Fast random string generator in Clojure

Question

I was wondering if there is a way to generate a fixed length random string in Clojure.

A quick search resulted in:

https://gist.github.com/rboyd/5053955

(defn rand-str [len]
  (apply str (take len (repeatedly #(char (+ (rand 26) 65))))))

After I had a look at the VisualVM CPU profile data I realized that is consumes a non-trivial amount of CPU:

I decided to rewrite it be similar to the Java function I have used to use:

(defn rand-str2-slow
  ^String [^Long len]
  (let [leftLimit 97
        rightLimit 122
        random (Random.)
        stringBuilder (StringBuilder. len)
        diff (- rightLimit leftLimit)]
    (dotimes [_ len]
      (let [ch (char (.intValue (+ leftLimit (* (.nextFloat random) (+ diff 1)))))]
        (.append stringBuilder ch)))
        (.toString stringBuilder)))

This resulted in even slower code, however the stack trace has much less depth:

I noticed it does a lot of Reflector.getMethods(). Is there a way to type hint the function to avoid that?

UPDATE1:

Relevant microbenchmarks:

rand-str

(with-progress-reporting (quick-bench (rand-str 5000) :verbose))

      Execution time sample mean : 1.483232 ms
             Execution time mean : 1.483547 ms
Execution time sample std-deviation : 31.161960 µs
    Execution time std-deviation : 31.651732 µs
   Execution time lower quantile : 1.441678 ms ( 2.5%)
   Execution time upper quantile : 1.531289 ms (97.5%)
                   Overhead used : 14.598226 ns

rand-str2-slow

(with-progress-reporting (quick-bench (rand-str2-slow 5000) :verbose))

      Execution time sample mean : 17.637256 ms
             Execution time mean : 17.647974 ms
Execution time sample std-deviation : 523.527242 µs
    Execution time std-deviation : 528.559280 µs
   Execution time lower quantile : 17.322583 ms ( 2.5%)
   Execution time upper quantile : 18.522246 ms (97.5%)
                   Overhead used : 14.598226 ns

rand-str2 (fast)

(with-progress-reporting (quick-bench (rand-str2 5000) :verbose))

      Execution time sample mean : 84.362974 µs
             Execution time mean : 84.355379 µs
Execution time sample std-deviation : 3.496944 µs
    Execution time std-deviation : 3.674542 µs
   Execution time lower quantile : 80.911920 µs ( 2.5%)
   Execution time upper quantile : 89.264431 µs (97.5%)
                   Overhead used : 14.598226 ns

Answer 1

Let me answer my own question:

(defn rand-str2
  ^String [^Long len]
  (let [leftLimit 97
        rightLimit 122
        random (Random.)
        stringBuilder (StringBuilder. len)
        diff (- rightLimit leftLimit)]
    (dotimes [_ len]
      (let [ch (char (.intValue ^Double (+ leftLimit (* (.nextFloat ^Random random) (+ diff 1)))))]
        (.append ^StringBuilder stringBuilder ch)))
        (.toString ^StringBuilder stringBuilder)))