexRotation(k, direction);

                tm = tn;
                tn <<= 1;

                for (int i = 0; i < tm; i++)
                {
                    Complex t = rotation[i];

                    for (int even = i; even < n; even += tn)
                    {
                        int odd = even + tm;
                        Complex ce = data[even];
                        Complex co = data[odd];

                        double tr = co.Real * t.Real - co.Imaginary * t.Imaginary;
                        double ti = co.Real * t.Imaginary + co.Imaginary * t.Real;

                        data[even] += new Complex(tr, ti);
                        data[odd] = new Complex(ce.Real - tr, ce.Imaginary - ti);
                    }
                }
            }

            if (direction == Direction.Forward)
            {
                for (int i = 0; i < data.Length; i++)
                    data[i] /= (double)n;
            }
        }

        /// <summary>
        ///   Performs the transformation over a double[] array.
        /// </summary>
        public static void FHT(double[] data, FourierTransform.Direction direction)
        {
            int N = data.Length;


            // Forward operation
            if (direction == FourierTransform.Direction.Forward)
            {
                // Copy the input to a complex array which can be processed
                //  in the complex domain by the FFT
                var cdata = new Complex[N];
                for (int i = 0; i < N; i++)
                {
                    cdata[i].Re = data[i];
                }

                // Perform FFT
                FourierTransform.FFT(cdata, FourierTransform.Direction.Forward);

                //double positive frequencies
                for (int i = 1; i < (N / 2); i++)
                {
                    cdata[i].Re *= 2.0;
                    cdata[i].Im *= 2.0;
                }

                // zero out negative frequencies
                //  (leaving out the dc component)
                for (int i = (N / 2) + 1; i < N; i++)
                {
                    cdata[i].Re = 0.0;
                    cdata[i].Im = 0.0;
                }

                // Reverse the FFT
                FourierTransform.FFT(cdata, FourierTransform.Direction.Backward);

                // Convert back to our initial double array
                for (int i = 0; i < N; i++)
                {
                    data[i] = cdata[i].Im;
                }
            }

            else // Backward operation
            {
                // The inverse Hilbert can be calculated by
                //  negating the transform and reapplying the
                //  transformation.
                //
                // H^–1{h(t)} = –H{h(t)}

                FHT(data, FourierTransform.Direction.Forward);

                for (int i = 0; i < data.Length; i++)
                {
                    data[i] = -data[i];
                }
            }
        }