Alternative Hyperloglog implementation for Cascalog

hyperloglog.clj

;; Possibly more performant version due the absence of multimethods of http://screen6.github.io/blog/2013/11/13/hyperloglog-with-cascalog.html
;; Needs to be tested
(defprotocol IHyperLogLogMerge 
  (hyperloglog-val [this])
  (merge [this other])
  (merge-with-hyperloglog [this other-hll]))

(extend-protocol IHyperLogLogMerge
  nil
  (hyperloglog-val [this] nil)
  (merge-with-hyperloglog [this other-hll] other-hll)
  (merge [this other] other)

  Object
  (hyperloglog-val [this] (doto (new-hyperloglog) (.offer this)))
  (merge-with-hyperloglog [this other-hll] (.offer other-hll this))
  (merge [this other]
    (merge (hyperloglog-val other) this))

  HyperLogLog
  (hyperloglog-val [this] this)
  (merge-with-hyperloglog [this other-hll] (.addAll this other-hll))
  (merge [this other]
    (merge-with-hyperloglog other this)))

;; As in the original blog post
(defn merge-n
   ([h1] h1)
   ([h1 h2] (merge h1 h2))
   ([h1 h2 & more]
    (reduce merge (merge h1 h2) more)))

(c/defparallelagg parallel-sum
                   :init-var #'identity
                   :combine-var #'merge-n)

This actually can't be done as is using deparallelagg, can it? Did you run this and have it work? Since the :init-var is identity you'll get a string returned instead of an HLL. When I try to do this I get an exception that there is no matching field "cardinality" for java.lang.String.

I'm not 100% sure, but I think that the :init-var function of a defparallelagg has to return the same type as the :combine-var function in order to work correctly. At least that's what I understood.