ZOJ 1010 Area 求任意多边形面积

#include <iostream>
#include <cmath>
#include <stdio.h>
using namespace std;
#define eps 1e-8
int sig(double x)
{
if(x<-eps) return -1;
if(x>eps) return 1;
return 0;
}
struct point
{
double x,y;
} p[1005];

struct line
{
point p;
point q;
line()
{

}
line(point aa,point bb)
{
p=aa;
q=bb;
}
};
int inside(point o,line L)
{
double xl=min(L.p.x,L.q.x);
double xr=max(L.p.x,L.q.x);
double yl=min(L.p.y,L.q.y);
double yr=max(L.p.y,L.q.y);
if(sig(o.x-xl)>=0&&sig(xr-o.x)>=0&&sig(o.y-yl)>=0&&sig(yr-o.y)>=0)
return 1;
else
return 0;
}

int banana(line u,line v)
{
point ret=u.p;
double t=((u.p.x-v.p.x)*(v.p.y-v.q.y)-(u.p.y-v.p.y)*(v.p.x-v.q.x))/((u.p.x-u.q.x)*(v.p.y-v.q.y)-(u.p.y-u.q.y)*(v.p.x-v.q.x));
ret.x+=(u.q.x-u.p.x)*t;
ret.y+=(u.q.y-u.p.y)*t;
if(inside(ret,u)&&inside(ret,v)) return 1;
return 0;
}
double det(point p0,point p1,point p2)
{
return (p1.x-p0.x)*(p2.y-p0.y)-(p1.y-p0.y)*(p2.x-p0.x);
}
//求多边形面积
double ploygon_area(int n)
{
double s=0.0;
for(int i=1; i < n-1; i++)
s += det(p[0],p[i],p[i+1]);
return 0.5*fabs(s);
}

int main()
{
int i,n,ca=1,j,yy=0;
while(scanf("%d",&n)!=EOF,n)
{
for(i=0; i<n; i++)
scanf("%lf%lf",&p[i].x,&p[i].y);
if(yy==1) printf("\n");
yy=1;
printf("Figure %d: ",ca++);
if(n<3)
{
printf("Impossible\n");
continue;
}
p
=p[0];
int flag=0;
for(i=0;i<n;i++)
{
for(j=i+2;j<n;j++)
{
if(i==(j+1)%n) continue;
if(banana(line(p[i],p[i+1]),line(p[j],p[j+1]))) {flag=1;break;}
}
if(flag) break;
}
if(flag)
{
printf("Impossible\n");
continue;
}
double ans=ploygon_area(n);
printf("%.2f\n",ans);
}
return 0;
}