FFT和iFFT，FFTiFFT,public class

FFT和iFFT，FFTiFFT,public class

public class FFT {    public static void fft(double pr[], double pi[], double fr[], double fi[]) {        int N = pr.length;        for (int i = 0; i < N; i++) {            fr[i] = 0;            fi[i] = 0;            for (int j = 0; j < N; j++) {                fr[i] += pr[j] * Math.cos(2 * Math.PI * i * j / N) + pi[j]                        * Math.sin(2 * Math.PI * i * j / N);                fi[i] += (-pr[j] * Math.sin(2 * Math.PI * i * j / N)) + pi[j]                        * Math.cos(2 * Math.PI * i * j / N);            }        }    }    public static void ifft(double fr[], double fi[], double pr[], double pi[]) {        int N = pr.length;        for (int i = 0; i < N; i++) {            pr[i] = 0;            pi[i] = 0;            for (int j = 0; j < N; j++) {                pr[i] += fr[j] * Math.cos(2 * Math.PI * i * j / N) / N - fi[j]                        * Math.sin(2 * Math.PI * i * j / N) / N;                pi[i] += fr[j] * Math.sin(2 * Math.PI * i * j / N) / N + fi[j]                        * Math.cos(2 * Math.PI * i * j / N) / N;            }        }    }    public static int greater2p2(int n) {        for (int i = 1;; i++) {            if (n < Math.pow(2, i)) {                return (int) Math.pow(2, i);            }        }    }    public static void main(String[] args) {        double pr[] = { 1, 2, 3, 4, 5 };        double pi[] = { 0, 0, 0, 0, 0 };        /*double fr[] = new double[5];        double fi[] = new double[5];        double pr1[] = new double[5];        double pi1[] = new double[5];        fft(pr, pi, fr, fi);        for (int i = 0; i < pr.length; i++) {            System.out.printf("%3.3f   %3.3f", fr[i], fi[i]);            System.out.println();        }        System.out.println();        ifft(fr, fi, pr1, pi1);        for (int i = 0; i < fr.length; i++) {            System.out.printf("%3.3f    %3.3f", pr1[i], pi1[i]);            System.out.println();        }*/        int KK = greater2p2(pr.length);        double pr2[] = new double[KK];        double pi2[] = new double[KK];        double fr2[] = new double[KK];        double fi2[] = new double[KK];        for (int i = 0; i < KK; i++) {            pr2[i] = 0;            pi2[i] = 0;            if (i < pr.length) {                pr2[i] = pr[i];                pi2[i] = pi[i];            }        }        fft(pr2, pi2, fr2, fi2);        for (int i = 0; i < KK; i++) {            System.out.printf("%3.3f   %3.3f", fr2[i], fi2[i]);            System.out.println();        }    }}//该片段来自于http://byrx.net