circle to circle collision detection

#!/usr/bin/ruby
#
# check for collisions between two circles

# we can compare the squared radius/distance instead of comparing squareroots because of math.

def circles_collide (x1, y1, radius1, x2, y2, radius2)
  # compare the distance to combined radii
  distance_x = x2 - x1
  distance_y = y2 - y1
  radii = radius1 + radius2
  # if the distance between circles is smaller than the sum of their radii, they collide
  if ( dx * dx ) + ( dy * dy ) < (radii * radii)
    return true
  else
    return false


# via http://cgp.wikidot.com/circle-to-circle-collision-detection