POJ 2451 半平面交nlogn

题意：

半平面交面积

题解：

果断上模板了。

尼玛。。。我输出-0.0给wa了半天，，，抑郁。。。

这个好像没什么用。。。

#include <iostream>
#include <cstring>
#include <cstdlib>
#include <cstdio>
#include <algorithm>
#include <cmath>

#define N 222222
#define EPS 1e-10
#define SIDE 10000

using namespace std;

struct PO
{
    double x,y;
    void prt()
    {
        printf("%lf       %lf\n",x,y);
    }
}p[N],o;

struct LI
{
    PO a,b;
    double angle;
    void in(double x1,double y1,double x2,double y2)
    {
        a.x=x1; a.y=y1; b.x=x2; b.y=y2;
    }
    void prt()
    {
        printf("%lf     %lf      %lf    %lf\n",a.x,a.y,b.x,b.y);
    }
}li[N],deq[N];

int n,m,cnt;

inline int dc(double x)
{
    if(x>EPS) return 1;
    else if(x<-EPS) return -1;
    return 0;
}

inline PO operator -(PO a,PO b)
{
    PO c;
    c.x=a.x-b.x; c.y=a.y-b.y;
    return c;
}

inline double cross(PO a,PO b,PO c)
{
    return (b.x-a.x)*(c.y-a.y)-(b.y-a.y)*(c.x-a.x);
}

inline bool cmp(const LI &a,const LI &b)//极角排序，内侧在前 
{
    if(dc(a.angle-b.angle)==0) return dc(cross(a.a,a.b,b.a))<0;
    return a.angle>b.angle;
}

inline PO getpoint(LI &a,LI &b)
{
    double k1=cross(a.a,b.b,b.a);
    double k2=cross(a.b,b.a,b.b);
    PO tmp=a.b-a.a,ans;
    ans.x=a.a.x+tmp.x*k1/(k1+k2);
    ans.y=a.a.y+tmp.y*k1/(k1+k2);
    return ans;
}

inline void read()
{
    for(int i=1;i<=n;i++)
    {
        scanf("%lf%lf%lf%lf",&li[i].a.x,&li[i].a.y,&li[i].b.x,&li[i].b.y);
        li[i].angle=atan2(li[i].b.y-li[i].a.y,li[i].b.x-li[i].a.x);
    }
    li[n+1].in(0,0,SIDE,0);
    li[n+2].in(0,SIDE,0,0);
    li[n+3].in(SIDE,0,SIDE,SIDE);
    li[n+4].in(SIDE,SIDE,0,SIDE);
    for(int i=n+1;i<=n+4;i++) li[i].angle=atan2(li[i].b.y-li[i].a.y,li[i].b.x-li[i].a.x);
    n+=4;
}

inline void getcut()
{
    sort(li+1,li+1+n,cmp);
    m=1;
    for(int i=2;i<=n;i++)
        if(dc(li[i].angle-li[m].angle)!=0)
            li[++m]=li[i];
    deq[1]=li[1]; deq[2]=li[2];
    int bot=1,top=2;
    for(int i=3;i<=m;i++)
    {
        while(bot<top&&dc(cross(li[i].a,li[i].b,getpoint(deq[top],deq[top-1])))<0) top--;
        while(bot<top&&dc(cross(li[i].a,li[i].b,getpoint(deq[bot],deq[bot+1])))<0) bot++;
        deq[++top]=li[i];
    }
    while(bot<top&&dc(cross(deq[bot].a,deq[bot].b,getpoint(deq[top],deq[top-1])))<0) top--;
    while(bot<top&&dc(cross(deq[top].a,deq[top].b,getpoint(deq[bot],deq[bot+1])))<0) bot++;
    if(bot==top) return;
    cnt=0;
    for(int i=bot;i<top;i++) p[++cnt]=getpoint(deq[i],deq[i+1]);
    if(top-1>bot) p[++cnt]=getpoint(deq[bot],deq[top]);
}

inline double getarea()
{
    double res=0.0;
    p[cnt+1]=p[1];
    for(int i=1;i<=cnt;i++) res+=cross(o,p[i],p[i+1]);
    return res;
}

inline void go()
{
    getcut();
    printf("%.1lf\n",fabs(getarea())*0.5);
}

int main()
{
    while(scanf("%d",&n)!=EOF) read(),go();
    return 0;
}