cod.lisp

;;; Idp
;;; Identity predicate. Test if all members of the vector are equals.
;;;
;;; CL-USER> (idp (list 2 3 4 5))
;;; NIL
;;; CL-USER> (idp (list 2 2 2 2))
;;; T
(defun idp (v)
  (if (null v)
    t
    (let ((r (reduce #'(lambda (x y) (if (eql x y) x nil)) v)))
      (if (not (null r)) t))))

;;; Average
;;; Compute the average value of a N^n vector v.
(defun average (v)
  (if (null v)
    (error "The average value of an empty list isn't defined.")
    (/ (reduce #'+ v)
        (length v))))

;;; Shrink
;;; Reduce a N^n vector v to the average av.
;;;
(defun shrink (v av)
  (if (null v)
    nil
    (cons (let ((a (car v)))
              (if (= (- a av) 0)
                a
                (- a (/ (- a av) (abs (- a av))))))
          (shrink (cdr v) av))))

;;; Print-to-file
;;; Print the list of vectors to a file.
(defun print-to-file (l)
  (with-open-file (strm "./output.txt" :direction :output :if-exists :supersede)
    (loop for vect in l do
         (loop for nbr in vect do
              (format strm (write-to-string nbr)))
         (format strm "~%")) ))

;;; COD
;;; Loop to reduce the vector.
(defun cod (v)
  (let ((av (round (average v))) (acc (cons v nil)))
    (loop
      (print (car acc))
      (when (idp (car acc)) (return acc))
      (setf acc (cons (shrink (car acc) av) acc)))
    (print-to-file (reverse acc))))



(cod (list 0 4 2 3 54 12 21 44 32 50))