
#include <math.h>

void drealft(data,n,isign)
double data[];
int isign;
unsigned long n;
{
	void dfour1();
	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);
	}
}
