double testBallCollision(Ball b, double t) {
        if (t == 0.0) {
            return -1;
        }
        
        // Calculate the movement vector a for t
        Vector movea = current.velocity * t;
        
        // Calculate the movement vector b for t
        Vector moveb = b.current.velocity * t;		
        
        double deltaX = b.current.position.x - current.position.x;
        double deltaXSquared = deltaX * deltaX;
        double deltaY = b.current.position.y - current.position.y;
        double deltaYSquared = deltaY * deltaY;
        int sumRadii = radius + b.radius;
        int sumRadiiSquared = sumRadii * sumRadii;
        double distSquared = deltaXSquared + deltaYSquared;
        
        // Reduce dynamic-dynamic collision to dynamic-static
        Vector movec = movea - moveb;
        
        // If the length of the movement vector is less
        // then the distance between the objects,
        // then there is no way they can hit
        if (movec.length() < sqrt(distSquared) - sumRadii) {
            return -1; // no collision
        }
        
        Vector N = movec.normalize();        
        
        // Find C, the vector from the center of the moving
        // ball A to the center of ball B
        Vector C = b.current.position - current.position;
        
        // D is the point along movec that is closest to ball b
        // D = N . C = ||C|| * cos(angle between N and C)
        double D = N.dot(C);
        
        // Find the lenght of the vector C
        double lengthC = C.length();
        
        // sqrt(F) is the distance between the center of ball b
        // ad the closest point on movec
        double F = (lengthC * lengthC) - (D * D);
        
        // If the closest that A will get to B
        // is more than the sum of their radii, there's no
        // way they are going collide
        if (F >= sumRadiiSquared) {
            return -1; // no collision
        }
        
        // We now have F and sumRadii, two sides of a right triangle.
        // Use these to find the third side, sqrt(T)
        double T = sumRadiiSquared - F;
        
        // If there is no such right triangle with sides length of
        // sumRadii and sqrt(f), T will probably be less than 0.
        // Better to check now than perform a square root of a negative number
        if (T < 0) {
            return -1; // no collision;
        }
        
        // Therefore the distance the circle has to travel along the
        // movement vector is D - sqrt(T)
        double distance = D - sqrt(T);
        
        // Get the magnitude of the movement vector
        double mag = movec.length();
        
        // Finally, make sure that the distance A has to move
        // to touch B is not greater than the magnitude of the
        // movement vector.
        if (mag < distance) {
            return -1; // no collision
        }
        
        // Return the time it takes ball A to travel this distance
        double collision_t = (distance / mag) * t;
        
        if (collision_t < 0.0 || collision_t > t) {
            return -1; // no collision
        }

        return collision_t;
    }