ZOJ 3728 Collision

#include<stdio.h>
#include<string.h>
#include<math.h>
#define eps 1e-9
struct point
{
    double x,y;
};

struct line
{
    point a,b;
};

struct circle
{
    point p;
    double r;
};

double distance(point p0,point p1)
{
    return sqrt((p0.x-p1.x)*(p0.x-p1.x)+(p0.y-p1.y)*(p0.y-p1.y));
}

double xmult(point p1,point p2,point p0)
{
    return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
}

double disptoline(circle p,line l)
{
    // printf("%lf",xmult(p.p,l.a,l.b));
    //printf("%lf",distance(l.a,l.b));
    return fabs(xmult(p.p,l.a,l.b))/distance(l.a,l.b);
}

int GetLineInsertion(line l,circle cir,point v,double &t1,double &t2)
{
    double a=v.x,c=v.y;
    double b=l.a.x-cir.p.x,d=l.a.y-cir.p.y;//p0-o
    double f=2*(b*a+d*c);//2*t*u*(po-o)
    double e=a*a+c*c;//射线方向的平方
    double g=b*b+d*d-cir.r*cir.r;//(po-o)^2-r^2
    double delta=f*f-4*e*g;
    if(delta<0) return 0;
    if(delta==0)
    {
        t1=t2=-f/(2*e);
    }
    t1=(-f-sqrt(delta))/(2*e);
    t2=(-f+sqrt(delta))/(2*e);
    if(t1<0 || t2<0) return 0;//当时间为负数的时候，射线反方向执行
    //虽然这样可以符合delta>0
    return 2;
}

int main()
{
    double R,RM,r;
    double k1,k2,k3,k4;
    point p1,v,p2;
    circle cir_R,cir_RM;
    cir_R.p.x=cir_R.p.y=cir_RM.p.x=cir_RM.p.y=0.0;
    line l;
    point t1,t2,t3,t4;
    while(scanf("%lf%lf%lf%lf%lf%lf%lf",&RM,&R,&r,&p1.x,&p1.y,&v.x,&v.y)!=EOF)
    {
        cir_R.r=R+r;
        cir_RM.r=RM+r;
        p2.x=p1.x+v.x*10000;
        p2.y=p1.y+v.y*10000;
        l.a.x=p1.x,l.a.y=p1.y;
        l.b.x=p2.x,l.b.y=p2.y;
        double dist=disptoline(cir_R,l);//圆心到直线的距离
        int flag=GetLineInsertion(l,cir_R,v,k1,k2);
        if(flag==0)
        {
            printf("0.000\n");
        }
        else
        {
            t1.x=p1.x+v.x*k1;
            t1.y=p1.y+v.y*k1;
            t2.x=p1.x+v.x*k2;
            t2.y=p1.y+v.y*k2;
            flag=GetLineInsertion(l,cir_RM,v,k3,k4);
            if(flag==0)//没有和小圆相交
            {
                double dist_time;
                dist_time=distance(t1,t2);
                dist_time/=sqrt(v.x*v.x+v.y*v.y);
                printf("%.3lf\n",dist_time);
            }
            else
            {
                t3.x=p1.x+v.x*k3;
                t3.y=p1.y+v.y*k3;
                t4.x=p1.x+v.x*k4;
                t4.y=p1.y+v.y*k4;
                double dist_time;
                dist_time=distance(t1,t2)-distance(t3,t4);
                dist_time/=sqrt(v.x*v.x+v.y*v.y);
                printf("%.3lf\n",dist_time);
            }
        }
    }
    return 0;
}