POJ 2451 Uyuw's Concert

把别人的代码和自己的代码不断对比，实在不知道问题出在哪里，太坑了

就改了两个地方

一个是把判断点在向量左右的方法改成了xmul函数，我仔细看过，代进去公式完全一样的

还有一个地方就是把极角排序改成了按从大到小

就改了这两个地方就AC了

总结就是以后还是用xmul好了

#include<stdio.h>
#include<math.h>
#include<algorithm>
#define zero(a) fabs(a)<eps
using namespace std;
const double eps=1e-10;
struct Point {
double x,y;
Point () {};
Point (double xx,double yy) {x=xx;y=yy;};
double operator^(const Point b)const{
return x*b.y-y*b.x;
}
Point operator-(const Point b)const{
return Point(x-b.x,y-b.y);
}
};
struct Line {
Point s,e;
double angle;
Line(){};
Line(Point ss,Point ee){s=ss;e=ee;}
Point operator &(const Line b)const {
Point res=s;
double t=((s-b.e)^(b.s-b.e))/((s-e)^(b.s-b.e));
res.x+=(e.x-s.x)*t;
res.y+=(e.y-s.y)*t;
return res;
}
void puts(){
printf("%.2f %.2f %.2f %.2f  %f\n",s.x,s.y,e.x,e.y,angle);
}
};
int n;
Line dq[20005],l[20005];
Point p[20005];
int cmp(Line a,Line b){
if (fabs(a.angle-b.angle)<eps) return ((a.e-a.s)^(b.s-a.s))<-eps;
else return a.angle>b.angle;
}
double xmul(Point p0,Point p1,Point p2){
return (p1.x-p0.x)*(p2.y-p0.y)-(p2.x-p0.x)*(p1.y-p0.y);
}
/*bool cmp(Line s1,Line s2){
//先按极角排序
if(s1.angle>s2.angle) return true;
//极角相等，内侧的在前
else if(zero(s1.angle-s2.angle)&&xmul(s2.s,s2.e,s1.e)>-eps) return true;
return false;
}*/
void HPI(){
sort(l,l+n,cmp);
int ln=0;
for(int i=1,j=0;i<n;i++) {
if (!zero(l[i].angle-l[j].angle)) l[++j]=l[i];
ln=j+1;
}
n=ln;
//for(int i=0;i<n;i++) l[i].puts();
int bot=0,top=1;
dq[0]=l[0];    dq[1]=l[1];
/*for(int i=2;i<n;i++) {
while(bot<top && ((l[i].e-l[i].s)^((dq[top]&dq[top-1])-l[i].s))<-eps) top--;
while(bot<top && ((l[i].e-l[i].s)^((dq[bot]&dq[bot+1])-l[i].s))<-eps) bot++;
dq[++top]=l[i];
}
while(bot<top && ((l[bot].e-l[bot].s)^((dq[top]&dq[top-1])-l[bot].s))<-eps) top--;
while(bot<top && ((l[top].e-l[top].s)^((dq[bot]&dq[bot+1])-l[top].s))<-eps) bot++;*/
for(int i=2;i<n;i++){
while(bot<top&&xmul(l[i].s,l[i].e,(dq[top]&dq[top-1]))<-eps) top--;
while(bot<top&&xmul(l[i].s,l[i].e, (dq[bot]&dq[bot+1]))<-eps) bot++;
dq[++top]=l[i];
}
while(bot<top&&xmul(dq[bot].s,dq[bot].e, (dq[top]&dq[top-1]))<-eps) top--;
while(bot<top&&xmul(dq[top].s,dq[top].e, (dq[bot]&dq[bot+1]))<-eps) bot++;

dq[top+1]=dq[bot];
int cnt=0;
//for(int i=bot;i<=top;i++) l[i].puts();
for(int i=bot;i<=top;i++) p[cnt++]=dq[i]&dq[i+1];
//printf("%d ",cnt);
p[cnt]=p[0];
if (top<=bot+1) cnt=0;
double s=0;
for(int i=0;i<cnt;i++) s+=p[i]^p[i+1];
s=fabs(s*0.5);
printf("%.1f\n",s);
}
int main() {
#ifndef ONLINE_JUDGE
freopen("in.txt","r",stdin);
#endif // ONLINE_JUDGE
while(scanf("%d",&n)!=EOF) {
l[0]=Line(Point(0,0),Point(10000,0));   l
.angle=atan2(l
.e.y-l
.s.y,l
.e.x-l
.s.x);
l[0+1]=Line(Point(10000,0),Point(10000,10000)); l[0+1].angle=atan2(l[0+1].e.y-l[0+1].s.y,l[0+1].e.x-l[0+1].s.x);
l[0+2]=Line(Point(10000,10000),Point(0,10000)); l[0+2].angle=atan2(l[0+2].e.y-l[0+2].s.y,l[0+2].e.x-l[0+2].s.x);
l[0+3]=Line(Point(0,10000),Point(0,0)); l[0+3].angle=atan2(l[0+3].e.y-l[0+3].s.y,l[0+3].e.x-l[0+3].s.x);
for(int i=0;i<n;i++) {
scanf("%lf%lf%lf%lf",&l[i+4].s.x,&l[i+4].s.y,&l[i+4].e.x,&l[i+4].e.y);
l[i+4].angle=atan2(l[i+4].e.y-l[i+4].s.y,l[i+4].e.x-l[i+4].s.x);
}

n=n+4;
HPI();
}
}