#include <math.h>void drealft(double data[], unsigned long n, int isign){	void dfour1(double data[], unsigned long nn, int isign);	unsigned long i,i1,i2,i3,i4,np3;	double c1=0.5,c2,h1r,h1i,h2r,h2i;	double wr,wi,wpr,wpi,wtemp,theta;	theta=3.141592653589793/(double) (n>>1);	if (isign == 1) {		c2 = -0.5;		dfour1(data,n>>1,1);	} else {		c2=0.5;		theta = -theta;	}	wtemp=sin(0.5*theta);	wpr = -2.0*wtemp*wtemp;	wpi=sin(theta);	wr=1.0+wpr;	wi=wpi;	np3=n+3;	for (i=2;i<=(n>>2);i++) {		i4=1+(i3=np3-(i2=1+(i1=i+i-1)));		h1r=c1*(data[i1]+data[i3]);		h1i=c1*(data[i2]-data[i4]);		h2r = -c2*(data[i2]+data[i4]);		h2i=c2*(data[i1]-data[i3]);		data[i1]=h1r+wr*h2r-wi*h2i;		data[i2]=h1i+wr*h2i+wi*h2r;		data[i3]=h1r-wr*h2r+wi*h2i;		data[i4] = -h1i+wr*h2i+wi*h2r;		wr=(wtemp=wr)*wpr-wi*wpi+wr;		wi=wi*wpr+wtemp*wpi+wi;	}	if (isign == 1) {		data[1] = (h1r=data[1])+data[2];		data[2] = h1r-data[2];	} else {		data[1]=c1*((h1r=data[1])+data[2]);		data[2]=c1*(h1r-data[2]);		dfour1(data,n>>1,-1);	}}