#  Singleton module that ensures only one object to be allocated.
#
# Usage:
#   class SomeSingletonClass
#     include Singleton
#    #....
#   end
#   a = SomeSingletonClass.instance
#   b = SomeSingletonClass.instance	# a and b are same object
#   p [a,b]
#   a = SomeSingletonClass.new		# error (`new' is private)

module Singleton
  def Singleton.included(klass)
    klass.private_class_method(:new)
    klass.instance_eval %{
      @__instance__ = nil
      def instance
	if defined? @__allocating__
	  until @__instance__
	    sleep 0.5
	  end
	elsif ! @__instance__
	  Thread.critical = true
	  @__allocating__ = true
	  Thread.critical = false
	  begin
	    @__instance__ = new
	  ensure
	    remove_instance_variable(:@__allocating__)
	  end
	end
	return @__instance__
      end
    }
  end
end

if __FILE__ == $0
  class SomeSingletonClass
    include Singleton
    #....
  end

  a = SomeSingletonClass.instance
  b = SomeSingletonClass.instance	# a and b are same object
  p [a,b]
  a = SomeSingletonClass.new		# error (`new' is private)
end