半平面求交 - 洛谷 - P3194 [HNOI2008] 水平可见直线

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题目大意

题目链接
https://www.luogu.com.cn/problem/P3194

在直角坐标系中给定一些直线，然后从Y轴无穷大处往0处看，问可以看到哪些直线。

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解析

在这里插入图片描述
通过观察可以发现，能看到的直线会形成一个开口凸包。

可以先对直线进行方向规定向右，然后进行半平面求交，凸包有效的边就是可以看到的直线。

代码

#include<stdio.h>
#include<cmath>
#include <algorithm>
#include <vector>
#include <list>
#include <cstring>
#include <deque>using namespace std;
const double EPS = 1e-12;const int N = 1e6 + 10;
const int M = 1e6 + 10;int cmp(double d) {if (abs(d) < EPS)return 0;if (d > 0)return 1;return -1;
}class Point {
public:double x, y;int id;Point() {}Point(double a, double b) :x(a), y(b) {}Point(const Point& p) :x(p.x), y(p.y), id(p.id) {}void in() {scanf("%lf %lf", &x, &y);}void out() {printf("%f %f\n", x, y);}double dis() {return sqrt(x * x + y * y);}double dis2() {return x * x + y * y;}Point operator -() const {return Point(-x, -y);}Point operator -(const Point& p) const {return Point(x - p.x, y - p.y);}Point operator +(const Point& p) const {return Point(x + p.x, y + p.y);}Point operator *(double d)const {return Point(x * d, y * d);}Point operator /(double d)const {return Point(x / d, y / d);}void operator -=(Point& p) {x -= p.x;y -= p.y;}void operator +=(Point& p) {x += p.x;y += p.y;}void operator *=(double d) {x *= d;y *= d;}void operator /=(double d) {this ->operator*= (1 / d);}bool operator<(const Point& a) const {return x < a.x || (abs(x - a.x) < EPS && y < a.y);}bool operator==(const Point& a) const {return abs(x - a.x) < EPS && abs(y - a.y) < EPS;}
};// 向量操作double cross(const Point& a, const Point& b) {return a.x * b.y - a.y * b.x;
}double dot(const Point& a, const Point& b) {return a.x * b.x + a.y * b.y;
}class Line {
public:Point front, tail;int ind;Line() {}Line(const Point& a, const Point& b) :front(a), tail(b) {}Line(const Point& a, const Point& b, int i) :front(a), tail(b), ind(i) {}
};int cmp(const Line& a, const Line& b) {auto ta = atan2(a.front.y - a.tail.y, a.front.x - a.tail.x);auto tb = atan2(b.front.y - b.tail.y, b.front.x - b.tail.x);return cmp(ta - tb);
}// 点在直线哪一边>0 左边，<0边
int SideJudge(const Line& a, const Point& b) {return cmp(cross(a.front - a.tail, b - a.tail));
}int LineSort(const Line& a, const Line& b) {int c = cmp(a, b);if (c)return c < 0;return SideJudge(b, a.front) > 0;
}/*
点p 到 p+r 表示线段1
点q 到 q+s 表示线段2
线段1 上1点用 p' = p+t*r (0<=t<=1)
线段2 上1点用 q' = q+u*s (0<=u<=1)
让两式相等求交点 p+t*r = q+u*s
两边都叉乘s
(p+t*r)Xs = (q+u*s)Xs
pXs + t*rXs = qXs
t = (q-p)Xs/(rXs)
同理，
u = (p-q)Xr/(sXr) -> u = (q-p)Xr/(rXs)以下分4种情况：
1. 共线，sXr==0 && (q-p)Xr==0, 计算 (q-p)在r上的投影在r长度上的占比t0，
计算(q+s-p)在r上的投影在r长度上的占比t1，查看[t0, t1]是否与范围[0，1]有交集。
如果t0>t1, 则比较[t1, t0]是否与范围[0，1]有交集。
t0 = (q-p)*r/(r*r)
t1 = (q+s-p)*r/(r*r) = t0 + s · r / (r · r)
2. 平行sXr==0 && (q-p)Xr!=0
3. 0<=u<=1 && 0<=t<=1 有交点
4. 其他u, t不在0到范围内，没有交点。
*/
pair<double, double> intersection(const Point& q, const Point& s, const Point& p, const Point& r) {// 计算 (q-p)Xrauto qpr = cross(q - p, r);auto qps = cross(q - p, s);auto rXs = cross(r, s);if (cmp(rXs) == 0)return { -1, -1 }; // 平行或共线// 求解t, u// t = (q-p)Xs/(rXs)auto t = qps / rXs;// u = (q-p)Xr/(rXs)auto u = qpr / rXs;return { u, t };
}Point LineCross(const Line& a, const Line& b) {Point dira = a.front - a.tail;Point dirb = b.front - b.tail;auto p = intersection(a.tail, dira, b.tail, dirb);return a.tail + dira * p.first;
}class HalfPlane {
public:vector<Line> lines;void addLine(const Line& a) {lines.push_back(a);}vector<int> run() {sort(lines.begin(), lines.end(), LineSort);vector<int> q(lines.size() + 10);vector<Point> t(lines.size() + 10);int l = -1, r = 0;q[0] = 0;for (int i = 1; i < lines.size(); ++i) {if (cmp(lines[i], lines[i - 1]) == 0)continue;while (r - l > 1 && SideJudge(lines[i], t[r]) <= 0)r--;while (r - l > 1 && SideJudge(lines[i], t[l + 2]) <= 0)l++;q[++r] = i;t[r] = LineCross(lines[q[r]], lines[q[r - 1]]);}/*while (r - l > 1 && SideJudge(lines[q[l + 1]], t[r]) < 0)r--;t[r+1] = LineCross(lines[q[l+1]], lines[q[r]]);r++;*/// 统计交点/*l++;vector<Point> ans(r-l);for (int i = 0; i < ans.size(); ++i) {ans[i] = t[i + l + 1];}*/vector<int> ans;for (int i = l + 1; i <= r; ++i) ans.push_back(lines[q[i]].ind);sort(ans.begin(), ans.end());return ans;}
};Point oiPs[N];void  solve() {int n;scanf("%d", &n);HalfPlane hp;int a, b;for (int i = 0; i < n; ++i) {scanf("%d%d", &a, &b);Line l(Point(1, a + b), Point(0, b), i + 1);hp.addLine(l);}auto ans = hp.run();for (auto x : ans) printf("%d ", x);
}int main() {solve();return 0;}/*
1
0 02
-1 0
-1 14
-1 0
-1 1
0 0
1 -110
1 2
5 8
7 6
-1 8
-8 9
-9 11
-3 -4
-5 7
8 7
9 1010
-8 10
-7 9
-6 8
-10 0
-5 7
-4 6
-3 5
-2 4
-1 3
0 0*/