periodically_cleared_hash.rb

# A Hash derivative that is automatically cleared every so often
class PeriodicallyClearedHash
  
  def initialize(period, &default_value_calculator)
    @period = period.to_i
    @default_value_calculator = default_value_calculator || lambda { nil }
  end
  
  attr_reader :period
  
  private 
  
  def default_value_for(key)
    @default_value_calculator.call(key)
  end
  
  def current_hash
    reset_if_time_slice_changed
    @current_hash ||= Hash.new do |h, key|
      h[key] = default_value_for(key)
    end
  end

  def reset_if_time_slice_changed
    time_slice = Time.now.to_i / period
    unless @current_time_slice == time_slice
      @current_time_slice = time_slice
      @current_hash = nil
    end
  end
  
  def method_missing(method, *args)
    current_hash.__send__(method, *args)
  end
  
end