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99 LISP problems: Problem #19

Solution to the 99 LISP Problems #19

Uses the two previous solutions.

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(defun slice (alist from to)
  (let ((i 1))
    (mapcan
      (lambda (x)
        (let ((j i))
          (setf i (1+ i))
          (if (and (>= j from) (<= j to))
            (list x) nil)))
      alist)))

(defun split (alist n)
  (list
    (slice alist 1 n)
    (slice alist (1+ n) (length alist))))

(defun rotate (alist n)
  (let* ((m (if (>= n 0) n (+ (length alist) n)))
         (pieces (split alist m)))
    (append (cadr pieces) (car pieces))))

Lisp dialect: Steel Bank Common Lisp