算法-线段是否相交判断(二维)

声明

	#define MAX(a, b) (((a)>(b))?(a):(b))
	#define MIN(a, b) (((a)<(b))?(a):(b))
	
	typedef struct _Point{
	    short x;
	    short y;
	    short z; // z: 忽略
	}Point;
	int lint(Point p1, Point p2, Point p3, Point p4);

定义

	/**
	* @brief: 判断两条线段(line segment)是否存在相交(二维)
	*       : LineSeg1:(p1, p2),  LineSeg1:(p3, p4)
	* return: 相交--1 , 不相交--0
	*/
	int lint(Point p1, Point p2, Point p3, Point p4)
	{
	
	    double z1, z2, z3, z4;
	    int    s1, s2, s3, s4;
	
	    /*Perfrom the quick rejection test.*/
	    if( !(MAX(p1.x, p2.x) > MIN(p3.x, p4.x) &&
	          MAX(p3.x, p4.x) >= MIN(p1.x, p2.x) &&
	          MAX(p1.y, p2.y) >= MIN(p3.y, p4.y) &&
	          MAX(p3.y, p4.y) >= MIN(p1.y, p2.y)
	          ))
	    {
	        return 0;
	    }
	
	    /*Determine whether the line segments srraddle each other.*/
	    if((z1 = ((p3.x-p1.x) * (p2.y-p1.y)) - ((p3.y-p1.y) * (p2.x-p1.x))) < 0)
	    {
	        s1 = -1;
	    }
	    else if(z1 > 0)
	    {
	        s1 = 1;
	    }
	    else {
	        s1 = 0;
	    }
	
	    if( (z2 = ((p4.x-p1.x)*(p2.y-p1.y)) - ((p4.y-p1.y)*(p2.x-p1.x))) < 0 )
	    {
	        s2 = -1;
	    }
	    else if(z2 > 0)
	    {
	        s2 = 1;
	    }
	    else {
	        s2 = 0;
	    }
	
	    if( (z3 = ((p1.x-p3.x)*(p4.y-p3.y)) - ((p1.y-p3.y)*(p4.x-p3.x))) < 0 )
	    {
	        s3 = -1;
	    }
	    else if(z3 > 0)
	    {
	        s3 = 1;
	    }
	    else {
	        s3 = 0;
	    }
	
	    if( (z4 = ((p2.x-p3.x)*(p4.y-p3.y)) - ((p2.y-p3.y)*(p4.x-p3.x))) < 0 )
	    {
	        s4 = -1;
	    }
	    else if(z4 > 0)
	    {
	        s4 = 1;
	    }
	    else {
	        s4 = 0;
	    }
	
	    if((s1*s2 <= 0) && (s3*s4 <= 0))
	    {
	        return 1;
	    }
	
	    return 0;
	}