计算几何模板

const double eps = 1e-8;
const double inf = 1e20;
const double pi = acos(-1.0);
const int maxp = 1010;
//判断正负
inline int sgn (double x) {
	if (fabs(x) < eps) return 0;
	if (x < 0) return -1;
	else return 1;
}
inline double sqr (double x) {return x * x;}
struct Point {
	double x, y;
	Point () {}
	Point (double _x, double _y) {
		x = _x;
		y = _y;
	}
	void input () {scanf("%lf%lf", &x, &y);}
	void output () {printf("%.2lf%.2lf\n", x, y);}
	bool operator == (Point b) const {return sgn(x - b.x) == 0 && sgn(y - b.y) == 0;}
	bool operator < (Point b) const {return sgn(x - b.x) == 0 ? sgn(y - b.y) < 0 : x < b.x;}
	Point operator + (const Point &b) const {return Point (x + b.x, y + b.y);}
	Point operator - (const Point &b) const {return Point (x - b.x, y - b.y);}
	Point operator * (const double &k) const {return Point(x * k, y * k);}
	Point operator / (const double &k) const {return Point(x / k, y / k);}
	double operator ^ (const Point &b) const {return x*b.y - y*b.x;} //叉积
	double operator * (const Point &b) const {return x * b.x + y * b.y;} //点乘
	double len () {return hypot(x, y);} //距离原点长度
	double len2 () {return x * x + y * y;} //距离原点长度的平方
	double distance (Point p) {return hypot(x - p.x, y - p.y);} //两点距离
	double rad (Point a, Point b) {
		Point p = *this;
		return fabs(atan2(fabs((a - p) ^ (b - p)), (a - p) * (b - p)));
	} //pa和pb所成的夹角
	Point trunc (double r) {
		double l = len();
		if (!sgn(l)) return *this;
		r /= l;
		return Point (x * r, y * r);
	} //化为长度为r的向量
	Point rotleft () {return Point(-y, x);} //逆时针旋转90°
	Point rotright () {return Point(y, -x);} //顺时针旋转90°
	Point rotate (Point p, double angle) {
		Point v = (*this) - p;
		double c = cos(angle), s = sin(angle);
		return Point (p.x + v.x * c - v.y * s, p.y + v.x * s + v.y * c);
	} //绕着p点逆时针旋转angle
};
struct Line {
	Point s, e;
	Line () {}
	Line (Point _s, Point _e) {
		s = _s;
		e = _e;
	}
	bool operator == (Line v) {return (s == v.s) && (e == v.e);}
	Line (Point p, double angle) {
		s = p;
		if (sgn(angle - pi / 2) == 0) e = (s + Point(0, 1));
		else e = (s + Point(1, tan(angle)));
	} //根据一个点和倾斜角angle确定直线， 0 <= angle < pi 
	Line (double a, double b, double c) {
		if (sgn(a) == 0) {
			s = Point(0, -c / b);
			e = Point(1, -c / b);
		} else if (sgn(b) == 0) {
			s = Point(-c / a, 0);
			e = Point(-c / a, 1);
		} else {
			s = Point(0, -c / b);
			e = Point(1, (-c - a) / b);
		}
	}
	void input () {
		s.input();
		e.input();
	}
	void adjust () {if (e < s) swap(s, e);}
	double length () {return s.distance(e);} //求线段长度
	double angle () {
		double k = atan2(e.y - s.y, e.x - s.x);
		if (sgn(k) < 0) k += pi;
		if (sgn(k - pi) == 0) k -= pi;
		return k;
	} //返回直线倾斜角
	int relation (Point p) {
		int c = sgn((p - s) ^ (e - s));
		if (c < 0) return 1;
		else if (c > 0) return 2;
		else return 3;
	} //点和直线的关系，1在左侧，2在右侧，3在直线上
	bool pointonseg (Point p) {return sgn((p - s) ^ (e - s)) == 0 && sgn((p - s) * (p - e)) <= 0;} //点在直线上判断
	bool parallel (Line v) {return sgn((e - s) ^ (v.e - v.s)) == 0;} //两向量平行（直线平行或重合）
	int segcrossseg (Line v) {
		int d1 = sgn((e - s) ^ (v.s - s));
		int d2 = sgn((e - s) ^ (v.e - s));
		int d3 = sgn((v.e - v.s) ^ (s - v.s));
		int d4 = sgn((v.e - v.s) ^ (e - v.s));
		if ((d1 ^ d2) == -2 && (d3 ^ d4) == -2) return 2;
		return (d1 == 0 && sgn((v.s - s) * (v.s - e)) <= 0) ||
			   (d2 == 0 && sgn((v.e - s) * (v.e - e)) <= 0) ||
			   (d3 == 0 && sgn((s - v.s) * (s - v.e)) <= 0) ||
			   (d4 == 0 && sgn((e - v.s) * (e - v.e)) <= 0);
	} //两线段相交判断，2规范相交，1非规范相交，0不相交
	int linecrossseg (Line v) {
		int d1 = sgn((e - s) ^ (v.s - s));
		int d2 = sgn((e - s) ^ (v.e - s));
		if ((d1 ^ d2) == -2) return 2;
		return (d1 == 0 || d2 == 0);
	} //直线（*this）与线段（v）相交判断，2规范相交，1非规范相交，0不相交
	int linecrossline (Line v) {
		if ((*this).parallel(v)) return v.relation(s) == 3;
		return 2;
	} //两直线关系，0平行，1重合，2相交
	Point crosspoint (Line v) {
		double a1 = (v.e - v.s) ^ (s - v.s);
		double a2 = (v.e - v.s) ^ (e - v.s);
		return Point((s.x * a2 - e.x * a1) / (a2 - a1), (s.y * a2 - e.y * a1) / (a2 - a1));
	} //两直线交点
	double dispointtoline (Point p) {return fabs((p - s) ^ (e - s) / length());} //点到直线距离
	double dispointtoseg (Point p) {
		if (sgn((p - s) * (e - s)) < 0 || sgn((p - e) * (s - e)) < 0)
			return min(p.distance(s), p.distance(e));
		return dispointtoline(p);
	} //点到线段距离 
	double dissegtosseg (Line v) {
		return min(min(dispointtoseg(v.s), dispointtoseg(v.e)), min(v.dispointtoseg(s), v.dispointtoseg(e)));
	} //线段到线段的距离
	Point lineprog (Point p) {
		return s + (((e - s) * ((e - s) * (p - s))) / ((e - s).len2()));
	} //返回点p在直线上的投影
	Point symmetrypoint (Point p) {
		Point q = lineprog(p);
		return Point(2 * q.x - p.x, 2 * q.y - p.y);
	} //返回点p关于直线的对称点
};

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来源： https://www.cnblogs.com/duoluoluo/p/15794729.html