cache.rb

class Cache
  attr_reader :limit, :store, :queue

  def [](key)
    heat(key) if result = store[key]

    result
  end

  def []=(key, value)
    store[key] = value

    heat(key)

    pop if length > limit

    value
  end

  def heat(key)
    queue.delete(key)
    queue.unshift(key)
  end

  def pop
    store.delete(queue.pop)
  end

  def length
    store.length
  end

private

  def initialize(limit=100)
    @limit = limit
    @store = {}
    @queue = []
  end
end