(ns clj-etl-utils.sequences)


(def random-sample-seq
     (let [rnd (java.util.Random.)]
       (fn self [[item & population :as population-seq] population-size remaining-samples-needed]
         (if (or (zero? remaining-samples-needed) (empty? population-seq))
           nil
           (if (< (.nextInt rnd population-size) remaining-samples-needed)
             (lazy-cat
              [item]
              (self population
                    (dec population-size)
                    (dec remaining-samples-needed)))
             (self population
                   (dec population-size)
                   remaining-samples-needed))))))

(comment

  (with-open [outp (ds/writer "/tmp/20k-sample.tab")]
    (doseq [line (clj-etl-utils.sequences/random-sample-seq
                  (ds/read-lines "data-to-be-sampled.tab")
                  390000000
                  20000
                  (clj-etl-utils.lang/make-periodic-invoker
                   10
                   #(printf "at %s out of %s: %3.2f\n" %1 100 (* 100 (/ %1 100.0)))))]
      (.println outp line)))

  )