判断两线段相交，并求交点

原文
http://blog.youkuaiyun.com/rickliuxiao/article/details/6259322

#include "math.h"
#include "stdio.h"

double E = 0.0001;

struct Point  
{   
    double x, y;  
};   

double min(double x1, double x2)
{
    return x1 < x2 ? x1 : x2;
}

double max(double x1, double x2)
{
    return x1 > x2 ? x1 : x2;
}

bool between(double X, double X1, double X2)  
{  
    return X >= min(X1, X2) && X <= max(X1, X2);
}  


// 判断两条直线段是否有交点，有则计算交点的坐标  
// p1,p2是直线一的端点坐标  
// p3,p4是直线二的端点坐标  
// line_x, line_y; //交点
bool detectIntersect(Point p1, Point p2, Point p3, Point p4, double &line_x, double &line_y)  
{  
    if ( (fabs(p1.x-p2.x)<E) && (fabs(p3.x-p4.x)<E) )  
    {  
        return false;  
    }  
    else if ( (fabs(p1.x-p2.x)<E) ) //如果直线段p1p2垂直与y轴  
    {  
        if (between(p1.x, p3.x, p4.x))  
        {  
            double k = (p4.y-p3.y)/(p4.x-p3.x);  
            line_x = p1.x;  
            line_y = k*(line_x-p3.x) + p3.y;  

            if (between(line_y,p1.y,p2.y))  
            {  
                return true;  
            }  
            else  
            {  
                return false;  
            }  
        }  
        else   
        {  
            return false;  
        }  
    }  
    else if ( (fabs(p3.x-p4.x)<E) ) //如果直线段p3p4垂直于y轴  
    {  
        if (between(p3.x,p1.x,p2.x))  
        {  
            double k = (p2.y-p1.y)/(p2.x-p1.x);  
            line_x = p3.x;  
            line_y = k*(line_x-p2.x) + p2.y;  

            if (between(line_y, p3.y, p4.y))  
            {  
                return true;  
            }  
            else  
            {  
                return false;  
            }  
        }  
        else   
        {  
            return true;  
        }  
    }  
    else  
    {  
        double k1 = (p2.y-p1.y)/(p2.x-p1.x);   
        double k2 = (p4.y-p3.y)/(p4.x-p3.x);  

        if (fabs(k1-k2) < E)  
        {  
            return false;  
        }  
        else   
        {  
            line_x = ((p3.y - p1.y) - (k2*p3.x - k1*p1.x)) / (k1 - k2);  
            line_y = k1*(line_x-p1.x) + p1.y;  
        }  

        if (between(line_x, p1.x, p2.x) && between(line_x, p3.x, p4.x))  
        {  
            return true;  
        }  
        else   
        {  
            return false;  
        }  
    }  
}  

int main()
{
    Point p1, p2, p3, p4;  
    p1.x = 5.0;  
    p1.y = 0.0;  
    p2.x = 0.0;  
    p2.y = 5.0;  
    p3.x = 0.0;  
    p3.y = 0.0;  
    p4.x = 5.0;  
    p4.y = 5.0;  

    double x, y;
    int ok = detectIntersect(p1, p2, p3, p4, x, y); 
    if(ok)
    {
        printf("相交，交点为：(%.3f, %.3f)\n", x, y);
    }
    else
    {
        printf("不相交\n");
    }
    return 0;
}