BZOJ1845 : [Cqoi2005] 三角形面积并

#include<cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
const int N=310;
const double eps=1e-9,inf=2000000;
struct P{
double x,y;
P(){x=y=0;}
P(double _x,double _y){x=_x,y=_y;}
P operator+(P v){return P(x+v.x,y+v.y);}
P operator-(P v){return P(x-v.x,y-v.y);}
P operator*(double v){return P(x*v,y*v);}
P operator/(double v){return P(x/v,y/v);}
double operator*(P v){return x*v.x+y*v.y;}
}tri
[4],seg
;
inline bool cmp(P a,P b){return a.x<b.x;}
double px[N*N],ans;
int n,i,j,k,l,m;
inline int sig(double x){
if(fabs(x)<eps)return 0;
return x>0?1:-1;
}
inline double cross(P a,P b){return a.x*b.y-a.y*b.x;}
inline bool has_intersection(P a,P b,P p,P q){
int d1=sig(cross(b-a,p-a)),d2=sig(cross(b-a,q-a)),
d3=sig(cross(q-p,a-p)),d4=sig(cross(q-p,b-p));
return d1*d2<0&&d3*d4<0;
}
inline P line_intersection(P a,P b,P p,P q){
double U=cross(p-a,q-p),D=cross(b-a,q-p);
return a+(b-a)*(U/D);
}
inline double cal(double x){
P D(x,-inf),U(x,inf);
int i,m=0;
for(i=0;i<n;i++){
int j=0,k=0;double y[2];
for(j=0;j<3;j++)if(has_intersection(tri[i][j],tri[i][j+1],D,U))
y[k++]=line_intersection(tri[i][j],tri[i][j+1],D,U).y;
if(k)seg[m++]=P(min(y[0],y[1]),max(y[0],y[1]));
}
if(m>1)sort(seg,seg+m,cmp);
double l=-inf,r=-inf,t=0;
for(i=0;i<m;i++){
if(sig(seg[i].x-r)>0)t+=r-l,l=seg[i].x;
r=max(r,seg[i].y);
}
return t+r-l;
}
int main(){
scanf("%d",&n);
for(i=0;i<n;i++){
for(j=0;j<3;j++)scanf("%lf%lf",&tri[i][j].x,&tri[i][j].y);
tri[i][3]=tri[i][0];
}
for(i=0;i<n;i++)for(j=0;j<3;j++)px[m++]=tri[i][j].x;
for(i=0;i<n;i++)for(j=0;j<i;j++)for(k=0;k<3;k++)for(l=0;l<3;l++)
if(has_intersection(tri[i][k],tri[i][k+1],tri[j][l],tri[j][l+1]))
px[m++]=line_intersection(tri[i][k],tri[i][k+1],tri[j][l],tri[j][l+1]).x;
sort(px,px+m);
for(i=1;i<m;i++)if(sig(px[i]-px[i-1]))ans+=(px[i]-px[i-1])*cal((px[i]+px[i-1])/2);
return printf("%.2f",ans-eps),0;
}