线段被圆截得距离模板

虽然是题看错了，并不用这么做……贴上模板以防万一：

(x1,y1,x2,y2)为线段的两个端点，(x3,y3)为圆心坐标，r为半径：

const double inf = -2139062144000.0;
const double pi = acos(-1.0);
double re[4];
const double eps = 1e-7;
bool oncircle(double xx,double yy,double xr,double yr,double r) {
    if(fabs(sqrt((xx-xr) * (xx-xr) + (yy-yr) * (yy-yr)) - r) <= eps) {
        return true;
    }
    return false;
}
bool incircle(double xx,double yy,double xr,double yr,double r) {
    if(sqrt((xx-xr) * (xx-xr) + (yy-yr) * (yy-yr)) <= r + eps) {
        return true;
    }
    return false;
}
void jiao(double x1,double y1,double x2,double y2,double x3,double y3,double r) {
    re[0] = inf;
    re[1] = inf;
    re[2] = inf;
    re[3] = inf;
    if(x1 > x2) {
        swap(x1,x2);
        swap(y1,y2);
    }
    double A = (x2-x1)*(x2-x1) + (y2-y1)*(y2-y1);
    double B = 2*((x2-x1)*(x1-x3)+(y2-y1)*(y1-y3));
    double C = x3*x3+y3*y3+x1*x1+y1*y1-2*(x3*x1+y3*y1)-r*r;
    double u1,u2;
    if(B*B-4.0*A*C >= 0) {
        u1 = (-B-sqrt(B*B-4.0*A*C))/(2.0*A);
        u2 = (-B+sqrt(B*B-4.0*A*C))/(2.0*A);
        double x5 = x1 + u1*(x2-x1);
        double y5 = y1 + u1*(y2-y1);
 
        double x6 = x1 + u2*(x2-x1);
        double y6 = y1 + u2*(y2-y1);
//        cout << x5 << " " << y5 << " " << x6 << " " << y6 << " " <<endl;
        if(x6 < x5) {
            swap(x5,x6);
            swap(y5,y6);
        }
        int coun = 0;
 
        if(x5 >= x1 && x5 <= x2) {
            re[coun*2] = x5;
            re[coun*2+1] = y5;
            coun++;
        }
        if(x6 >= x1 && x6 <= x2) {
            re[coun*2] = x6;
            re[coun*2+1] = y6;
            coun++;
        }
 
 
        if(coun == 2) {
            if(re[0] > re[2]) {
                swap(re[0],re[2]);
                swap(re[1],re[3]);
            }
            return;
        }
 
        if(coun == 0) {
            if(incircle(x1,y1,x3,y3,r) && incircle(x2,y2,x3,y3,r)) {
                re[0] = x1;
                re[1] = y1;
                re[2] = x2;
                re[3] = y2;
            }
            if(re[0] > re[2]) {
                swap(re[0],re[2]);
                swap(re[1],re[3]);
            }
            return;
        }
        if(coun == 1) {
            if((x2-x3) * (x2-x3) + (y2-y3) * (y2-y3) > (x1-x3) * (x1-x3) + (y1-y3) * (y1-y3)) {
                re[2] = x1;
                re[3] =y1;
            } else {
                re[2] = x2;
                re[3] =y2;
            }
            if(re[0] > re[2]) {
                swap(re[0],re[2]);
                swap(re[1],re[3]);
            }
            return;
        }
    } else {
        return;
    }
}