How to dechunk(unchunk) sequence in clojure. Chunked and dechunked sequence realization count example.

seq_test.clj

(ns seq-test)

(ns seq-test
  (:require [clojure.test :as test]))

;; clarification 1
;; be carefull this functions guarantees one-by-one fetching for client of the dechunk function,
;; but NOT guarantees that source seq will not be evaluated forward by chunks
;; given tests demonstrates it

;; clarirication 2
;; given tests actually check how map function works (check source of map function before)

(defn dechunk [s]
  
  (lazy-seq
   (when-let [[x] (seq s)]
     (cons x (dechunk (rest s))))))

(defn get-realization-count [seq-length drop-count chunked?]
  (let [realization-agent (agent 0)]
    (first (take 1
                 (drop drop-count
                       (seq (map #(do (send-off realization-agent inc)
                                      %)
                                 (if chunked?
                                   (range seq-length)
                                   (dechunk (range seq-length)))))
                       )
                 ))
    (await realization-agent)
    @realization-agent))

(test/deftest chunked-seq-realized-count
  (test/testing "number of realized items for chunked seq"
    (test/are [seq-length drop-count expected-realized-count]
        (= expected-realized-count
           (get-realization-count seq-length drop-count true))
      ;; precaclulation size = 32 (up to 32 elements realized even if one requested)
      1 0 1
      2 0 2
      4 2 4
      8 3 8
      15 7 15
      32 5 32
      64 1 32 ; precalculate 32
      64 32 64
      64 33 64; precalculate 64
      )))

(test/deftest dechunked-seq-realized-count
  (test/testing "number of realized items for dechunked seq"
    (test/are [seq-length drop-count expected-realized-count]
        (= expected-realized-count
           (get-realization-count seq-length drop-count false))
      ;; only needed element realized
      1 0 1
      2 0 1
      4 2 3
      8 3 4
      15 7 8
      32 5 6
      64 1 2
      64 32 33
      64 33 34)))