【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(-1);
const double eps = 1e-9;
int dcmp(double x){
    if(fabs(x) < eps) return 0;
    return x > 0? 1 : -1;
}
struct Point{
    double x, y;
    Point(double x = 0, double y = 0):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) > 0;
}
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 = 0] = L[0];
    for(int i = 1; i < n; i++){
        while(first < last&&!Onleft(L[i], p[last-1])) last--;
        while(first < last&&!Onleft(L[i], p[first])) first++;
        q[++last] = L[i];
        if(fabs(Cross(q[last].v, q[last-1].v)) < eps){
            last--;
            if(Onleft(q[last], L[i].p)) q[last] = L[i];
        }
        if(first < last) p[last-1] = GetIntersection(q[last-1], q[last]);
    }
    while(first < last&&!Onleft(q[first], p[last-1])) last--;
    if(last-first <= 1) return 0;
    p[last] = GetIntersection(q[last], q[first]);

    int m = 0;
    for(int i = first; i <= last; i++)
        poly[m++] = p[i];
    return m;
}
double PolygonArea(Point* p, int n){
    double ans = 0;
    for(int i = 1; i < n-1; i++)
        ans += Cross(p[i]-p[0], p[i+1]-p[0]);
    return ans/2.0;
}

Line L[100], q[100];
Point p[100], poly[100];

double x[10], y[10];
int main(){
    while(~scanf("%lf%lf", x+1, y+1)){
        for(int i = 2; i <= 4; i++)
            scanf("%lf%lf", x+i, y+i);
        p[0] = Point(x[1], y[1]);
        p[1] = Point(x[1], y[2]);
        p[2] = Point(x[2], y[1]);
        if(Onleft(Line(p[0], p[1]-p[0]), p[2]))
            L[0] = Line(p[0], p[1]-p[0]);
        else
            L[0] = Line(p[1], p[0]-p[1]);

        if(Onleft(Line(p[0], p[2]-p[0]), p[1]))
            L[1] = Line(p[0], p[2]-p[0]);
        else
            L[1] = Line(p[2], p[0]-p[2]);

        if(Onleft(Line(p[1], p[2]-p[1]), p[0]))
            L[2] = Line(p[1], p[2]-p[1]);
        else
            L[2] = Line(p[2], p[1]-p[2]);
        ///////////////////////////////////////
        L[3] = Line(Point(x[3], y[3]), Point(1, 0));
        L[4] = Line(Point(x[4], y[3]), Point(0, 1));
        L[5] = Line(Point(x[4], y[4]), Point(-1, 0));
        L[6] = Line(Point(x[3], y[4]), Point(0, -1));
        int tot = Hpi(L, 7, poly, p, q);
        if(tot == 0) printf("%.8f
", 0.0);
        else
            printf("%.8f
", PolygonArea(poly, tot));
    }
    return 0;
}