POJ 1410.Intersection

447


题目



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You are to write a program that has to decide whether a given line segment intersects a given rectangle.

An example:
line: start point: (4,9)
end point: (11,2)
rectangle: left-top: (1,5)
right-bottom: (7,1)


Figure 1: Line segment does not intersect rectangle

The line is said to intersect the rectangle if the line and the rectangle have at least one point in common. The rectangle consists of four straight lines and the area in between. Although all input values are integer numbers, valid intersection points do not have to lay on the integer grid.


The input consists of n test cases. The first line of the input file contains the number n. Each following line contains one test case of the format:
xstart ystart xend yend xleft ytop xright ybottom

where (xstart, ystart) is the start and (xend, yend) the end point of the line and (xleft, ytop) the top left and (xright, ybottom) the bottom right corner of the rectangle. The eight numbers are separated by a blank. The terms top left and bottom right do not imply any ordering of coordinates.


For each test case in the input file, the output file should contain a line consisting either of the letter "T" if the line segment intersects the rectangle or the letter "F" if the line segment does not intersect the rectangle.


1
4 9 11 2 1 5 7 1


F




题解


判断一个直线是否与矩形相交或者在矩形内部


先针对四条边分别判断一下线段关系,再判断一下线段是否在矩形内部
需要注意的是描述多边形点要有顺序

代码


点击显/隐代码
#include <algorithm>
#include <complex>
#include <cstdio>
#include <cstring>
using namespace std;

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

/* 计算几何模板 */
const double eps = 1e-8;
const double INF = 9e50;

inline int sgn(const double &x) {
    if (fabs(x) < eps)
        return 0;
    return x > 0 ? 1 : -1;
}
struct Vector {
    double x, y;
    Vector(double _x = 0, double _y = 0) : x(_x), y(_y) {}

    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(x - rhs.x) == 0)
            return sgn(y - rhs.y) < 0;
        return sgn(x - rhs.x) < 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);
    }
    double squre() const { return x * x + y * y; }
    double distance() const { return sqrt(squre()); }
    void print() const { Log("(%.f %.f)", x, y); }
};

typedef Vector Point;
struct Segment {
    Point a, b;
    Segment() {}
    Segment(Point _a, Point _b) : a(_a), b(_b) {}
    Vector toVector() const { return b - a; }
    double distance() const { return (b - a).distance(); }
    void print() const {
        a.print();
        Log(" -> ");
        b.print();
    }
};
/**
    * 读入一个点的坐标
    * @return   读入的点
    */
inline Point read_Point() {
    double x, y;
    scanf("%lf%lf", &x, &y);
    return Point(x, 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) {
    p.print();
    Log("     ");
    L.print();
    Log("      %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(Point p, Point polygon[], 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;
}

/**
    * 求点集的凸包
    * @param p              点集
    * @param numOfPoint     点集内的点的个数
    * @param ans            返回的凸包
    *
void Convex_Hull(Point p[], int numOfPoint, Point ans[]) {
    sort(p, p + numberOfPoint);
    bool *vis = new bool[numOfPoint + 5];
    ans[0] = p[0];
    for (int i = 0; i < numberOfPoint; ++i) {
    }
}
*/
int main() {
    int T;
    scanf("%d", &T);
    while (T--) {
        Segment s;
        s.a = read_Point();
        s.b = read_Point();

        Point p1 = read_Point();
        Point p2 = read_Point();

        Point pots[4] = {p1, p2, Point(p1.x, p2.y), Point(p2.x, p1.y)};
        sort(pots, pots + 4);

        Segment segs[4] = {Segment(pots[0], pots[2]), Segment(pots[2], pots[3]),
                           Segment(pots[3], pots[1]),
                           Segment(pots[1], pots[0])};
        swap(pots[1], pots[2]);
        swap(pots[2], pots[3]);

        // 判断是否在多边形内部
        bool ans = Point_Polygon(s.a, pots, 4) && Point_Polygon(s.b, pots, 4);

        // 判断相交
        for (int i = 0; i < 4; ++i)
            ans |= Segment_Segment(segs[i], s) > 0;

        printf("%c\n", ans ? 'T' : 'F');
    }
    return 0;
}
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