求两圆相交部分面积（C++)

已知两圆圆心坐标和半径，求相交部分面积：

 #include <iostream>
 using namespace std;
 #include<cmath>
 #include<stdio.h>
 #define PI 3.141593
 struct point//点
 {
     double x,y;
 };
 struct circle//圆
 {
     point center;
     double r;
 };
 float dist(point a,point b)//求圆心距
 {
     return sqrt((a.x-b.x)*(a.x-b.x)+(a.y-b.y)*(a.y-b.y));
 }

 int main()
 {
     circle a,b;
     while(cin>>a.center.x>>a.center.y>>a.r>>b.center.x>>b.center.y>>b.r)
     {
         if((dist(a.center,b.center)+min(a.r,b.r))<=max(a.r,b.r))//两圆内交或包含
             {
                 if(a.r<b.r) printf("%.3lf\n",PI*a.r*a.r);
                 else printf("%.3lf\n",PI*b.r*b.r);
             }
         else if(dist(a.center,b.center)>=(a.r+b.r))//两圆外交或相离
             cout<<"0.000"<<endl;
         else
         {
             double length=dist(a.center,b.center);
             double d1=*acos((a.r*a.r+length*length-b.r*b.r)/(*a.r*length)); //求两扇形圆心角
             double d2=*acos((b.r*b.r+length*length-a.r*a.r)/(*b.r*length));
             double area1=a.r*a.r*d1/-a.r*a.r*sin(d1)/;
             //根据圆心角求扇形面积，减去三角形面积，得到相交部分面积
             //扇形面积：S=PI*r*r*θ/(2*PI)  三角形面积：S=1/2*a*c*sin(B)
             double area2=b.r*b.r*d2/-b.r*b.r*sin(d2)/;
             double area=area1+area2;
             printf("%.3lf\n",area);
         }
     }
     return ;
 }