Zcosft1.cin

// zcosft1.cin is C++ routine required for compilation of examples with DCTI, Discrete Fourier transform of the First kind.

void zcosft1(z_type y[], int n)
{ /* void zrealft(z_type data[], unsigned long n, int isign);*/
       int j,n2;  z_type sum,y1,y2;
       double theta,wi=0.0,wpi,wpr,wr=1.0,wtemp;
       theta=M_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;
       }
       zrealft(y,n,1);
       y[n+1]=y[2];
       y[2]=sum;
       for(j=4;j<=n;j+=2) {sum += y[j]; y[j]=sum;}
}

References

http://tori.ils.uec.ac.jp/TORI/index.php/Zcosft1.cin