程序原理见附件三张图片（懒得打字^_^）程序代码如下，运行无误,红色部分看不懂，看了好久了，请高手帮忙解释一下好吗？
子程序：gain(b,a,m,n,x,y,len,sign)

#include "math.h"
void gain(b,a,m,n,x,y,len,sign)
int m,n,len,sign;
double b[],a[],x[],y[];
{int i,k;
    double ar,ai,br,bi,zr,zi,im,re,den,numi,numr,freq,temp;
for(k=0;k<len;k++)
    {   
        
        freq=k*0.5/(len-1);
           zr=cos(-8.0*atan(1.0)*freq);
           zi=sin(-8.0*atan(1.0)*freq);
           
           
           br=0.0;
           bi=0.0;            
           for(i=m;i>0;i--)
              {re=br;
                  im=bi;
                  br=(re+b[i])*zr-im*zi;
                  bi=(re+b[i])*zi+im*zr;
              }
           ar=0.0;
           ai=0.0;
           for(i=n;i>0;i--)
              {re=ar;
                  im=ai;
                  ar=(re+a[i])*zr-im*zi;
                  ai=(re+a[i])*zi+im*zr;             
              }  
              
              
              
            br=br+b[0];
            ar=ar+1.0;
            numr=ar*br+ai*bi;
            numi=ar*bi-ai*br;
            den=ar*ar+ai*ai;
            x[k]=numr/den;
            y[k]=numi/den;           
            switch(sign)
               {case 1:
                     {temp=sqrt(x[k]*x[k]+y[k]*y[k]);
                      y[k]=atan2(y[k],x[k]);
                      x[k]=temp;
                      break;
                     }
                case 2:
                     {temp=x[k]*x[k]+y[k]*y[k];
                         y[k]=atan2(y[k],x[k]);
                         x[k]=10.0*log10(temp);/*???????估计应该是20.0*/
                         }      
               }
              
    }
    
}