require 'drb/drb'
require 'monitor'

module DRb
  class TimerIdConv < DRbIdConv
    class TimerHolder2
      include MonitorMixin

      class InvalidIndexError < RuntimeError; end

      def initialize(timeout=600)
	super()
	@sentinel = Object.new
	@gc = {}
	@curr = {}
	@renew = {}
	@timeout = timeout
	@keeper = keeper
      end

      def add(obj)
	synchronize do
	  key = obj.__id__
	  @curr[key] = obj
	  return key
	end
      end

      def fetch(key, dv=@sentinel)
	synchronize do
	  obj = peek(key)
	  if obj == @sentinel
	    return dv unless dv == @sentinel
	    raise InvalidIndexError
	  end
	  @renew[key] = obj # KeepIt
	  return obj
	end
      end

      def include?(key)
	synchronize do
	  obj = peek(key)
	  return false if obj == @sentinel
	  true
	end
      end

      def peek(key)
	synchronize do
	  return @curr.fetch(key, @renew.fetch(key, @gc.fetch(key, @sentinel)))
	end
      end

      private
      def alternate
	synchronize do
	  @gc = @curr       # GCed
	  @curr = @renew
	  @renew = {}
	end
      end

      def keeper
	Thread.new do
	  loop do
	    alternate
	    sleep(@timeout)
	  end
	end
      end
    end

    def initialize(timeout=600)
      @holder = TimerHolder2.new(timeout)
    end

    def to_obj(ref)
      return super if ref.nil?
      @holder.fetch(ref)
    rescue TimerHolder2::InvalidIndexError
      raise "invalid reference"
    end

    def to_id(obj)
      return @holder.add(obj)
    end
  end
end

# DRb.install_id_conv(TimerIdConv.new)