Fastest way to generate random strings

Question

I'm new to Clojure and functional programming. I'd like to create a list of 100,000 keys in the format: XXXXX-XXXXX-XXXXX-XXXXX-XXXXX

I do something like this:

(defn get-key [chunk-size, key-length] 
 (apply str 
  (flatten
   (interpose "-" 
    (partition chunk-size 
     (take key-length 
      (repeatedly #(rand-nth "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ"))))))))

(defn dump-keys [n, chunk-size, key-length] 
 (with-open [wrt (io/writer "keys.txt")]
  (doseq [i (range n)]
   (.write wrt (str (get-key chunk-size key-length) "\n")))))

Which produces

KYFL0-7YO6J-30XMV-ZIGE7-MK009
MNQZH-K7L8I-35C0K-7DS7Q-OTZWI
MVB9D-GHME9-IMGCL-YPAKX-4YZVD
... etc

However, it takes around 5 seconds, which is comparatively long compared to a similar imperative-style algorithm.

What's considered an idiomatic (and quick) way to do what I'm trying to do?

Answer 1

To get maximum speed for this, I would suggest the following techniques:

• Construct the keys using a pre-allocated (char-array 29)
• Use aset to set the character at each position in the array
• Use randomz to get very fast random numbers (about 150% faster than java.util.Random)
• Look up the character with .charAt, e.g. something like (.charAt "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ" (int (Rand/r 36)))
• Use dotimes for your loop - this is generally faster than mapping / anything with sequences

If you do all the above, you should get very efficient code, probably as fast as you could write it in pure Java.