Quickly generate pretty referral codes through Base-N encoding

referral_codes.rb

# Want to give your users nice referral URLs, like http://www.service.com/?r=2ab3 ?
# 
# A fast and easy 1-1 method is to use base-n encoded numbers.
# 
# For instance, encode the user's id in base 36.  In Ruby, this is easy:
# > 272927.to_s(36)
# => "5ulb" 
#
# To get the base 10 id back again is just as easy:
# > "5ulb".to_i(36)
# => 272927 
#
# What if you have a new service and your user ids start with 1, 2, 3, and you don't want that?
# One solution is to add something to the id or multiply it by something, or both.
#
# Here's something you can dump in your user model:

###
## Sharing
#

# If you change these constants, existing values will not be accurate
SHARING_ID_ADDITION = 1693
SHARING_ID_MULTIPLIER = 7
SHARING_ID_BASE = 36

def sharing_id
  self.class.to_sharing_id(id)
end

class << self
  def to_sharing_id(decimal_id)
    ((decimal_id + SHARING_ID_ADDITION)*SHARING_ID_MULTIPLIER).to_s(SHARING_ID_BASE)
  end
  
  def from_sharing_id(encoded_base_id)
    encoded_base_id.to_i(SHARING_ID_BASE) / SHARING_ID_MULTIPLIER - SHARING_ID_ADDITION
  end
end

# > u = User.new(1)
# => #<User:0x100351f50 @id=1> 
# > u.sharing_id
# => "95e" 
# > User.from_sharing_id("95e")
# => 1