Is there an elegant way to store a dual relationship (i.e. user 1 and user 2 are friends)

Question

I\'ve run into the same problem in two different pieces of work this month:

Version 1: User 1 & User 2 are friends
Version 2: Axis 1 & Axis 2 when gr         


        

Answer 1

For anyone that's interested, I played around with a few bitwise operations and found that the following seems to fulfill the criteria for f(x,y):

#Python, returns 3 tuple
def get_hash(x, y):
  return (x & y, x | y, x * y)

I can't prove it, though.