rollup.clj

(ns rollup.core
  (:require [clojure.core.async :refer [go chan <! put! timeout alt! alts!]]))


(def ch (chan))

(go
 (loop [stash []]
   (let [[v c] (alts! [ch (timeout 3000)])]
     (if (= c ch)
       (when v
         (when (< (count stash) 5)
           (println "A" v))
         (recur (conj stash v)))
       (do
         (when (not-empty stash)
           (println "B" stash))
         (recur []))))))

(dotimes [n 20] (put! ch n))

(clojure.core.async/close! ch)