lru_cache.rb

class Array
  def move_to_last(val)
    self.delete(val)
    self.push(val)
  end
end

class LRUCache

  attr_reader :cache_size
  attr_accessor :store_seconds

  def initialize(options)
    @cache_size    = options[:cache_size]    || 2
    @store_seconds = options[:store_seconds] || 1
    @queue = []
  end

  def cache_size=(size)
    if size < @cache_size
      @queue = @queue[(@cache_size - size) .. -1]
    end
    @cache_size = size
  end

  #[[key3, atime, value], [key1, atime, value3], [key4, atime, value], [key1, atime, value]]
  def put(key, value)
    new_item = [key, Time.now, value]

    if (existing_item = get_item(key))
      @queue.delete(existing_item)
    end
    @queue << new_item
    delete_oldest_cache
    new_item
  end
  # alias :[]= :put

  def get(key)
    delete_old_cache
    item = get_item(key)
    item ? item.last : nil
  end
  # alias :[] :get

private
  def get_item(key)
    value = @queue.assoc(key)
    return nil unless value
    @queue.move_to_last(value)
    value
  end

  def get_atime(key)
    value = @queue.assoc(key)
    return nil unless value
    value[1]
  end

  def delete_oldest_cache
    if @queue.size > @cache_size
      @queue.shift
    end
  end

  def delete_old_cache
    now = Time.now
    @queue.delete_if do |item|
      (now - item[1]).to_i > @store_seconds
    end
  end
end