equality_map.rb

class EqualityMap
  def initialize
    @keys = {}
    @map  = ObjectSpace::WeakMap.new
  end

  def fetch(*key)
    identity = @keys[key.hash]
    key      = identity == key ? identity : key
    @map[key] ||= track(key, yield)
  end

  def track(key, object)
    ObjectSpace.define_finalizer(object, finalizer(hash))
    @keys[key.hash] = key
    object
  end

  def finalizer(hash)
    proc { @keys.delete(hash) }
  end

  private :track, :finalizer
end