# 题目

{% fold 点击显/隐题目 %}

The art galleries of the new and very futuristic building of the Center for Balkan Cooperation have the form of polygons (not necessarily convex). When a big exhibition is organized, watching over all of the pictures is a big security concern. Your task is that for a given gallery to write a program which finds the surface of the area of the floor, from which each point on the walls of the gallery is visible. On the figure 1. a map of a gallery is given in some co-ordinate system. The area wanted is shaded on the figure 2.
The number of tasks T that your program have to solve will be on the first row of the input file. Input data for each task start with an integer N, 5 <= N <= 1500. Each of the next N rows of the input will contain the co-ordinates of a vertex of the polygon ? two integers that fit in 16-bit integer type, separated by a single space. Following the row with the co-ordinates of the last vertex for the task comes the line with the number of vertices for the next test and so on.
For each test you must write on one line the required surface - a number with exactly two digits after the decimal point (the number should be rounded to the second digit after the decimal point).
1 7 0 0 4 4 4 7 9 7 13 -1 8 -6 4 -4
80.00
{% endfold %}

# 代码

{% fold 点击显/隐代码 %}```cpp Art Gallery https://github.com/OhYee/sourcecode/tree/master/ACM 代码备份
#include
#include
#include
#include

using namespace std;

#define Log(format, ...) // printf(format, ##VA_ARGS)

/* 计算几何模板 */

const double eps = 1e-8;
const double INF = 0x4242424242424242;
inline int sgn(const double &x) {
if (fabs(x) < eps)
return 0;
return x > 0 ? 1 : -1;
}

struct Vector;
struct Segment;
double Cross(const Vector &, const Vector &);
int Point_Segment(const Vector &, const Segment &);

struct Vector {
double x, y;
int n;
Vector(double _x = 0, double _y = 0, int _n = 0) : x(_x), y(_y), n(_n) {}
bool operator==(const Vector &rhs) const {
return sgn(x - rhs.x) == 0 && sgn(y - rhs.y) == 0;
}
bool operator!=(const Vector &rhs) const { return !(this == rhs); }
bool operator<(const Vector &rhs) const {
if (sgn(y - rhs.y) == 0)
return sgn(x - rhs.x) < 0;
return sgn(y - rhs.y) > 0;
}
Vector operator+(const Vector &rhs) const {
return Vector(x + rhs.x, y + rhs.y);
}
Vector operator-(const Vector &rhs) const {
return Vector(x - rhs.x, y - rhs.y);
}
Vector operator
(const double &rhs) const {
return Vector(x * rhs, y * rhs);
}
Vector operator/(const double &rhs) const {
return Vector(x / rhs, y / rhs);
}
double getAngle() { return atan2(y, x); }
double squre() const { return x * x + y * y; }
double distance() const { return sqrt(squre()); }
void print(bool flag = 0) const {
Log("(%.2f %.2f)", x, y);
if (flag)
Log("\n");
}
};
typedef Vector Point;
struct Segment {
Point a, b;
Segment() {}
Segment(Point _a, Point _b) : a(_a), b(_b) {}
bool operator<(const Segment &rhs) const {
double angle1 = getAngle();
double angle2 = rhs.getAngle();
if (sgn(angle1 - angle2) == 0)
return Point_Segment(a, rhs) > 0;
return sgn(angle1 - angle2) < 0;
}
Vector toVector() const { return b - a; }
void print(bool flag = 0) const {
a.print();
Log(" -> ");
b.print();
if (flag)
Log("\n");
}
};

/**
* 读入一个点的坐标
* @return 读入的点
*/
double x, y;
scanf("%lf%lf", &x, &y);
return Point(x, y);
}

inline double xmult(const Vector &a, const Vector &b, const Vector &c) {
return (a.x - c.x) * (b.y - c.y) - (b.x - c.x) * (a.y - c.y);
}

/**
* 计算两个向量的叉积
* @param a 向量1
* @param b 向量2
* @return 叉积
*/
inline double Cross(const Vector &a, const Vector &b) {
return a.x * b.y - a.y * b.x;
}

