gistfile1.rb

class WeakHash
  def initialize(cache = Hash.new)
    @cache = cache
    @key_map = {}
    @reclaim_key = lambda do |key_id|
      delete(@key_map[key_id]) unless @cache.empty?
    end
  end

   def [](key)
     value_id = @cache[key]
     value_id ? ObjectSpace._id2ref(value_id) : nil
     rescue RangeError
       nil
   end

   def []=(key, value)
     case key
     when Fixnum, Symbol, true, false
       key2 = key
     else
       key2 = key.dup
     end
     @cache[key2] = value.object_id
     @key_map[key.object_id] = key2

     ObjectSpace.define_finalizer(key, @reclaim_key)
   end

   def clear
     @key_map.clear
     @cache.clear
   end

   def delete(key)
     @cache.delete(key)
   end
end