AForge.Math.FourierTransform.GetComplexRotation C# (CSharp) Method

        private static Complex[] GetComplexRotation(int numberOfBits, Direction direction)
        {
            int directionIndex = (direction == Direction.Forward) ? 0 : 1;

            // check if the array is already calculated
            if (complexRotation[numberOfBits - 1, directionIndex] == null)
            {
                int n = 1 << (numberOfBits - 1);
                double uR = 1.0;
                double uI = 0.0;
                double angle = System.Math.PI/n*(int) direction;
                double wR = System.Math.Cos(angle);
                double wI = System.Math.Sin(angle);
                double t;
                var rotation = new Complex[n];

                for (int i = 0; i < n; i++)
                {
                    rotation[i] = new Complex(uR, uI);
                    t = uR*wI + uI*wR;
                    uR = uR*wR - uI*wI;
                    uI = t;
                }

                complexRotation[numberOfBits - 1, directionIndex] = rotation;
            }
            return complexRotation[numberOfBits - 1, directionIndex];
        }

        /// <summary>
        /// One dimensional Fast Fourier Transform.
        /// </summary>
        ///
        /// <param name="data">Data to transform.</param>
        /// <param name="direction">Transformation direction.</param>
        ///
        /// <remarks><para><note>The method accepts <paramref name="data"/> array of 2<sup>n</sup> size
        /// only, where <b>n</b> may vary in the [1, 14] range.</note></para></remarks>
        ///
        /// <exception cref="ArgumentException">Incorrect data length.</exception>
        ///
        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.Re * t.Re - co.Im * t.Im;
                        double ti = co.Re * t.Im + co.Im * t.Re;

                        data[even].Re += tr;
                        data[even].Im += ti;

                        data[odd].Re = ce.Re - tr;
                        data[odd].Im = ce.Im - ti;
                    }
                }
            }

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