public static Vector2D Cross(Vector2D startA, Vector2D vectorA, Vector2D startB, Vector2D vectorB)
        {
            Vector2D outVector = new Vector2D();

            double a1 = startA.y;
            double a2 = startB.y;
            double b1 = vectorA.y;
            double b2 = vectorB.y;
            double c1 = startA.x;
            double c2 = startB.x;
            double d1 = vectorA.x;
            double d2 = vectorB.x;
            double k1 = b1 * d2;
            double k2 = b2 * d1;

            outVector.x = (d1 * d2 * (a2 - a1) + k1 * c1 - k2 * c2) / (k1 - k2);
            outVector.y = (b1 * (outVector.x - c1) / d1) + a1;

            return outVector;
        }

private static Vec intersectionVec = new Vec(0., 0.);

private static Vec lineIntersection(Vec s1, Vec e1, Vec s2, Vec e2){
        double denom = ((e1.x - s1.x) * (e2.y - s2.y)) - ((e1.y - s1.y) * (e2.x - s2.x));
        //  AB & CD are parallel 
        if (denom == 0)
        	return null;
        double numer = ((s1.y - s2.y) * (e2.x - s2.x)) - ((s1.x - s2.x) * (e2.y - s2.y));
        double r = numer / denom;
        double numer2 = ((s1.y - s2.y) * (e1.x - s1.x)) - ((s1.x - s2.x) * (e1.y - s1.y));
        double s = numer2 / denom;
        if ((r < 0 || r > 1) || (s < 0 || s > 1))
        	return null;
        // Find intersection point
        intersectionVec.x = s1.x + (r * (e1.x - s1.x));
        intersectionVec.y = s1.y + (r * (e1.y - s1.y));
        return intersectionVec;
}

bool Maths::RealEq(real a, real b) {
    return std::fabs(a-b) < std::numeric_limits<real>::epsilon();
}

// ...

if (RealEq(denom, 0))

#define NaV2 Vector2(NaN,NaN) // Not a Vector2: Not a Number, Not a Number