凸多边形并 [ECNU 1624] 求交集多边形面积

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
#define EPS 1e-8
#define N 1010

int n;
int dq[N];
int top,bot,pn;

int dump(double x)
{
    if(fabs(x)<EPS) return 0;
    return x>0?1:-1;
}

struct Point
{
    double x,y;
    Point (){}
    Point (double x,double y):x(x),y(y){}
    Point operator - (Point p){
        return Point(x-p.x,y-p.y);
    }
    Point operator + (Point p){
        return Point(x+p.x,y+p.y);
    }
    Point operator * (double d){
        return Point(x*d,y*d);
    }
    double operator ^ (Point p){
        return x*p.y-y*p.x;
    }
};

struct Line
{
    Point s,e;
    double k;
    Line(){}
    Line(Point s,Point e):s(s),e(e){
        k=atan2(e.y-s.y,e.x-s.x);
    }
    Point operator & (Line l){       
        return s+(e-s)*(((l.e-l.s)^(l.s-s))/((l.e-l.s)^(e-s)));
    }
};

Line l[N];
Point p[N];

bool HPIcmp(Line a,Line b)
{
    int d=dump(a.k-b.k);
    if(!d) return dump((b.s-a.s)^(b.e-a.s))>0;
    return d<0;
}

bool Judge(Line a,Line b,Line c)  
{
    Point p=b&c;
    return dump((a.s-p)^(a.e-p))<0;   
}

void HPI(int n)
{
    int i,j;
    sort(l,l+n,HPIcmp);       
    for(i=0,j=0;i<n;i++)  
    {
        if(dump(l[i].k-l[j].k)>0) l[++j]=l[i];
    }
    n=j+1;
    dq[0]=0;  
    dq[1]=1;
    top=1;
    bot=0;
    for(i=2;i<n;i++)
    {
        while(top>bot && Judge(l[i],l[dq[top]],l[dq[top-1]])) top--;
        while(top>bot && Judge(l[i],l[dq[bot]],l[dq[bot+1]])) bot++;
        dq[++top]=i;
    }
    while(top>bot && Judge(l[dq[bot]],l[dq[top]],l[dq[top-1]])) top--;
    while(top>bot && Judge(l[dq[top]],l[dq[bot]],l[dq[bot+1]])) bot++;
    dq[++top]=dq[bot];
    for(pn=0,i=bot;i<top;i++,pn++)
    {
        p[pn]=l[dq[i+1]]&l[dq[i]];
    }
}

double GetArea()
{
    double marea=0;
    if(pn<3) return 0;
    for(int i=2;i<pn;i++)
    {
        marea+=(p[i]-p[0])^(p[i-1]-p[0]);
    }
    return fabs(marea)/2;
}

int main()
{
    int n,m,tot;
    while(scanf("%d%d",&n,&m)!=EOF)
    {
        tot=0;
        Point p1[N],p2[N];
        for(int i=0;i<n;i++) scanf("%lf%lf",&p1[i].x,&p1[i].y);
        for(int i=0;i<m;i++) scanf("%lf%lf",&p2[i].x,&p2[i].y);
        for(int i=0;i<n;i++) l[tot++]=Line(p1[i],p1[(i-1+n)%n]);
        for(int i=0;i<m;i++) l[tot++]=Line(p2[i],p2[(i-1+m)%m]);
        HPI(tot);
        printf("%.2f\n",GetArea());
    }
    return 0;
}