(defpackage :webutils.captcha (:use :cl)             (:export :expire-captchas :generate-captcha :check-captcha)) (in-package :webutils.captcha) (webutils::export-all :webutils.captcha) (defvar *captchas* nil) (defvar *captcha-counter* 0) (defstruct (captcha (:type list))   id answer generated-time) (defun expire-captchas (&optional (timeout (* 60 60)))   "Expire active captchas older than TIMEOUT seconds."   (let ((now (get-universal-time)))     (setf *captchas*           (remove-if #'(lambda (c)                          (< (+ (captcha-generated-time c) timeout)                             now)) *captchas*)))) (defparameter *male-names*   '("Bill" "Bob" "James" "Jim" "Eric")) (defparameter *female-names*   '("Mary" "Sally" "Lucy" "Lizzie" "Natalie")) (defun random-elt (list)   (elt list (random (length list)))) (defun random-name ()   (ecase (random 2)     (1 (list (random-elt *male-names*) "his" "he"))     (0 (list (random-elt *female-names*) "her" "she")))) (defun generate-captcha ()   "Generate a new captcha. Returns two values: the id of the captcha and the text to present to the user."   (let ((id (incf *captcha-counter*)))     (ecase (random 2)       (0        (let* ((width (1+ (* 2 (+ (random 10) 1))))               (ratio (+ (random 8) 2))               (height (* width ratio))               (area (* width height))               (perimeter (+ (* width 2) (* height 2)))               (name (random-name)))          (push (make-captcha :id id :answer ratio :generated-time (get-universal-time)) *captchas*)          (values id                  (format nil "Farmer ~A needs to surround ~A bunny pen with a fence. The fence needs to cover ~A square meters and must be made from ~A total meters of fencing. What is the ratio of the larger of the dimensions of the bunny pen to the smaller?" (first name) (second name) area perimeter))))       (1        (let* ((speed1 (* 10 (1+ (* 2 (+ (random 10) 1)))))               (speed2 (* 10 (1+ (* 2 (+ (random 10) 1)))))               (delay (+ (random 4) 1))               (time (+ delay (+ (random 8) 2)))               (distance (+ (* speed1 time) (* speed2 (- time delay))))               (name1 (random-name))               (name2 (random-name)))          (push (make-captcha :id id :answer time :generated-time (get-universal-time)) *captchas*)          (values id                  (format nil "~A starts driving due east towards ~A and travelling ~A kph. ~A starts driving due west at ~A kph ~A hours later. They start off ~A kilometers apart. How many hours after ~A started will they meet?" (first name1) (first name2) speed1 (first name2) speed2 delay distance (first name1)))))))) (defun check-captcha (id answer)   (let ((captcha (find id *captchas* :key #'captcha-id)))     (if captcha         (setf *captchas* (remove captcha *captchas*))         (return-from check-captcha nil))     (eql answer (captcha-answer captcha))))