直线由一个点P和一个方向向量v表示，比如L=P+tv
#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <complex>
using namespace std;/*Point模板部分*/
typedef complex<double> Point;
typedef Point Vector;
const double eps = 1e-10;
int dcmp(double x){if(fabs(x) < eps) return 0;else return x < 0 ? -1: 1;
}
double Dot(Vector A, Vector B) { return real(conj(A)*B); }
double Cross(Vector A, Vector B) { return imag(conj(A)*B); }
Vector Rotate(Vector A, double rad) { return A*exp(Point(0,rad)); }
double Length(Vector A) { return sqrt(Dot(A,A)); }
double Angle(Vector A, Vector B) { return acos(Dot(A, B)/Length(A)/Length(B)); } //叉积
double Area2(Point A, Point B, Point C) { return Cross(B-A,C-A); } //有向面积
/*Point模板部分*/Point GetLineIntersection(Point P, Vector v, Point Q, Vector w){ //两直线的交点，前提是Cross(v,w)!=0Vector u=P-Q;double t=Cross(w,u)/Cross(v,w);return P+v*t;
}
double DistanceToLine(Point P, Point A, Point B){ //P到直线A-B距离Vector v1=B-A, v2=P-A;return fabs(Cross(v1,v2) / Length(v1));
}
double DistanceToSegment(Point P, Point A, Point B){ //P到线段A-B距离if(A==B) return Length(P-A);Vector v1=B-A, v2=P-A, v3=P-B;if(dcmp(Dot(v1,v2)) < 0) return Length(v2);else if(dcmp(Dot(v1,v3)) > 0) return Length(v3);else return fabs(Cross(v1,v2)) / Length(v1);
}
Point GetLineProjection(Point P, Point A, Point B){ //求P到直线A-B垂点Vector v=B-A;return A+v*(Dot(v,P-A)) / Dot(v,v);
}
bool SegmentProPerIntersection(Point a1, Point a2, Point b1,Point b2){ //两线是否规范相交double c1=Cross(a2-a1,b1-a1), c2=Cross(a2-a1,b2-a1),c3=Cross(b2-b1,a1-b1), c4=Cross(b2-b1,a2-b1);return dcmp(c1)*dcmp(c2)<0 && dcmp(c3)*dcmp(c4)<0;
}
bool OnSegment(Point p, Point a1, Point a2){ //p点是否在a1,a2线段上(不在a1,a2点上)return dcmp(Cross(a1-p,a2-p))==0 && dcmp(Dot(a1-p,a2-p)) < 0;
}