/**
* 计算两个向量的点积
* @param a 向量1
* @param b 向量2
* @return 点积
*/
inline double Dot(const Vector &a, const Vector &b) {
return a.x * b.x + a.y * b.y;
}

/**
* 计算两点之间的距离
* @param a 线段L1
* @param b 线段L2
* @return 两点间的距离
*/
inline double Distance(const Point &a, const Point &b) {
return (a - b).distance();
}

/**
* 点和直线的关系
* @param p 目标点
* @param L 目标直线
* @return 1 在左侧,0 在直线上,-1在右侧
*/
inline int Point_Segment(const Vector &p, const Segment &L) {
// printf("Point_segment %d\n", sgn(Cross(L.b - L.a, p - L.a)));
return sgn(Cross(L.b - L.a, p - L.a));
}

/**
* 计算两个线段(直线)是否平行
* @param L1 L1的向量
* @param L2 L2的向量
* @return 返回是否平行
*/
bool parallel(const Vector &L1, const Vector &L2) {
return sgn(Cross(L1, L2)) == 0;
}

/**
* 计算两个直线的交点(需要确保不平行、重合)
* @param L1 L1的向量
* @param L2 L2的向量
* @return 返回是否平行
*/
Point getIntersection(const Segment &L1, const Segment &L2) {
Point ret = L1.a;
double t = ((L1.a.x - L2.a.x) * (L2.a.y - L2.b.y) -
(L1.a.y - L2.a.y) * (L2.a.x - L2.b.x)) /
((L1.a.x - L1.b.x) * (L2.a.y - L2.b.y) -
(L1.a.y - L1.b.y) * (L2.a.x - L2.b.x));
ret.x += (L1.b.x - L1.a.x) * t;
ret.y += (L1.b.y - L1.a.y) * t;
return ret;
}

/**
* 计算两个线段的位置关系
* @param L1 线段L1
* @param L2 线段L2
* @param p 返回交点坐标
* @return 2 重叠
1 相交
0 延长线相交
-1 平行
-2 共线不交
*/
inline int Segment_Segment(const Segment &L1, const Segment &L2,
Point *p = NULL) {
double a = L1.b.x - L1.a.x;
double b = L2.b.x - L2.a.x;
double c = L1.b.y - L1.a.y;
double d = L2.b.y - L2.a.y;
double f = a * d - b * c;
// 平行或重叠
if (sgn(f) == 0) {
if (Point_Segment(L1.a, L2)) {
// 平行
return -1;
} else {
// 共线
int len = max(max(Distance(L1.a, L2.a), Distance(L1.a, L2.b)),
max(Distance(L1.b, L2.a), Distance(L1.b, L2.b)));
if (sgn(len - L1.distance() - L2.distance()) > 0) {
// 共线不交
return -2;
} else {
// 重叠
return 2;
}
}
}
double g = L2.b.x - L1.a.x;
double h = L2.b.y - L1.a.y;
double t = (d * g - b * h) / f;
double s = (-c * g + a * h) / f;
if (p != NULL)
*p = Point(L1.a.x + t * a, L1.a.y + t * c);
// 在延长线上
if (t < 0 || t > 1 || s < 0 || s > 1)
return 0;
// 线段相交
return 1;
}

/**
* 判断点是否在多边形内部
* @param p 需要判断的点
* @param polygon 多边形点集,需要保证有序
* @param numberOfSide 多边形边数
* @return true 点在多边形内,false 点不在多边形内
*/
bool Point_Polygon(const Point &p, const Point polygon[],
const int &numberOfSide) {
bool ok =
Point_Segment(p, Segment(polygon[numberOfSide - 1], polygon[0])) >= 0;
for (int i = 1; i < numberOfSide && ok; ++i) {
if (!(Point_Segment(p, Segment(polygon[i - 1], polygon[i])) >= 0))
ok = false;
}
return ok;
}

/**
* 求多边形面积
* @param p 点集
* @param numOfSide 多边形边数
* @return 返回多边形面积
*/
double getArea(const Point p[], const int numberOfSide) {
if (numberOfSide < 3)
return 0.0;
double area = 0.0;
for (int i = 2; i < numberOfSide; ++i)
area += fabs(0.5 * Cross(p[i] - p[0], p[i - 1] - p[0]));
return area;
}

