cosft1.c

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#include <math.h>
#define PI 3.141592653589793

void cosft1(float y[], int n)
{
	void realft(float data[], unsigned long n, int isign);
	int j,n2;
	float sum,y1,y2;
	double theta,wi=0.0,wpi,wpr,wr=1.0,wtemp;

	theta=PI/n;
	wtemp=sin(0.5*theta);
	wpr = -2.0*wtemp*wtemp;
	wpi=sin(theta);
	sum=0.5*(y[1]-y[n+1]);
	y[1]=0.5*(y[1]+y[n+1]);
	n2=n+2;
	for (j=2;j<=(n>>1);j++) {
		wr=(wtemp=wr)*wpr-wi*wpi+wr;
		wi=wi*wpr+wtemp*wpi+wi;
		y1=0.5*(y[j]+y[n2-j]);
		y2=(y[j]-y[n2-j]);
		y[j]=y1-wi*y2;
		y[n2-j]=y1+wi*y2;
		sum += wr*y2;
	}
	realft(y,n,1);
	y[n+1]=y[2];
	y[2]=sum;
	for (j=4;j<=n;j+=2) {
		sum += y[j];
		y[j]=sum;
	}
}
#undef PI