sinft.c

Numerical Recipes in C的源代码

#include <math.h>

void sinft(y,n)
float y[];
int n;
{
	void realft();
	int j,n2=n+2;
	float sum,y1,y2;
	double theta,wi=0.0,wr=1.0,wpi,wpr,wtemp;

	theta=3.14159265358979/(double) n;
	wtemp=sin(0.5*theta);
	wpr = -2.0*wtemp*wtemp;
	wpi=sin(theta);
	y[1]=0.0;
	for (j=2;j<=(n>>1)+1;j++) {
		wr=(wtemp=wr)*wpr-wi*wpi+wr;
		wi=wi*wpr+wtemp*wpi+wi;
		y1=wi*(y[j]+y[n2-j]);
		y2=0.5*(y[j]-y[n2-j]);
		y[j]=y1+y2;
		y[n2-j]=y1-y2;
	}
	realft(y,n,1);
	y[1]*=0.5;
	sum=y[2]=0.0;
	for (j=1;j<=n-1;j+=2) {
		sum += y[j];
		y[j]=y[j+1];
		y[j+1]=sum;
	}
}