65 lines
1.3 KiB
C
65 lines
1.3 KiB
C
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#include <math.h>
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#define PI 3.141592653589793
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void cosft2(float y[], int n, int isign)
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{
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void realft(float data[], unsigned long n, int isign);
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int i;
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float sum,sum1,y1,y2,ytemp;
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double theta,wi=0.0,wi1,wpi,wpr,wr=1.0,wr1,wtemp;
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theta=0.5*PI/n;
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wr1=cos(theta);
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wi1=sin(theta);
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wpr = -2.0*wi1*wi1;
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wpi=sin(2.0*theta);
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if (isign == 1) {
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for (i=1;i<=n/2;i++) {
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y1=0.5*(y[i]+y[n-i+1]);
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y2=wi1*(y[i]-y[n-i+1]);
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y[i]=y1+y2;
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y[n-i+1]=y1-y2;
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wr1=(wtemp=wr1)*wpr-wi1*wpi+wr1;
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wi1=wi1*wpr+wtemp*wpi+wi1;
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}
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realft(y,n,1);
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for (i=3;i<=n;i+=2) {
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wr=(wtemp=wr)*wpr-wi*wpi+wr;
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wi=wi*wpr+wtemp*wpi+wi;
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y1=y[i]*wr-y[i+1]*wi;
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y2=y[i+1]*wr+y[i]*wi;
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y[i]=y1;
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y[i+1]=y2;
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}
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sum=0.5*y[2];
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for (i=n;i>=2;i-=2) {
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sum1=sum;
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sum += y[i];
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y[i]=sum1;
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}
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} else if (isign == -1) {
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ytemp=y[n];
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for (i=n;i>=4;i-=2) y[i]=y[i-2]-y[i];
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y[2]=2.0*ytemp;
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for (i=3;i<=n;i+=2) {
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wr=(wtemp=wr)*wpr-wi*wpi+wr;
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wi=wi*wpr+wtemp*wpi+wi;
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y1=y[i]*wr+y[i+1]*wi;
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y2=y[i+1]*wr-y[i]*wi;
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y[i]=y1;
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y[i+1]=y2;
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}
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realft(y,n,-1);
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for (i=1;i<=n/2;i++) {
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y1=y[i]+y[n-i+1];
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y2=(0.5/wi1)*(y[i]-y[n-i+1]);
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y[i]=0.5*(y1+y2);
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y[n-i+1]=0.5*(y1-y2);
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wr1=(wtemp=wr1)*wpr-wi1*wpi+wr1;
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wi1=wi1*wpr+wtemp*wpi+wi1;
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}
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}
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}
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#undef PI
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