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gen-symmetrical (with rnd-unique)

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;;; alternative function for GEN-SYMMETRICAL: in combination 
;;; with FIND-UNIQUE => symmetries with unique items (except 
;;; what is generated by symmetry)


(defun gen-symmetrical* (n list)
  (let ((list (rnd-order list))
        (newlist (rest list))
        (center (car list)))   

    (if (> n (* 2 (length list)))
      'list-has-too-few-items
      (if (evenp n)
        (progn
          (setf list (rnd-unique (/ n 2) newlist))
          (append list (reverse list)))
        (progn 
          (setf list (rnd-unique (/ (1- n) 2) (rest newlist)))
          (append list (list center) (reverse list)))))))
  

(gen-symmetrical* 5 '(1 2 3 4 5 6 7 8))
(gen-symmetrical* 9 '(1 2 3 4 5 6 7 8))

(gen-symmetrical* 30 '(1 2 3 4 5 6 7 8))
=> list-has-too-few-items ; error-message

 

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