fast-random.lisp

(defvar *fast-prng-cipher* nil)
(defun fast-random (nbytes)
  (unless *fast-prng-cipher*
    (setf *fast-prng-cipher*
	  (ironclad:make-cipher 'ironclad:salsa20
				:key (ironclad:random-data 32) :mode :stream)))
  (let ((result (make-array nbytes :element-type '(unsigned-byte 8))))
    (ironclad:encrypt *fast-prng-cipher* (make-array nbytes :element-type '(unsigned-byte 8) :initial-element 0) result)
    result))
#|
For best multi-core performance, make *fast-prng-cipher* thread-local.
3.0GHz Ryzen 1700 generates 10M random 16-byte strings in 2 seconds on a single core:
CL-USER> (time (loop repeat 10000000 do (fast-random 16)))
Evaluation took:
  2.082 seconds of real time
  2.082169 seconds of total run time (1.975959 user, 0.106210 system)
  100.00% CPU
  7,065,394,718 processor cycles
  639,991,792 bytes consed
|#