判断两线段是否相交 模板

struct point
{
    double x, y;
    point( double _x = 0, double _y = 0 )
    {
        x = _x;
        y = _y;
    }
    point operator-( point t )
    {
        return point( x - t.x, y - t.y );
    }
    double operator*( point t )
    {
        return x * t.y - y * t.x;
    }
};

//快速排斥试验模板
bool quickExclude( point a, point b, point c, point d )
{
    int x1 = a.x, x2 = b.x, x3 = c.x, x4 = d.x;
    int y1 = a.y, y2 = b.y, y3 = c.y, y4 = d.y;
    if (  min(x1,x2) <= max(x3,x4) && min(x3,x4) <= max(x1,x2) &&
           min(y1,y2) <= max(y3,y4) && min(y3,y4) <= max(y1,y2)      )
           return true;
    else    return false;
}

//跨立试验模板（两线段是否相交）
bool ifIntersect( point a, point b, point c, point d )
{
    if ( quickExclude( a, b, c, d ) )
    {
        if (  ( ( a - c ) * ( c - d ) ) * ( ( b - c ) * ( c - d ) ) <= 0 && ( ( c - a ) * ( a - b ) ) * ( ( d - a ) * ( a - b ) ) <= 0  )
        return true;
    }
    return false;
}

//若已判断两线段相交求交点或者求两相交直线交点模板（四个点为a, b, c, d），a1, b1, c1, a2, b2, c2为方程系数，交点为x0和y0
double a1 = a.y - b.y;
double b1 = b.x - a.x;
double c1 = a.x * b.y - b.x * a.y;
double a2 = c.y - d.y;
double b2 = d.x - c.x;
double c2 = c.x * d.y - d.x*c.y;
double x0 = ( b1 * c2 - b2 * c1 ) / ( a1 * b2 - a2 * b1 );
double y0 = ( a2 * c1 - a1 * c2 ) / ( a1 * b2 - a2 * b1 );