///<summary>Returns the first non-negative time, if any, where a moving circle will intersect a fixed extended line.</summary>
public static double? WhenMovingCircleWillIntersectExtendedLine(Point center, double radius, Vector velocity, Point pointOnLine, Vector displacementAlongLine)
{
var a = (center - pointOnLine).PerpOnto(displacementAlongLine);
if (a * a - radius * radius <= 0) return 0; // already touching at t=0?
var b = velocity.PerpOnto(displacementAlongLine);
var kissTime = QuadraticRoots(b * b, a * b * 2, a * a - radius * radius)
.Where(e => e >= 0)
.Cast<double?>()
.FirstOrDefault();
return kissTime;
}