/**
* 求点集的凸包
* @param p 点集,需要保证已经排序,并且点需要有不同的编号
* @param numOfPoint 点集内的点的个数
* @param vis 记录点对应的编号是否可以选择
* @param ans 返回的凸包
* @param begin 起始位置
* @return 返回凸包上点的个数
*/
int Convex_Hull(Point p[], int numOfPoint, bool vis[], Point ans[],
int begin = 0) {
int pos = begin - 1;
// 右链
for (int i = 0; i < numOfPoint; ++i) {
if (!vis[p[i].n]) {
while (pos > 0 &&
Point_Segment(p[i], Segment(ans[pos], ans[pos - 1])) > 0) {
vis[ans[pos--].n] = false;
}
ans[++pos] = p[i];
vis[ans[pos].n] = true;
}
}
// 左链
for (int i = numOfPoint - 2; i > 0; --i) {
if (!vis[p[i].n]) {
while (pos > 0 &&
Point_Segment(p[i], Segment(ans[pos], ans[pos - 1])) > 0) {
vis[ans[pos--].n] = false;
}
ans[++pos] = p[i];
vis[ans[pos].n] = true;
}
}
return pos + 1;
}

/**
* 求直线集合的半平面交
* @param s 线段集合
* @param numOfSide 线段数目
* @param ans 返回的多边形点
* @param dequeue 需要的中间数组(记录用到的)
* @return 返回结果多边形点的个数
*/
int Half_Plane(Segment s[], int numOfSide, Point ans[], Segment dequeue[]) {
sort(s, s + numOfSide);

``````int del = 1;
for (int i = 1; i < numOfSide; ++i) {
if (sgn(s[i].getAngle() - s[del - 1].getAngle()) != 0)
s[del++] = s[i];
}
numOfSide = del;

for (int i = 0; i < numOfSide; ++i) {
s[i].print(1);
}

int bot = 0, top = 1;
dequeue[0] = s[0];
dequeue[1] = s[1];
for (int i = 2; i < numOfSide; i++) {
if (parallel(dequeue[top].toVector(), dequeue[top - 1].toVector()) ||
parallel(dequeue[bot].toVector(), dequeue[bot + 1].toVector())) {
dequeue[top].print(), dequeue[top - 1].print(1);
dequeue[bot].print(), dequeue[bot + 1].print(1);
return 0;
}

while (bot < top &&
Point_Segment(getIntersection(dequeue[top], dequeue[top - 1]),
s[i]) < 0)
top--;
while (bot < top &&
Point_Segment(getIntersection(dequeue[bot], dequeue[bot + 1]),
s[i]) < 0)
bot++;
dequeue[++top] = s[i];
}

while (bot < top &&
Point_Segment(getIntersection(dequeue[top], dequeue[top - 1]),
dequeue[bot]) < 0)
top--;
while (bot < top &&
Point_Segment(getIntersection(dequeue[bot], dequeue[bot + 1]),
dequeue[top]) < 0)
bot++;

//计算交点(注意不同直线形成的交点可能重合)
if (bot + 1 >= top)
return 0;

int len = 0;
for (int i = bot; i < top; i++)
ans[len++] = getIntersection(dequeue[i], dequeue[i + 1]);
if (bot < top + 1)
ans[len++] = getIntersection(dequeue[bot], dequeue[top]);

return len;
``````

}

const int maxn = 1505;
bool vis[maxn];
Point p[maxn];
Point ans[maxn];
Segment s[maxn];
Segment deq[maxn];

int main() {
int T;
scanf("%d", &T);
while (T--) {
int n;
scanf("%d", &n);
for (int i = 0; i < n; ++i) {
p[i].n = i;
}
for (int i = 0; i <= n; ++i)
s[i] = Segment(p[(i + 1) % n], p[i]);

``````    int len = Half_Plane(s, n, ans, deq);

for (int i = 0; i < len; ++i)
ans[i].print(1);

if (len == 0) {
printf("0.00\n");
} else {
double area = getArea(ans, len);
if (sgn(area) <= 0)
area = 0.0;
printf("%.2f\n", area);
}
}
return 0;
``````

}

`{% endfold %}`