app_cache.rb

class AppCache
  def initialize
    @store = Store.new
  end

  def cache(*keys, &block)
    options = keys.extract_options!
    @store.cache(keys, options[:expires_in], &block)
  end

  class Store
    def initialize
      @cached_objects = CachedObjects.new
    end

    def cache(keys, expires_in, &block)
      cached_object = @cached_objects.find(keys)
      return nil if not cached_object and not block
      if not cached_object or cached_object.expired?
        block = block || cached_object.block
        cached_object = CachedObject.new(block, expires_in)
        @cached_objects.add keys, cached_object
      end
      cached_object.object
    end

    class CachedObjects
      def initialize(cached_objects = {})
        @cached_objects = cached_objects
      end

      def find(keys)
        keys.inject(@cached_objects) {|memo, key| Hash === memo ? memo[convert_key(key)] : return }
      end

      def add(keys, cached_object)
        @cached_objects.merge! keys.reverse.inject(cached_object) {|memo, key| {convert_key(key) => memo} }
      end

      private

      def convert_key(key)
        Symbol === key ? key.to_s : key
      end
    end

    class CachedObject
      attr_reader :object, :block

      def initialize(block, expires_in)
        @expires_in = expires_in.try(:to_f)
        @created_at = Time.now.to_f
        @object = block.call
        @block = block
      end

      def expired?
        @expires_in && @created_at + @expires_in <= Time.now.to_f
      end
    end
  end
end