[OTFFT: High Speed FFT library]
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## [Introduction to FFT -- Cooley-Tukey Algorithm]

This page is a homepage explaining the Cooley-Tukey FFT algorithm which is a kind of fast Fourier transforms. Fast Fourier transform, it is an algorithm that calculates discrete Fourier transform very fast. It is heavily used as a basic process in the field of scientific and technical computing.

Let's first define the discrete Fourier transform. The discrete Fourier transform $$F_N$$ whose sequence length is $$N$$ is defined as follows. $$x[p]$$ is the input array, $$X[k]$$ is the array of transformation result.

$X[k] = F_N(x[p]) = \sum_{p=0}^{N-1}x[p]W_N^{kp},~W_N = \exp\left(-j\frac{2\pi}{N}\right),~j = \sqrt{-1}\\$ $k = 0,1,\ldots,N-1 \\$

Here, we think in the way that we does not attach the coefficient $$N^{-1}$$ to the discrete Fourier transform. Also, we consider $$N$$ to be limited to $$N=2^L$$ ($$L=0,1,2,3,\ldots$$).

FFT is an algorithm based on divide and conquer. We divide the given problem into smaller problems and solve the original problem by integrating the processing results. Therefore, in order to calculate the discrete Fourier transform $$X[k]$$, let's calculate the even position component $$X[2k]$$ and the odd position component $$X[2k+1]$$ by assuming that $$N \ge 2$$. In other words, it is a division into smaller problems.

Using the relationship $$W_n^{kn}=1$$($$n$$ is a natural number, $$k$$ is an integer) and $$W_{2n}^n=-1$$($$n$$ is a natural number), each component can be transformed as follows.

$\begin{eqnarray*} X[2k] & = & \sum_{p=0}^{N-1}x[p]W_N^{2kp} = \sum_{p=0}^{N-1}x[p]W_{N/2}^{kp} \\ & = & \sum_{p=0}^{N/2-1}x[p]W_{N/2}^{kp}+\sum_{p=N/2}^{N-1}x[p]W_{N/2}^{kp} \\ & = & \sum_{p=0}^{N/2-1}x[p]W_{N/2}^{kp}+\sum_{p=0}^{N/2-1}x[p+N/2]W_{N/2}^{k(p+N/2)} \\ & = & \sum_{p=0}^{N/2-1}x[p]W_{N/2}^{kp}+\sum_{p=0}^{N/2-1}x[p+N/2]W_{N/2}^{kp} \\ & = & \sum_{p=0}^{N/2-1}(x[p]+x[p+N/2])W_{N/2}^{kp} \\ & = & F_{N/2}(x[p]+x[p+N/2]) \\ X[2k+1] & = & \sum_{p=0}^{N-1}x[p]W_N^{(2k+1)p} = \sum_{p=0}^{N-1}x[p]W_N^pW_{N/2}^{kp} \\ & = & \sum_{p=0}^{N/2-1}x[p]W_N^pW_{N/2}^{kp}+\sum_{p=N/2}^{N-1}x[p]W_N^pW_{N/2}^{kp} \\ & = & \sum_{p=0}^{N/2-1}x[p]W_N^pW_{N/2}^{kp}+\sum_{p=0}^{N/2-1}x[p+N/2]W_N^{p+N/2}W_{N/2}^{k(p+N/2)} \\ & = & \sum_{p=0}^{N/2-1}x[p]W_N^pW_{N/2}^{kp}-\sum_{p=0}^{N/2-1}x[p+N/2]W_N^pW_{N/2}^{kp} \\ & = & \sum_{p=0}^{N/2-1}(x[p]-x[p+N/2])W_N^pW_{N/2}^{kp} \\ & = & F_{N/2}\left((x[p]-x[p+N/2])W_N^p\right) \\ \end{eqnarray*}$

Now, If $$x^0[p]=x[p]+x[p+N/2]$$ and $$x^1[p]=(x[p]-x[p+N/2])W_N^p$$, $$F_N(x[p])$$ can be calculated by integrating discrete Fourier transforms of half sequence length $$F_{N/2}(x^0[p])$$, $$F_{N/2}(x^1[p])$$. In other words, $$F_N(x[p])$$ can be calculated using recursive calls. When we continue recursive calls, the sequence length of the discrete Fourier transform is halved one after another, and the sequence length will become 1 sometime. Since the discrete Fourier transform whose sequence length is 1 is just an identity transform, we can finish the recursion by returning the input as is.

When we program the calculation process of this recursive call, it becomes as follows.

