rand-int

user=> (rand-int 30)
10

user=> (rand-int 30)
7

; is equivalent to
user=> (int (rand 30))

(require '[clojure.set :as set])

; random generation of unique series of random numbers from 0 to n-1 

(defn unique-random-numbers [n]
  (let [a-set (set (take n (repeatedly #(rand-int n))))]
    (concat a-set (set/difference (set (take n (range)))
                                  a-set))))

user=> (unique-random-numbers 20)
(0 1 3 6 7 8 9 12 14 16 17 19 2 4 5 10 11 13 15 18)

;; It'll allow you to supply an arg > Integer/MAX_VALUE, but the incidence of
;; integer overflow is proportional to how far you go over…
(dotimes [n 10]
  (try (println (rand-int (* Integer/MAX_VALUE 2.0)))
       (catch ArithmeticException _ (println "A"))))
A
A
1854263459
83487894
629497223
A
1311192557
A
A
2112963130

;; The docstring uses the word "integer" to mean a Java integer, rather than a
;; math integer, confusingly unlike `integer?`

;; If you want to extend into the full Long range, repimplement like so…
(defn rand-long [n] (long (rand n)))

(dotimes [n 10]
  (try (println (rand-long (* Integer/MAX_VALUE 2.0)))
       (catch ArithmeticException _ (println "A"))))
2843734446
3705237344
2260635666
1353888982
2353595107
1511365943
2605201990
2533357711
2501246267
1330427847