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=2*acos((a.r*a.r+length*length-b.r*b.r)/(2*a.r*length)); //求两扇形圆心角
            double d2=2*acos((b.r*b.r+length*length-a.r*a.r)/(2*b.r*length));
            double area1=a.r*a.r*d1/2-a.r*a.r*sin(d1)/2;
            //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*d2/2-b.r*b.r*sin(d2)/2;
            double area=area1+area2;
            printf("%.3lf\n",area);
        }
    }
    return 0;
}