Below is a Common Lisp implementation of the Minimum Standard Random Number Generator described by Park and Miller in 1988.

**Problem statement**

Read the problem statement here at Programming Praxis. Be warned that the blog post mixes up the values for *lo* and *hi*. It took me ages to figure out why my program wasn’t returning correct results. I finally got the correct formulas from the paper the post links to.

**Solution**

Programming Praxis talks about 3 different implementations but I just implemented the first two:

(defun get-msrn-naive (x n)
(let ((a 16807) (m 2147483647))
(mod (*
a
;Multiply either by x or recursive call
(if (eq n 1)
x
(get-msrn-naive x (- n 1))))
m)))
(defun get-next-msrn (x)
(let (
(a 16807)
(m 2147483647)
(q 127773)
(r 2836)
)
(let
(
(result
(-
(* a (mod x q))
(* r (floor (/ x q)))
)
)
)
(if
(<= result 0)
(+ result m)
result
)
)
)
)
(defun get-msrn-schrage (x n)
(if (eq n 1)
(get-next-msrn x)
(get-msrn-schrage (get-next-msrn x) (- n 1))))

It can also be viewed here on Github Gist.

**Sample use**

(get-msrn-schrage 1 10000)

1043618065

(get-msrn-schrage 1 5)

1144108930

(get-msrn-naive 1 5)

1144108930

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valbonneconsultingWARNING: programming in Lisp has been linked to accelerated growth of facial hair🙂 http://wp.me/p35rFe-38