「HDU - 2857」Mirror and Light（点关于直线的对称点）

#include <cstdio>
#include <cmath>
#include <algorithm>
using namespace std;
const double eps=1e-8;
int sgn(double x){
    if(fabs(x)<eps)return 0;
    return (x<0)?-1:1;
}
struct Point{
    double x,y;
    Point(double _x=0,double _y=0):x(_x),y(_y){}
    Point operator -(const Point &b)const{
        return Point(x-b.x,y-b.y);
    }
    double operator ^(const Point &b)const{
        return x*b.y-y*b.x;
    }
    double operator *(const Point &b)const{
        return x*b.x+y*b.y;
    }
};
struct Line{
    Point s,e;
    Line(){}
    Line(Point _s,Point _e):s(_s),e(_e){}
    pair<int,Point> operator &(const Line& b)const{
        Point res=s;
        if(sgn((s-e)^(b.s-b.e))==0){
            if(sgn((s-b.e)^(b.s-b.e))==0)
                return make_pair(0,res);
            return make_pair(1,res);
        }
        double t=((s-b.s)^(b.s-b.e))/((s-e)^(b.s-b.e));
        res.x+=(e.x-s.x)*t;
        res.y+=(e.y-s.y)*t;
        return make_pair(2,res);
    }
};
double dist(Point a,Point b){
    return sqrt((a-b)*(a-b));
}
void format(Line L,double& A,double& B,double& C){
    A=(L.e.y-L.s.y)/dist(L.s,L.e);
    B=(L.s.x-L.e.x)/dist(L.s,L.e);
    C=-L.s.y*B-L.s.x*A;
}
Point mirror(Point P,Line L){
    double A,B,C;
    format(L,A,B,C);
    double A2=A*A,B2=B*B,AB=A*B;
    return Point(((B2-A2)*P.x-AB*P.y*2-A*C*2)/(A2+B2),
            ((A2-B2)*P.y-AB*P.x*2-B*C*2/(A2+B2)));
}
int t;
double d[8];
int main(){
    scanf("%d",&t);
    while(t--){
        for(int i=0;i<8;++i)scanf("%lf",d+i);
        Line L(Point(d[0],d[1]),Point(d[2],d[3]));
        Point s(d[4],d[5]);
        Point e=mirror(s,L);
        Line L2(e,Point(d[6],d[7]));
        Point c=(L&L2).second;
        printf("%.3f %.3f\n",c.x,c.y);
    }
    return 0;
}