poj 1279 Art Gallery

#include <cstdio>#include <algorithm>#include <iostream>using namespace std;#define vector pointusing std::swap;struct point{    double x,y;    point(double xx = 0,double yy = 0)    {        x = xx;        y = yy;    }    point operator - (const point& s)    {        return point(x - s.x, y - s.y);    }    point operator + (const point& s)    {        return point(x + s.x,y + s.y);    }};struct polygon{    point p[2000];    int size;};struct line{    point first,second;    line(point p1 = point(),point p2 = point())    {        first = p1;        second = p2;    }};double cross_product(vector v1,vector v2){    return v1.x * v2.y - v1.y * v2.x;}//求两直线交点point line_intersection(line ln1,line ln2){    double a1,b1,c1,a2,b2,c2;    a1 = ln1.first.y - ln1.second.y;    b1 = ln1.second.x - ln1.first.x;    c1 = ln1.first.x * ln1.second.y - ln1.second.x * ln1.first.y;    a2 = ln2.first.y - ln2.second.y;    b2 = ln2.second.x - ln2.first.x;    c2 = ln2.first.x * ln2.second.y - ln2.second.x * ln2.first.y;    double d = a1 * b2 - a2 * b1;    return point((b1 * c2 - b2 * c1) / d,(c1 * a2 - c2 * a1) / d);}//一个多边形与一个半平面的交集polygon hpi(polygon& poly,line& ln){    int m = 0;    polygon hull;    point p1 = ln.first,p2 = ln.second;    //穷举多边形的所有点，判断是否在半平面上    //如果凸多边形hull与直线ln有交点就求交点    for(int i = 0;i < poly.size;i++)    {        double c = cross_product(p2 - p1,poly.p[i] - p1);        double d = cross_product(p2 - p1,poly.p[(i + 1) % poly.size] - p1);        //点poly.p[i]在半平面上        if(c >= 0) hull.p[m++] = poly.p[i];        //有交点        if(c * d < 0) hull.p[m++] = line_intersection(line(poly.p[(i + 1) % poly.size],poly.p[i]),ln);    }    hull.size = m;    return hull;}//poly的顶点顺时针bool polygon_kernel(polygon& poly,polygon& knl){    //初始化核为无限大    knl.p[0] = point(-1e5,-1e5);    knl.p[1] = point(1e5,-1e5);    knl.p[2] = point(1e5,1e5);    knl.p[3] = point(-1e5,1e5);    knl.size = 4;    line ln;    point pre = poly.p[0];    for(int i = 1;i <= poly.size;i++)    {        ln.first = poly.p[i % poly.size];        ln.second = poly.p[i - 1];        knl = hpi(knl,ln);        if(knl.size == 0) return false;    }    return true;}double polygon_area(polygon& poly){    double s = 0;    for(int i = 0;i < poly.size - 1;i++)        s += cross_product(poly.p[i],poly.p[i + 1]);    s += cross_product(poly.p[poly.size - 1],poly.p[0]);    return s / 2;}int main(){    int t,n;    polygon gallery,kernel;    scanf("%d",&t);    while(t--)    {        scanf("%d",&n);        for(int i = 0;i < n;i++) scanf("%lf%lf",&gallery.p[i].x,&gallery.p[i].y);        gallery.size = n;        if(!polygon_kernel(gallery,kernel))        { int left = 0,right = n - 1; while(left < right)     swap(gallery.p[left++],gallery.p[right--]); polygon_kernel(gallery,kernel);        }        double s;        s = polygon_area(kernel);        printf("%.2lfn",s);    }    return 0;}