la3890(半平面交)

蓝书半平面交例题

#include<iostream>
#include<cstring>
#include<cmath>
#include<cstdio>
#include<algorithm>
using namespace std;
const int maxn=150;
const double eps=1e-12;
int n;
struct vec{
    double x,y;
    vec(double x=0,double y=0):x(x),y(y){}
    vec operator-(vec& a){
        return vec(x-a.x,y-a.y);
    }
    vec operator+(vec&a){
        return vec(x+a.x,y+a.y);
    }
}po[maxn],v[maxn],v2[maxn],g[maxn];
vec operator*(vec a,double t){return vec(a.x*t,a.y*t);}
double cross(vec a,vec b){return a.x*b.y-b.x*a.y;}
struct lin{
    vec p,v;
    double ang;
    lin(){}
    lin(vec p,vec v):p(p),v(v){ang=atan2(v.y,v.x);}
    bool operator<(const lin&a)const{
        return ang<a.ang;
    }
}ll[maxn],q[maxn];
bool onl(lin L,vec p){
    return cross(L.v,p-L.p)>0;
}
vec qj(lin a,lin b){
    vec u=a.p-b.p;
    double t=cross(b.v,u)/cross(a.v,b.v);
    return a.v*t+a.p;
}
vec nor(vec a){
    double len=sqrt(a.x*a.x+a.y*a.y);
    return vec(-a.y/len,a.x/len);
}
int halfj(){
    sort(ll,ll+n);
    int head,tail;
    q[head=tail=0]=ll[0];
    for(int i=1;i<n;++i){
        while(head<tail&&!onl(ll[i],g[tail-1]))tail--;
        while(head<tail&&!onl(ll[i],g[head]))head++;
        q[++tail]=ll[i];
        if(fabs(cross(q[tail].v,q[tail-1].v))<eps){
            --tail;if(onl(q[tail],ll[i].p))q[tail]=ll[i];
        }
        if(head<tail)g[tail-1]=qj(q[tail-1],q[tail]);
    }
    while(head<tail&&!onl(q[head],g[tail-1]))--tail;
    if(tail-head<=1)return 0;
    return 1;
}
int main(){
    while(scanf("%d",&n)==1&&n){
        int m,x,y;
        for(int i=0;i<n;++i){
            scanf("%d%d",&x,&y);po[i]=vec(x,y);
        }
        for(int i=0;i<n;++i){v[i]=po[(i+1)%n]-po[i];v2[i]=nor(v[i]);}
        double l=0,r=20000,mid;
        while(r-l>1e-6){
            mid=(l+r)/2;
            for(int i=0;i<n;++i)ll[i]=lin(v2[i]*mid+po[i],v[i]);//这里必须先写乘法再写加法才能过编译，可能是我这种重定义的问题；
            if(halfj())l=mid;else r=mid;
        }
        printf("%.6lf\n",l);
    }
    system("pause");
    return 0;
}
/*
4
0 0
10000 0
10000 10000
0 10000
3
0 0
10000 0
7000 1000
6
0 40
100 20
250 40
250 70
100 90
0 70
3
0 0
10000 10000
5000 5001
0
*/