#include <complex>
#include <cmath>

typedef std::complex<double> complex_t;

void F(int N, int q, complex_t* x)
// N : sequence length
// q : block start point (initial value is 0)
// x : input/output sequence
{
const int m = N/2;
const double theta0 = 2*M_PI/N;

if (N > 1) {
for (int p = 0; p < m; p++) {
const complex_t wp = complex_t(cos(p*theta0), -sin(p*theta0));
const complex_t a = x[q + p + 0];
const complex_t b = x[q + p + m];
x[q + p + 0] =  a + b;
x[q + p + m] = (a - b) * wp;
}
F(N/2, q + 0, x); // even position components
F(N/2, q + m, x); // odd position components
}
}


If we calculate the discrete Fourier transform using the original definition as is, the complexity of multiplication is $$O(N^2)$$. But, we suppose that we calculate $$W_N^{kp}$$ in advance and refer to it from the table. However, if you view this program carefully, the complexity of multiplication is $$O(N\log_2 N)$$. The computational complexity has been dramatically improved.

However, this program is still not enough as a discrete Fourier transform. In Cooley-Tukey algorithm, the phenomenon called bit reversal sorting occurs, so in this program the results are not sorted in natural order.

So, from here on, we will explain bit reversal sorting. First, we suppose that $$X^0[k]$$ represents an array of even position components of $$X[k]$$. Also, we suppose that $$X^1[k]$$ represents an array of odd position components of $$X[k]$$.

$\begin{eqnarray*} X^0[k] & = & X[2k] & = & F_{N/2}(x^0[p]) & = & F_{N/2}(x[p]+x[p+N/2]) \\ X^1[k] & = & X[2k+1] & = & F_{N/2}(x^1[p]) & = & F_{N/2}\left((x[p]-x[p+N/2])W_N^p\right) \\ \end{eqnarray*}$ $k = 0,1,\ldots,N/2-1 \\$

Similarly, $$X^{00}[k]$$ represents an array of even position components of $$X^0[k]$$. $$X^{01}[k]$$ represents an array of odd position components of $$X^0[k]$$. Like this example, we define the symbols. Then we can decompose $$X^0[k]$$,$$~X^1[k]$$ into $$X^{00}[k]$$,$$~X^{01}[k]$$,$$~X^{10}[k]$$,$$~X^{11}[k]$$ as follows.

$\begin{eqnarray*} X^{00}[k] & = & X^0[2k] & = & X[4k] & = & F_{N/4}(x^0[p]+x^0[p+N/4]) \\ X^{01}[k] & = & X^0[2k+1] & = & X[4k+2] & = & F_{N/4}\left((x^0[p]-x^0[p+N/4])W_{N/2}^p\right) \\ X^{10}[k] & = & X^1[2k] & = & X[4k+1] & = & F_{N/4}(x^1[p]+x^1[p+N/4]) \\ X^{11}[k] & = & X^1[2k+1] & = & X[4k+3] & = & F_{N/4}\left((x^1[p]-x^1[p+N/4])W_{N/2}^p\right) \\ \end{eqnarray*}$

When $$N$$ of $$F_N$$ is 8, if we illustrate the above decomposition, it becomes as follows.

 $$X[k]$$ $$X^0[k]=X[2k]$$ $$X^1[k]=X[2k+1]$$ $$X^{00}[k]=X^0[2k]$$ $$X^{01}[k]=X^0[2k+1]$$ $$X^{10}[k]=X^1[2k]$$ $$X^{11}[k]=X^1[2k+1]$$ $$X^{000}[k]=X^{00}[2k]$$ $$X^{001}[k]=X^{00}[2k+1]$$ $$X^{010}[k]=X^{01}[2k]$$ $$X^{011}[k]=X^{01}[2k+1]$$ $$X^{100}[k]=X^{10}[2k]$$ $$X^{101}[k]=X^{10}[2k+1]$$ $$X^{110}[k]=X^{11}[2k]$$ $$X^{111}[k]=X^{11}[2k+1]$$

The sequence length is halved if we decompose the sequence into even position components and odd position componebts. And so, when N is 8, the eight $$X^{abc}[k]$$ at the bottom which was decomposed with 3 step decomposition become the array whose length is 1. In other words, the value only exists on $$k=0$$.

Let's confirm what value $$X^{abc}$$ takes. Confirmation is done from the bottom of the figure. Here, the numerical values are represented by binary numbers.

 $$X$$,  $$X$$,  $$X$$,  $$X$$,  $$X$$,  $$X$$,  $$X$$,  $$X$$ $$X^0=X$$,  $$X^0=X$$,  $$X^0=X$$,  $$X^0=X$$ $$X^1=X$$,  $$X^1=X$$,  $$X^1=X$$,  $$X^1=X$$ $$X^{00}=X^0$$,  $$X^{00}=X^0$$ $$X^{01}=X^0$$,  $$X^{01}=X^0$$ $$X^{10}=X^1$$,  $$X^{10}=X^1$$ $$X^{11}=X^1$$,  $$X^{11}=X^1$$ $$X^{000}=X^{00}$$ $$X^{001}=X^{00}$$ $$X^{010}=X^{01}$$ $$X^{011}=X^{01}$$ $$X^{100}=X^{10}$$ $$X^{101}=X^{10}$$ $$X^{110}=X^{11}$$ $$X^{111}=X^{11}$$

Confirmation will be as follows if you omit the transformation in the middle and extract only the result.

 $$X^{000}=X$$ $$X^{001}=X$$ $$X^{010}=X$$ $$X^{011}=X$$ $$X^{100}=X$$ $$X^{101}=X$$ $$X^{110}=X$$ $$X^{111}=X$$

$$abc$$ on the left side $$X^{abc}$$ of each result represents the position of the calculation result. Looking at the right side of the calculated value, It corresponds to the value of the place just inverted the position bit pattern. As a result, when we use the Cooley-Tukey algorithm for calculating discrete Fourier transform, bit reversal sorting occurs.

