//Get collision of ray (x1,y1)->(x2,y2)
//  with rectangle (x3,y3)->(x4,y4).
Collision getCollision(double x1, double y1, double x2, double y2, double x3, double y3, double x4, double y4) {
    //A struct to hold our collision information.
    Collision c;
    c.exists=0;

    //Remove collision if we're looking the wrong way.
    if((x1<x3 && x1<x4 && x2<x1) || (x1>x3 && x1>x4 && x2>x1) || (y1<y3 && y1<y4 && y2<y1) || (y1>y3 && y1>y4 && y2>y1)){
        return c;
    }

    //Calculate slope and y-intersept.
    double m = (y2 - y1) / (x2 - x1);
    double b = y1 - m * x1;

    //Calculate 4 intersection points.
    double xa = (y3 - b) / m;
    double ya = m * x3 + b;
    double xb = (y4 - b) / m;
    double yb = m * x4 + b;

    //Calculate distances.
    double dxa = pow(xa - x1, 2) + pow(y3 - y1, 2);
    double dya = pow(ya - y1, 2) + pow(x3 - x1, 2);
    double dxb = pow(xb - x1, 2) + pow(y4 - y1, 2);
    double dyb = pow(yb - y1, 2) + pow(x4 - x1, 2);

    //Get closest intersection point.
    double dist = INFINITY;

    if (xa >= x3 && xa <= x4 && dxa < dist) {
        dist = dxa;
        c.x = xa;
        c.y = y3;
        c.exists=1;
    }
    if (xb >= x3 && xb <= x4 && dya < dist) {
        dist = dya;
        c.y = ya;
        c.x = x3;
        c.exists=1;
    }
    if (ya >= y3 && ya <= y4 && dxb < dist) {
        dist = dxb;
        c.x = xb;
        c.y = y4;
        c.exists=1;
    }
    if (yb >= y3 && yb <= y4 && dyb < dist) {
        dist = dyb;
        c.y = yb;
        c.x = x4;
        c.exists=1;
    }

    //Return the collision information.
    return c;
}