Random prime numbers

gistfile1.lisp

(defun random-number (length &key (current 0) odd)
    (cond
        ((and odd (<= length 1)) (if (= 1 (mod current 2)) current (+ 1 current)))
        ((<= length 1) current)
        (t (random-number (- length 1) :current (+ (ash current 1) (random 2)) :odd odd))))

(defun check-prime (n rounds)
    (let 
        ((basic-primes '(2 3 5 7 9 11 13 17 19 23 29 31 37 41)))
        (if (loop for i in basic-primes 
                do (when (= 0 (mod n i)) (return t))
                finally (return nil))
            nil
            (loop for i from 1 to rounds do
                (if (= 0 (mod n (random-number rounds)))
                    (return nil))
                finally (return t)))))

(defun random-prime (length &optional (current nil current-p))
    (cond
        ((and current-p (check-prime current (- length 1))) current)
        (current-p (random-prime length (+ 2 current)))
        (t (random-prime length (random-number length :odd t)))))