circular-member-p.lisp

(defun circular-mapc (function list)
  (loop for fast = list then (cddr fast)
        for slow = list then (cdr slow)
        do
        (funcall function (car slow))
        (when (eq (cddr fast) (cdr slow))
          (return-from circular-mapc nil))))

(defun circular (items)
  (check-type items cons)
  (setf (cdr (last items)) items)
  items)

(defun circular-member-p (item list)
  (circular-mapc (lambda (e)
                   (when (eql item e)
                     (return-from circular-member t)))
                 list))