LRU caching code kata

lru.rb

class Cache
  attr_accessor :max_size
  def initialize
    @max_size = 99
    @cache = {}
  end
  def store(key,object)
    @cache[key] = [0,object]
    purge_old
    key
  end

  def fetch(key)
    @cache[key][1] if @cache[key]
  end

  private
  def purge_old
    update_keys
    delete_oldest if @max_size < @cache.size
  end

  def update_keys
    @cache.each { |k,v| @cache[k][0] +=1 }
  end
  def delete_oldest
    oldest  = @cache.values.map { |v| v[0]}.max
    @cache.reject! { |k,v| v[0] ==oldest}
end
describe Cache do
  let(:cache) { Cache.new}
  let(:object) { "object" }
  let(:object2) { "object2"}
  let(:object3) { "object3"}
  let(:object4) { "object4"}
  it "stores an object item using a key" do
    cache.store(:key,object)
    cache.fetch(:key).should eql(object)
  end

  it "stores multiple objects by keys" do
    cache.store(:key,object)
    cache.store(:key2,object2)
    cache.fetch(:key2).should eql(object2)
  end

  it "removes the oldest cached object when the maximum cache is reached" do
    cache.max_size = 3
    cache.store(:key1,object)
    cache.store(:key2,object2)
    cache.store(:key3,object3)
    cache.fetch(:key1).should eql(object)
    cache.store(:key4,object4)
    cache.fetch(:key4).should eql(object4)
    cache.fetch(:key1).should eql(nil)
  end
end