lru_hash.rb

require 'forwardable'

class LRUHash
  extend Forwardable
  attr_reader :max_size

  rb_hash_has_key = %w[has_key? include? key? member?]
  def_delegators '@storage', :size, *rb_hash_has_key

  def initialize(max_size=16)
    @max_size = max_size
    @storage = {}
    @key_order = []
  end

  def []=(key, arg)
    @storage.delete(@key_order.shift) if size >= @max_size
    @key_order << key
    @storage[key] = arg
  end

  def [](key)
    @storage[key].tap{ |x| @key_order << @key_order.delete(key) if x }
  end
end