几何多边形面积交模板

求连个多边形相交的面积

#include<bits/stdc++.h>  
using namespace std;  
  
const int maxn=555;  
const int maxisn=10;  
const double eps=1e-8;  
const double pi=acos(-1.0);  
  
int dcmp(double x){  
    if(x>eps) return 1;  
    return x<-eps ? -1 : 0;  
}  
inline double Sqr(double x){  
    return x*x;  
}  
struct Point{  
    double x,y;  
    Point(){x=y=0;}  
    Point(double x,double y):x(x),y(y){};  
    friend Point operator + (const Point &a,const Point &b) {  
        return Point(a.x+b.x,a.y+b.y);  
    }  
    friend Point operator - (const Point &a,const Point &b) {  
        return Point(a.x-b.x,a.y-b.y);  
    }  
    friend bool operator == (const Point &a,const Point &b) {  
        return dcmp(a.x-b.x)==0&&dcmp(a.y-b.y)==0;  
    }  
    friend Point operator * (const Point &a,const double &b) {  
        return Point(a.x*b,a.y*b);  
    }  
    friend Point operator * (const double &a,const Point &b) {  
        return Point(a*b.x,a*b.y);  
    }  
    friend Point operator / (const Point &a,const double &b) {  
        return Point(a.x/b,a.y/b);  
    }  
    friend bool operator < (const Point &a, const Point &b) {  
        return a.x < b.x || (a.x == b.x && a.y < b.y);  
    }  
    inline double dot(const Point &b)const{  
        return x*b.x+y*b.y;  
    }  
    inline double cross(const Point &b,const Point &c)const{  
        return (b.x-x)*(c.y-y)-(c.x-x)*(b.y-y);  
    }  
  
};  
  
Point LineCross(const Point &a,const Point &b,const Point &c,const Point &d){  
    double u=a.cross(b,c),v=b.cross(a,d);  
    return Point((c.x*v+d.x*u)/(u+v),(c.y*v+d.y*u)/(u+v));  
}  
double PolygonArea(Point p[],int n){  
     if(n<3) return 0.0;  
     double s=p[0].y*(p[n-1].x-p[1].x);  
     p[n]=p[0];  
     for(int i=1;i<n;i++){  
        s+=p[i].y*(p[i-1].x-p[i+1].x);  
     }  
     return fabs(s*0.5);  
}  
double CPIA(Point a[],Point b[],int na,int nb){  
    Point p[maxisn],temp[maxisn];  
    int i,j,tn,sflag,eflag;  
    a[na]=a[0],b[nb]=b[0];  
    memcpy(p,b,sizeof(Point)*(nb+1));  
    for(i=0;i<na&&nb>2;++i){  
        sflag=dcmp(a[i].cross(a[i+1],p[0]));  
        for(j=tn=0;j<nb;++j,sflag=eflag){  
            if(sflag>=0) temp[tn++]=p[j];  
            eflag=dcmp(a[i].cross(a[i+1],p[j+1]));  
            if((sflag^eflag)==-2)  
                temp[tn++]=LineCross(a[i],a[i+1],p[j],p[j+1]);  
        }  
        memcpy(p,temp,sizeof(Point)*tn);  
        nb=tn,p[nb]=p[0];  
    }  
    if(nb<3) return 0.0;  
    return PolygonArea(p,nb);  
}  
double SPIA(Point a[],Point b[],int na,int nb){  
    int i,j;  
    Point t1[4],t2[4];  
    double res=0.0,if_clock_t1,if_clock_t2;  
    a[na]=t1[0]=a[0];  
    b[nb]=t2[0]=b[0];  
    for(i=2;i<na;i++){  
        t1[1]=a[i-1],t1[2]=a[i];  
        if_clock_t1=dcmp(t1[0].cross(t1[1],t1[2]));  
        if(if_clock_t1<0) swap(t1[1],t1[2]);  
        for(j=2;j<nb;j++){  
            t2[1]=b[j-1],t2[2]=b[j];  
            if_clock_t2=dcmp(t2[0].cross(t2[1],t2[2]));  
            if(if_clock_t2<0) swap(t2[1],t2[2]);  
            res+=CPIA(t1,t2,3,3)*if_clock_t1*if_clock_t2;  
        }  
    }  
    return res;//面积交  
    //return PolygonArea(a,na)+PolygonArea(b,nb)-res;//面积并  
}  
Point a[4],b[5];  
  
int main()  
{  
    double x1,x2,x3,x4,y1,y2,y3,y4;  
    while(scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2)!=EOF)  
    {  
       scanf("%lf%lf%lf%lf",&x3,&y3,&x4,&y4);  
       a[0]=Point(x1,y1);  
       a[1]=Point(x2,y1);  
       a[2]=Point(x1,y2);  
       b[0]=Point(x3,y3);  
       b[1]=Point(x3,y4);  
       b[2]=Point(x4,y4);  
       b[3]=Point(x4,y3);  
       printf("%.8lf
",fabs(SPIA(a,b,3,4)));  
    }  
}