[poj2451]Uyuw's Concert

半平面交滴裸题，但是要求nlogn，练练手

#include<iostream>
#include<cstdio>
#include<cmath>
#include<algorithm>
#define MN 50000
#define eps 1e-17
#define ld long double
using namespace std;
inline int read()
{
    int x =  , f = ; char ch = getchar();
    while(ch < '' || ch > ''){ if(ch == '-') f = -;  ch = getchar();}
    while(ch >= '' && ch <= ''){x = x *  + ch - '';ch = getchar();}
    return x * f;
}

struct P
{
    ld x,y;
    P(ld _x=,ld _y=):x(_x),y(_y){}
    P operator +(P b){return P(x+b.x,y+b.y);}
    P operator -(P b){return P(x-b.x,y-b.y);}
    P operator *(ld b){return P(x*b,y*b);}
    ld operator^(P b){return x*b.y-b.x*y;}
}p[MN+];
struct L
{
    P p,v;ld slop;
    L(){}
    L(P x,P y):p(x),v(y){slop=atan2(y.y,y.x);}
    P operator *(L y){P b=p-y.p;ld t=(y.v^b)/(v^y.v);return p+v*t;}
    bool operator <(const L&y)const{return slop<y.slop;}
    bool left(P y){return (v^(y-p))>eps;}
}q[MN+],s[MN+];
int n,top,tail;

void solve()
{
    q[top=tail=]=s[];
    for(int i=;i<=n;i++)
    {
        while(top>tail&&!s[i].left(p[top])) --top;
        while(top>tail&&!s[i].left(p[tail+])) ++tail;
        if(fabs(s[i].slop-q[top].slop)<eps)
            q[top]=s[i].left(q[top].p)?q[top]:s[i];
        else
            q[++top]=s[i];
        p[top]=q[top]*q[top-];
    }
    while(top>tail&&!q[tail].left(p[top])) --top;
}

int main()
{
    n=read();
    for(int i=;i<=n;i++)
    {
        double x,y,x2,y2;scanf("%lf%lf%lf%lf",&x,&y,&x2,&y2);
        s[i]=L(P(x,y),P(x2-x,y2-y));
    }
    s[++n]=L(P(,),P(,));
    s[++n]=L(P(,),P(,));
    s[++n]=L(P(,),P(-,));
    s[++n]=L(P(,),P(,-));
    sort(s+,s+n+);
    solve();p[tail]=q[top]*q[tail];
    if(top-tail<) return *puts("0.0");
    ld ans=p[top]^p[tail];
    for(int i=tail;i<top;i++) ans+=p[i]^p[i+];
    printf("%.1lf\n",(double)fabs(ans)/2.0);
    return ;
}