; needed:
;  get input til negative, print max
;  is perfect, and perfect looper
;  3n+1 problem

(defun stupid-max (&optional (max 0))
    (progn
      (format t "Enter a number, stupid: ")
      (let ((n (read)))
        (if (< n 0)
            max
          (stupid-max (if (> n max) n max))))))
(defun idiot-max (&optional (n 1) (lst ()))
  (if (< n 0)
      (reduce #'max lst) ;(car (sort lst #'>))
    (progn
     (format t "Enter a number, fool: ")
     (let ((x (read)))
       (idiot-max x (append lst (list x)))))))


; usable: (LEN RND); (length return) = LEN, (reduce #'max return) < RND
(defun random-list (len &optional (rnd 100) (cur 0) (lst ()))
  (if (= len cur) lst
    (random-list len rnd (1+ cur) (append lst (list (random rnd))))))


(defun perfect (n)
  (if (= n (reduce #'+ (ffactor n))) t nil))

(defun ffactor (n &optional (i 1) (fac ())) ;then this came 2nd
  (if (< n 2) (list 1) (if (= n i) fac
    (ffactor n (1+ i) (if (zerop (mod n i)) (append fac (list i)) fac)))))

(defun factors (n) ;this came first
  (mapcar #'(lambda (x) (if (zerop (mod n x)) x)) (range 1 n))) 


(defun rrange (max &optional (min 0) (lst '(0)))
  (if (null lst)
      (rrange max '(0))
    (if (= (1+ (car (last lst))) max)
        lst
      (rrange max (append lst (list (1+ (car (last lst)))))))))
(defun arange (max &optional (min 0) (lst '(0)))
  (cond ((= min max) (list min))
        ((> min max)
         (arange (1+ min) max (list max)))
        (t
         (if (= (1+ (car (last lst))) max)
             lst
           (arange max min (append lst (list (1+ (car (last lst))))))))))

;pythonesque range() with benefits; ends with provided end, goes both ways,...
(defun range (start &optional end lst)
  (if (null end) (range 0 start '(0))
    (if (null lst) (range start end (list start))
      (if (= (car (last lst)) end) lst
        (range start end (append lst (list (+ (if (minusp (- start end)) 1 -1) (car (last lst))))))))))



(defun 3nloop (a &optional b)
  (cond ((equal a b) (format t "~A: ~S~%" b (3n b)))
        ((null b)
         (3nloop 2 a))
        (t
         (format t "~A: ~S~%" a (3n a))
         (3nloop (1+ a) b))))

; sort list of 2-tuple lists by the 2nd tuple
(defun 2sortl (lst)
  (sort lst #'(lambda (a b) (< (cadr a) (cadr b)))))
(defun 2sortg (lst)
  (sort lst #'(lambda (a b) (> (cadr a) (cadr b)))))

(defun 3nmax2 (a b)
  (car (2sortg (3nlen a b))))

(defun 3nmax (a b)
  (reduce #'max (mapcar #'cadr (3nlen a b))))

(defun 3nlen-print (lst)
  (if (null lst) t
    (progn
      (format t "N(~A) = ~A~%" (caar lst) (cadar lst))
      (3nlen-print (cdr lst)))))

(defun 3nlen (a b &optional ls)
  (if (= a (1+ b))
      ls
    (3nlen (1+ a) b (append ls (list (list a (length (3n a))))))))

  
(defun 3n (n &optional ns)
  (if (= n 1) (append ns (list 1))
    (3n (if (evenp n) (/ n 2) (+ 1 (* 3 n))) (append ns (list n)))))

(defun 5n (n &optional ns)
  (if (= n 1) (append ns (list 1))
    (3n (if (evenp n) (/ n 2) (+ 1 (* 5 n))) (append ns (list n)))))