csu1812求两多边形的交面积

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直接用模板就可以了，拿的别人的模板，，，，

/* * 多边形的交，多边形的边一定是要按逆时针方向给出 * 还要判断是凸包还是凹包，调用相应的函数 * 面积并，只要和面积减去交即可 */#include <iostream>#include <cstdio>#include <cstdlib>#include <cstring>#include <algorithm>#include <cmath>using namespace std;const int maxn = 300;const double eps = 1e-8;struct Point//点 向量{    double x,y;    Point(double x=0,double y=0):x(x),y(y) {}};typedef  Point  Vector;//向量使用点作为表示方法 结构相同 为了代码清晰int dcmp(double x) //三态函数 处理与double零有关的精度问题{    if(fabs(x) < eps)    return 0;    return x<0 ? -1 : 1;}//向量运算Vector operator + (Vector A, Vector B){    return Vector(A.x+B.x, A.y+B.y);}Vector operator - (Vector A, Vector B){    return Vector(A.x-B.x, A.y-B.y);}Vector operator * (Vector A, double p){    return Vector(A.x*p, A.y*p);}Vector operator / (Vector A, double p){    return Vector(A.x/p, A.y/p);}bool operator == (const Vector& A, const Vector& B){    return dcmp(A.x-B.x)==0 && dcmp(A.y-B.y)==0;}bool operator < (const Point&a,const Point &b){    return a.x<b.x||(a.x==b.x&&a.y<b.y);}double cross(Point a,Point b,Point c) ///叉积{    return (a.x-c.x)*(b.y-c.y)-(b.x-c.x)*(a.y-c.y);}Point intersection(Point a,Point b,Point c,Point d){    Point p = a;    double t =((a.x-c.x)*(c.y-d.y)-(a.y-c.y)*(c.x-d.x))/((a.x-b.x)*(c.y-d.y)-(a.y-b.y)*(c.x-d.x));    p.x +=(b.x-a.x)*t;    p.y +=(b.y-a.y)*t;    return p;}//计算多边形面积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)//ConvexPolygonIntersectArea{    Point p[20], tmp[20];    int tn, sflag, eflag;    a[na] = a[0], b[nb] = b[0];    memcpy(p,b,sizeof(Point)*(nb + 1));    for(int i = 0; i < na && nb > 2; i++)    {        sflag = dcmp(cross(a[i + 1], p[0],a[i]));        for(int j = tn = 0; j < nb; j++, sflag = eflag)        {            if(sflag>=0) tmp[tn++] = p[j];            eflag = dcmp(cross(a[i + 1], p[j + 1],a[i]));            if((sflag ^ eflag) == -2)                tmp[tn++] = intersection(a[i], a[i + 1], p[j], p[j + 1]); ///求交点        }        memcpy(p, tmp, 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)///SimplePolygonIntersectArea 调用此函数{    int i, j;    Point t1[4], t2[4];    double res = 0, num1, num2;    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];        num1 = dcmp(cross(t1[1], t1[2],t1[0]));        if(num1 < 0) swap(t1[1], t1[2]);        for(j = 2; j < nb; j++)        {            t2[1] = b[j - 1], t2[2] = b[j];            num2 = dcmp(cross(t2[1], t2[2],t2[0]));            if(num2 < 0) swap(t2[1], t2[2]);            res += CPIA(t1, t2, 3, 3) * num1 * num2;        }    }    return res;}Point p1[maxn], p2[maxn];int n1, n2;int main(){    double x1,x2,x3,x4,y1,y2,y3,y4;    while(~scanf("%lf %lf %lf %lf %lf %lf %lf %lf",&x1,&y1,&x2,&y2,&x3,&y3,&x4,&y4))    {        p1[0]=Point(x1,y1);        p1[1]=Point(x1,y2);        p1[2]=Point(x2,y1);        p2[0]=Point(x3,y3);         p2[1]=Point(x3,y4);          p2[2]=Point(x4,y4);           p2[3]=Point(x4,y3);             double Area = SPIA(p1, p2, 3, 4);             printf("%.8lf\n",fabs(Area));    }}