gistfile1.txt

;; no consing one iteration
(defun rotatel (list)
  (let ((new (cdr list)))
    (setf (cdr list) nil)
    (setf new (nconc new list))))

;; no consing two iterations
(defun rotatel (list)
    (let ((first (pop list))
          (rlist (nreverse list)))
          (push first rlist)
          (nreverse rlist)))

;; one cons one iteration
(defmacro while (test &rest body)
  `(do () ((not ,test) (values)) ,@body))
     
(defun rotatel (list)
    (let ((first (pop list))
          (new list))
        (while (cdr list) (setf list (cdr list)))
        (setf (cdr list) (cons first nil))
        new))

;; no consing, no iteration, circular
(defun rotatel (list)
    (setf list (cdr list)))

(defun make-circle (list)
    (setf (cdr (last list)) list))

;; test
(setf *print-circle* t)
(setq mylist  '((a . 1) (b . 2) (c . 3)))
(make-circle mylist)

(format t "~a~%" mylist)
(setf mylist (rotatel mylist))
(setf mylist (rotatel mylist))
(setf mylist (rotatel mylist))
(setf mylist (rotatel mylist))