Uyuw's Concert POJ

题目链接：https://cn.vjudge.net/problem/POJ-2451

朱泽圆06年论文–半平面交

#include<iostream>#include<algorithm>#include<cmath>using namespace std;const int maxn=20010;const int bound=10000;const double esp= 1e-10;class point{public:    double x,y;};class node{public:    point a,b;};node pp[maxn];int cnt,id[maxn],n;double at[maxn];int q[maxn*2],front,rear,dg;point dpg[maxn];void add(double x1,double y1,double x2,double y2){    pp[cnt].a.x =x1;pp[cnt].a .y =y1;    pp[cnt].b.x =x2;pp[cnt++].b.y =y2;}void read(){    scanf("%d",&n);    cnt=0;    double x1,x2,y1,y2;    for(int i=0;i<n;i++)    {        scanf("%lf%lf%lf%lf",&x1,&y1,&x2,&y2);        add(x1,y1,x2,y2);    }    n+=4;    add(0,0,bound,0);    add(bound,0,bound,bound);    add(bound,bound,0,bound);    add(0,bound,0,0);}int is0(double a){    if(a<=esp&&a>=-esp) return 0;    if(a>esp) return 1;    return -1;}double det(double x1,double y1,double x2,double y2){    return x1*y2-x2*y1;}double ana(point p0,point p1,point p2){    return det(p1.x-p0.x ,p1.y -p0.y ,p2.x -p0.x ,p2.y -p0.y );}bool cmp(int a,int b){    if(at[a]<at[b]) return true;    if(is0(at[a]-at[b])==0)        if(is0(ana(pp[b].a,pp[b].b,pp[a].a ))>=0) return true;    return false;}void deal(){    for(int i=0;i<n;i++)    {        at[i]=atan2(pp[i].b.y-pp[i].a.y ,pp[i].b.x -pp[i].a.x );        id[i]=i;    }    sort(id,id+n,cmp);    int temp=1;    for(int i=1;i<n;i++)        if(is0(at[id[i-1]]-at[id[i]])!=0)  id[temp++]=id[i];    n=temp;}point APP(node &t1,node &t2){    double d1,d2;    point s1,e1,s2,e2;    s1=t1.a ;e1=t1.b ;s2=t2.a ;e2=t2.b ;    point temp;    d1=ana(s2,e1,s1);d2=ana(e1,e2,s1);    temp.x =(s2.x *d2+e2.x *d1)/(d2+d1);    temp.y =(s2.y *d2+e2.y *d1)/(d2+d1);    return temp;}bool judge(int x,int y,int now){    point temp=APP(pp[x],pp[y]);    double d=ana(pp[now].a,pp[now].b,temp);    if(is0(d)<0) return true;    return false;}void solve(){    int i;    front=0;rear=1;    q[0]=id[0];q[1]=id[1];    for(i=2;i<n;i++)    {        while(front<rear&&judge(q[rear-1],q[rear],id[i]))            rear--;        while(front<rear&&judge(q[front],q[front+1],id[i]))            front++;        q[++rear]=id[i];    }    while(front<rear&&judge(q[rear-1],q[rear],q[front]))        rear--;    while(front<rear&&judge(q[front],q[front+1],q[rear]))        front++;    q[++rear]=q[front];    dg=0;    for(i=front+1;i<=rear;i++)        dpg[dg++]=APP(pp[q[i-1]],pp[q[i]]);}double  area(){    double ans=0;    for(int i=0;i<dg;i++)        ans+=dpg[i].x*dpg[(i+1)%dg].y -dpg[(i+1)%dg].x *dpg[i].y ;    ans=fabs(ans)/2.0;    return ans;}int main(){    read();    deal();    solve();    printf("%.1f\n",area());    return 0;}