freqs.clj

(defn freqs [^bytes blocks]
  (loop [idx 0
         ret (cycle (repeatedly 128 #(transient {})))]
    (if (< idx (alength blocks))
      (do
        (update! (first ret) (aget blocks idx) (fnil inc 0))
        (recur (inc idx) (next ret)))
      (map persistent! (take 128 ret)))))

(defn freqs [^bytes blocks]
  (map persistent! (persistent!
       (areduce blocks idx ret (transient (vec (repeatedly 128 #(transient {}))))
           (let [layer (rem idx 128)]
             (update-in! ret [layer (aget blocks idx)] (fnil inc 0)))))))

(defn freqs [blocks]
  (->> blocks
    (partition 128)
    (reduce (fn [counts col]
              (doall (map #(assoc! %1 %2 (inc (get %1 %2 0))) counts col)))
            (repeatedly 128 #(transient {})))
       (map persistent!)))

(defn freqs [blocks]
  (->> blocks
    (partition 128)
    (apply map vector)
    (pmap frequencies)))