【CSU1812】三角形和矩形 【半平面交】

检验半平面交的板子。

 #include <stdio.h>
 #include <bits/stdc++.h>
 using namespace std;
 #define gg puts("gg");
 #define ll long long
 const double pi = acos(-);
 const double eps = 1e-;
 int dcmp(double x){
     if(fabs(x) < eps) return ;
     return x > ?  : -;
 }
 struct Point{
     double x, y;
     Point(double x = , double y = ):x(x), y(y){}
     Point operator +(const Point& a){ return Point(x+a.x, y+a.y);}
     Point operator -(const Point& a){ return Point(x-a.x, y-a.y);}
     Point operator *(const Point& a){ return Point(x*a.x, y*a.y);}
     Point operator *(const double a){ return Point(x*a, y*a); }
 };
 struct Line{
     Point p, v; double a;
     Line(){}
     Line(Point p, Point v):p(p), v(v){ a = atan2(v.y, v.x);}
     bool operator <(const Line& b) const{
         return a < b.a;
     }
 };

 double Cross(const Point& a, const Point& b){
     return a.x*b.y-a.y*b.x;
 }
 bool Onleft(Line L, Point P){
     return Cross(L.v, P-L.p) > ;
 }
 Point GetIntersection(Line a, Line b){
     Point u = a.p-b.p;
     double t = Cross(b.v, u)/Cross(a.v, b.v);
     return a.p+a.v*t;
 }
 int Hpi(Line* L, int n, Point* poly, Point* p, Line* q){
     sort(L, L+n);
     int first, last;
     q[first = last = ] = L[];
     for(int i = ; i < n; i++){
         while(first < last&&!Onleft(L[i], p[last-])) last--;
         while(first < last&&!Onleft(L[i], p[first])) first++;
         q[++last] = L[i];
         if(fabs(Cross(q[last].v, q[last-].v)) < eps){
             last--;
             if(Onleft(q[last], L[i].p)) q[last] = L[i];
         }
         if(first < last) p[last-] = GetIntersection(q[last-], q[last]);
     }
     while(first < last&&!Onleft(q[first], p[last-])) last--;
     if(last-first <= ) return ;
     p[last] = GetIntersection(q[last], q[first]);

     int m = ;
     for(int i = first; i <= last; i++)
         poly[m++] = p[i];
     return m;
 }
 double PolygonArea(Point* p, int n){
     double ans = ;
     for(int i = ; i < n-; i++)
         ans += Cross(p[i]-p[], p[i+]-p[]);
     return ans/2.0;
 }

 Line L[], q[];
 Point p[], poly[];

 double x[], y[];
 int main(){
     while(~scanf("%lf%lf", x+, y+)){
         for(int i = ; i <= ; i++)
             scanf("%lf%lf", x+i, y+i);
         p[] = Point(x[], y[]);
         p[] = Point(x[], y[]);
         p[] = Point(x[], y[]);
         if(Onleft(Line(p[], p[]-p[]), p[]))
             L[] = Line(p[], p[]-p[]);
         else
             L[] = Line(p[], p[]-p[]);

         if(Onleft(Line(p[], p[]-p[]), p[]))
             L[] = Line(p[], p[]-p[]);
         else
             L[] = Line(p[], p[]-p[]);

         if(Onleft(Line(p[], p[]-p[]), p[]))
             L[] = Line(p[], p[]-p[]);
         else
             L[] = Line(p[], p[]-p[]);
         ///////////////////////////////////////
         L[] = Line(Point(x[], y[]), Point(, ));
         L[] = Line(Point(x[], y[]), Point(, ));
         L[] = Line(Point(x[], y[]), Point(-, ));
         L[] = Line(Point(x[], y[]), Point(, -));
         int tot = Hpi(L, , poly, p, q);
         if(tot == ) printf("%.8f\n", 0.0);
         else
             printf("%.8f\n", PolygonArea(poly, tot));
     }
     return ;
 }