【计算几何】计算任意多边形的面积

struct Point2d{
	double x,y;
};

//计算任意多边形的面积，顶点按照顺时针或者逆时针方向排列
double ComputePolygonArea(const vector<Point2d> &points) {
	int point_num = points.size();
	if(point_num < 3)return 0.0;
	double s = points[0].y * (points[point_num-1].x - points[1].x);
	for(int i = 1; i < point_num; ++i)
		s += points[i].y * (points[i-1].x - points[(i+1)%point_num].x);
	return fabs(s/2.0);
}