BZOJ2618 CQOI2006 凸多边形

乱搞.jpg

直接看成直线半平面交qwq

挺好写的w

//Love and Freedom.
#include<cstdio>
#include<cmath>
#include<algorithm>
#include<cstring>
#define ll long long
#define inf 20021225
#define db double
#define eps 1e-8
#define N 1010
using namespace std;

struct poi
{
    db x,y;
    poi(){}
    poi(db _x,db _y){x=_x;y=_y;}
};
typedef poi vec;
vec operator+(vec a,vec b){return vec(a.x+b.x,a.y+b.y);}
vec operator-(vec a,vec b){return vec(a.x-b.x,a.y-b.y);}
vec operator*(vec a,db b){return vec(a.x*b,a.y*b);}
db cross(vec a,vec b){return a.x*b.y-a.y*b.x;}
db angle(vec a){return atan2(a.y,a.x);}

struct line 
{
    poi p; vec v; db ang;
    line(){}
    line(poi p1,poi p2){p = p1; v = p2-p1; ang = angle(v);}
}l[N];
void put(poi a)
{
    printf("%lf %lf\n",a.x,a.y);
}

poi section(line a,line b)
{
    db k = cross(a.v+a.p-b.p,a.p-b.p)/(cross(a.v+a.p-b.p,b.v)+cross(b.v,a.p-b.p));
    return b.p+b.v*k;
}
bool onleft(line a,poi p)
{
    return cross(a.p-p,a.p+a.v-p)>-eps;
}
int stk[N],hd,tl,n; poi sec[N];
bool cmp(line a,line b)
{
    return a.ang<b.ang || (abs(a.ang-b.ang) < eps && cross(b.v+b.p-a.p,a.v)>eps);
}
void halfplane()
{
    sort(l+1,l+n+1,cmp);
    hd = tl = 1; stk[1] = 1;
    for(int i=1;i<=n;i++)
    {
        while(tl>hd && !onleft(l[i],sec[tl-1]))    tl--;
        while(hd<tl && !onleft(l[i],sec[hd]))    hd++;
        stk[++tl] = i;    sec[tl-1] = section(l[i],l[stk[tl-1]]);
    }
    while(hd<tl && !onleft(l[stk[hd]],sec[tl-1]))    tl--;
    if(hd<tl) sec[tl] = section(l[stk[hd]],l[stk[tl]]); 
}

db calc()
{
    db ans = 0;
    //for(int i=hd;i<=tl;i++)    put(sec[i]); 
    for(int i=hd+1;i<tl;i++)    ans+=cross(sec[i]-sec[hd],sec[i+1]-sec[hd]);
    return ans/2.0;
}

int main()
{
    int nn,mm; poi poly[N];
    scanf("%d",&nn);
    while(nn--)
    {
        scanf("%d",&mm);
        for(int i=1;i<=mm;i++)
            scanf("%lf%lf",&poly[i].x,&poly[i].y);
        for(int i=1;i<mm;i++)
            l[++n] = line(poly[i],poly[i+1]);
        l[++n] = line(poly[mm],poly[1]);
    }
    halfplane(); printf("%.3lf\n",calc());
    return 0;
}