If we look at how bit reversal sorting occurs using an example $$X^{011}$$, it becomes as follows. Here, the binary number is represented by adding a suffix of $$b$$.

$X^{011}[0_b] = X^{01}[2\times 0_b+1] = X^{01}[1_b] = X^0[2\times 1_b+1] = X^{0}[11_b] = X[2\times 11_b] = X[110_b]$

Generally, it becomes as follows.

$X^{abc} = X^{ab}[c] = X^{a}[cb] = X[cba]$

You can understand that the inverse transform of bit reversal sorting is bit reversal sorting if you think a little. So, if we bit-reversal-sort the processing results above, we will get discrete Fourier transform results sorted in natural order.

The program of bit reversal sorting is as follows.

#include <complex>
#include <utility>

typedef std::complex<double> complex_t;

void bit_reverse(int N, complex_t* x) // bit reversal sorting
// N : sequence length
// x : input/output sequence
{
for (int i = 0, j = 1; j < N-1; j++) {
for (int k = N >> 1; k > (i ^= k); k >>= 1);
if (i < j) std::swap(x[i], x[j]); // swap x[i] and x[j]
}
}


If we summarize the above, the FFT program is as follows. The inverse Fourier transform can be calculated with $$N^{-1}\overline{F_N(\overline{x[p]})}$$ if we assume that $$\overline{x}$$ is the complex conjugate of $$x$$.

FFT program 1
#include <complex>
#include <cmath>
#include <utility>

typedef std::complex<double> complex_t;

void F(int N, int q, complex_t* x)
// N : sequence length
// q : block start point (initial value is 0)
// x : input/output sequence
{
const int m = N/2;
const double theta0 = 2*M_PI/N;

if (N > 1) {
for (int p = 0; p < m; p++) {
const complex_t wp = complex_t(cos(p*theta0), -sin(p*theta0));
const complex_t a = x[q + p + 0];
const complex_t b = x[q + p + m];
x[q + p + 0] =  a + b;
x[q + p + m] = (a - b) * wp;
}
F(N/2, q + 0, x); // even position components
F(N/2, q + m, x); // odd position components
}
}

void bit_reverse(int N, complex_t* x) // bit reversal sorting
// N : sequence length
// x : input/output sequence
{
for (int i = 0, j = 1; j < N-1; j++) {
for (int k = N >> 1; k > (i ^= k); k >>= 1);
if (i < j) std::swap(x[i], x[j]); // swap x[i] and x[j]
}
}

void fft(int N, complex_t* x) // Fourier transform
// N : sequence length
// x : input/output sequence
{
F(N, 0, x);
bit_reverse(N, x);
}

void ifft(int N, complex_t* x) // Inverse Fourier transform
// N : sequence length
// x : input/output sequence
{
for (int p = 0; p < N; p++) x[p] = conj(x[p]);
F(N, 0, x);
bit_reverse(N, x);
for (int k = 0; k < N; k++) x[k] = conj(x[k])/double(N);
}


Or if we try to do bit reversal sorting in the function F(), the program will be as follows.

FFT program 2
#include <complex>
#include <cmath>
#include <utility>

typedef std::complex<double> complex_t;

void F(int N, int s, int q, int d, complex_t* x)
// N : sequence length
// s : stride
// q : block start point (initial value is 0)
// d : bit reversal point of q
// x : input/output sequence
{
const int m = N/2;
const double theta0 = 2*M_PI/N;

if (N > 1) {
for (int p = 0; p < m; p++) {
const complex_t wp = complex_t(cos(p*theta0), -sin(p*theta0));
const complex_t a = x[q + p + 0];
const complex_t b = x[q + p + m];
x[q + p + 0] =  a + b;
x[q + p + m] = (a - b) * wp;
}
F(N/2, 2*s, q + 0, d + 0, x); // even position components
F(N/2, 2*s, q + m, d + s, x); // odd position components
}
else if (q > d) std::swap(x[q], x[d]); // bit reversal sorting
}

void fft(int N, complex_t* x) // Fourier transform
// N : sequence length
// x : input/output sequence
{
F(N, 1, 0, 0, x);
}

void ifft(int N, complex_t* x) // Inverse Fourier transform
// N : sequence length
// x : input/output sequence
{
for (int p = 0; p < N; p++) x[p] = conj(x[p]);
F(N, 1, 0, 0, x);
for (int k = 0; k < N; k++) x[k] = conj(x[k])/double(N);
}