Just an idea about how to memoize some calculations

memoizing_toggles_with_a_weakmap.rb

class MemoizedToggles
  attr_reader :memoizations

  def initialize(&constructor)
    @constructor = constructor
    @memoizations = ObjectSpace::WeakMap.new
  end

  def enabled?(user, toggle)
    @memoizations[user] ||= Hash.new do |hash, key|
      hash[:toggle] = @constructor[user] unless hash.key? :toggle
      hash[key] = hash[:toggle].enabled? key
    end
    @memoizations[user][toggle]
  end
end


class Toggles
  def self.enabled?(user, toggle)
    memoized.enabled? user, toggle
  end

  def self.memoized
    @memoized ||= MemoizedToggles.new { |user| new user }
  end

  # just a stub definition, we will say it is enabled if the user, a string,
  # contains the toggle name as a substring
  def initialize(user)
    @user = user
  end
  def enabled?(toggle)
    toggle # => "j", "J", "garbage-collected", "j", "J"
    @user.include? toggle
  end
end

user = 'josh'
# second invocations here will be returned from the memo
Toggles.enabled? user, 'j' # => true
Toggles.enabled? user, 'j' # => true
Toggles.enabled? user, 'J' # => false
Toggles.enabled? user, 'J' # => false

GC.start
Toggles.enabled? user, 'garbage-collected'

# The memoization got GC'd b/c there is no reference to the hash storing this
# user's memoizations, so now we will it re-query "j" and "J"
Toggles.enabled? user, 'j' # => true
Toggles.enabled? user, 'j' # => true
Toggles.enabled? user, 'J' # => false
Toggles.enabled? user, 'J' # => false

# Could possiby be improved by storing the memoization hash as an ivar on the
# user because then it wouldn't invalidate the user's cache unless the user got GC'd.