This small operation calcuates the distance between two points. The routine can work in any number of dimensions, so you cold apply it to 2D or 3D.
Define your two points. Point 1 at (x1, y1) and Point 2 at (x2, y2).
xd = x2-x1 yd = y2-y1 Distance = SquareRoot(xd*xd + yd*yd)In 3D
xd = x2-x1 yd = y2-y1 zd = z2-z1 Distance = SquareRoot(xd*xd + yd*yd + zd*zd)As you can see, this requires that you perform a square root. Square roots should be avoided like the plague if you want to write fast code. Only perform a Square Root if you really need to.
Ways to avoid Square Roots:
if SquareRoot(xd*xd + yd*yd) < Diameterto:
if (xd*xd + yd*yd) < (Diameter*Diameter)