Fast algorithm to find the x closest points to a given point on a plane

Question

I would like to find a fast algorithm in order to find the x closest points to a given point on a plane.

We are actually dealing with not too many points (between 1

Answer 1

For a given point (not all of them) and as the number of points is not extreme, you could calculate the distance from each point:

var points = new List();
Point source = ...
....
var closestPoints = points.Where(point => point != source).
                           OrderBy(point => NotReallyDistanceButShouldDo(source, point)).
                           Take(20);

private double NotReallyDistanceButShouldDo(Point source, Point target)
{
   return Math.Pow(target.X - source.X, 2) + Math.Pow(target.Y - source.Y, 2);
}

(I've used x = 20)

The calculation are based on doubles so the fpu should be able to do a decent job here. Note that you might get better performance if Point is a class rather than a struct.