POJ 1279

发现好多半平面交用N^2的增量法都能过诶。。。

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

using namespace std;

const int MAXN=1550;
const double eps=1e-8;
struct point {
	double x,y;
};
point pts[MAXN],p[MAXN],q[MAXN];
int ansCnt,curCnt,n;

double DB(double d){
	if(d>eps) return 1;
	if(d<-eps) return -1;
	return 0;
}

double getArea(point *tmp,int n){
	double ans=0;
	for(int i=1;i<=n;i++)
	ans+=(tmp[i].x*tmp[i+1].y-tmp[i+1].x*tmp[i].y);
	return ans/2;
}

void Adjust(){
	for(int i=1;i<(n+1)/2;i++)
	swap(pts[i],pts[n-i]);
}

void initial(){
	double area=getArea(pts,n);
	if(DB(area)>=0) Adjust();
	for(int i=1;i<=n;i++)
	p[i]=pts[i];
	p[n+1]=p[1];
	p[0]=p[n];
	ansCnt=n;
}

void getline(point x,point y,double &a,double &b,double &c){
	a=y.y-x.y;
	b=x.x-y.x;
	c=x.y*y.x-x.x*y.y;
}

point intersect(point x,point y,double a,double b,double c){
	double u=fabs(a*x.x+b*x.y+c);
	double v=fabs(a*y.x+b*y.y+c);
	point pt;
	pt.x=(x.x*v+y.x*u)/(u+v);
	pt.y=(x.y*v+y.y*u)/(u+v);
	return pt;
}

void cut(double a,double b,double c){
	curCnt=0;
	for(int i=1;i<=ansCnt;i++){
		if(DB(a*p[i].x+b*p[i].y+c)>=0) q[++curCnt]=p[i];
		else {
			if(DB(a*p[i-1].x+b*p[i-1].y+c)>0) q[++curCnt]=intersect(p[i],p[i-1],a,b,c);
			if(DB(a*p[i+1].x+b*p[i+1].y+c)>0) q[++curCnt]=intersect(p[i],p[i+1],a,b,c);
		}
	}
	for(int i=1;i<=curCnt;i++){
		p[i]=q[i];
	}
	ansCnt=curCnt;
	p[ansCnt+1]=p[1]; p[0]=p[ansCnt];
}

void slove(){
	initial();
	for(int i=1;i<=n;i++){
		double a,b,c;
		getline(pts[i],pts[i+1],a,b,c);
		cut(a,b,c);
	}
}

int main(){
	int cas=0;
	int t;
	scanf("%d",&t);
	while(t--){
		scanf("%d\n",&n);
		cas++;
		for(int i=1;i<=n;i++)
		scanf("%lf%lf",&pts[i].x,&pts[i].y);
		pts[n+1]=pts[1];
		slove();
		double as=getArea(p,ansCnt);
		printf("%.2lf\n",fabs(as));
	}
	return 0;
}