solution.cpp

#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
const double pi = acos(-1);
const double eps = 1e-8;
double xa, ya, xb, yb, l, x[5], y[5], _y[5], res = 0, site;
double dis(double a, double b, double c, double d)
{
    return sqrt((c - a) * (c - a) + (d - b) * (d - b));
}
bool findtuo(double now)
{
    double jx = (xa + xb) / 2, jy = (ya + yb) / 2;
    double aa = l / 2;
    double cc = dis(xa, ya, xb, yb) / 2;
    double bb = sqrt(aa * aa - cc * cc);
    double mid = 1.0 - (now - jx) * (now - jx) / (aa * aa);
    if (mid <= 0)
        return 0;
    mid = sqrt(mid) * bb;
    _y[1] = jy - mid, _y[2] = jy + mid;
    return 1;
}
double check(double now, int pos1, int pos2)
{
    if (fabs(x[pos1] - x[pos2]) <= eps && fabs(x[pos1] - now) <= eps)
        return 1001.0;
    if (now <= min(x[pos1], x[pos2]) || now >= max(x[pos1], x[pos2]))
        return 1001.0;
    double k = (y[pos2] - y[pos1]) / (x[pos2] - x[pos1]);
    return k * (now - x[pos2]) + y[pos2];
}
bool findsan(double now)
{
    int cnt = 2;
    for (int i = 1; i <= 3; i++)
    {
        int nx = i + 1;
        if (nx == 4)
            nx = 1;
        double mid = check(now, i, nx);
        if (mid >= 1000.5)
            continue;
        if (cnt == 3 && fabs(mid - _y[cnt]) <= eps)
            continue;
        _y[++cnt] = mid;
    }
    if (cnt != 4)
        return 0;
    return 1;
}
//1 0 0 1 2          -1 0 1 0 0 1.732
double anglee(double a, double b, double c, double d)
{
    double now = acos((c - a) / dis(a, b, c, d));
    if (d - b < 0)
        return pi + pi - now;
    return now;
}
void zhuan(int pos)
{
    double sitepos = anglee(0, 0, x[pos], y[pos]);
    double val = dis(0, 0, x[pos], y[pos]);
    double diff = sitepos - site;
    x[pos] = val * cos(diff);
    y[pos] = val * sin(diff);
    //printf("%lf %lf\n",x[pos],y[pos]);
}
void zhuantuo()
{
    double sitepos1 = anglee(0, 0, xa, ya), sitepos2 = anglee(0, 0, xb, yb);
    double val1 = dis(0, 0, xa, ya), val2 = dis(0, 0, xb, yb);
    double diff1 = sitepos1 - site, diff2 = sitepos2 - site;
    //printf("%lf %lf %lf\n",site,sitepos1,sitepos2);
    xa = val1 * cos(diff1);
    ya = val1 * sin(diff1);
    xb = val2 * cos(diff2);
    yb = val2 * sin(diff2);
    //printf("%lf %lf %lf %lf\n",xa,ya,xb,yb);
}
double solve(double now)
{
    if (findtuo(now) == 0)
        return 0;
    if (findsan(now) == 0)
        return 0;
    double mx = max(min(_y[1], _y[2]), min(_y[3], _y[4]));
    double mn = min(max(_y[1], _y[2]), max(_y[3], _y[4]));
    //printf("%.4lf %.4lf %.4lf %.4lf %.4lf %.4lf\n",now,_y[1],_y[2],_y[3],_y[4],mn-mx);
    if (mn - mx <= eps)
        return 0;
    return mn - mx;
}

int main()
{
    scanf("%lf%lf%lf%lf%lf", &xa, &ya, &xb, &yb, &l);
    site = anglee(xa, ya, xb, yb);
    for (int i = 1; i <= 3; i++)
        scanf("%lf%lf", &x[i], &y[i]), zhuan(i);
    zhuantuo();
    if (l <= eps || dis(xa, ya, xb, yb) >= l)
        return 0 * printf("0.00\n");
    for (double i = -1000; i <= 1000; i += 0.001)
        res += solve(i) * 0.001;
    printf("%.2lf\n", res);
    return 0;
}