; hw2.lisp
; CS 381 homework 2
; Jacob Lundberg

; ADD2 returns 2 plus op1.
(defun ADD2 (op1)
  (cond ((eql nil op1) 2)
        (T (+ 2 op1))
  )
)

; THREE-LAST returns the last element of a list of three elements.
(defun THREE-LAST (list1)
  (cond ((< (list-length list1) 3) nil)
        (T (car (cddr list1)))
  )
)

; THREEA returns ((nil 400 . 400) 500).
(defun THREEA ()
  (cons (cons nil (cons 400 400)) (cons 500 nil))
)

; THREEB returns (30 (10 . #\b)).
(defun THREEB ()
  (cons 30 (cons (cons 10 #\b) nil))
)

; MY-NTHCDR returns the n'th cdr of a list.
(defun MY-NTHCDR (n1 list1)
  (cond ((null list1) nil)    ; no need to take cdr of nil
        ((< n1 0) nil)        ; the real nthcdr would give an error
        ((= n1 0) list1)      ; whup, we're done
        (T (MY-NTHCDR (- n1 1) (cdr list1)))
  )
)

; MY-MEMBER returns a sub-list starting at a member.
(defun MY-MEMBER (mem1 list1)
  (cond ((null list1) nil)
        ((eq mem1 (car list1)) list1)
        (T (MY-MEMBER mem1 (cdr list1)))
  )
)

; MY-UNION returns a set that is the union of the given sets, unordered.
; *NOTE* that if you don't give it true sets it may not behave like UNION.
(defun MY-UNION (set1 set2)
  (cond ((null set2) set1)
        ((MY-MEMBER (car set2) set1) (MY-UNION set1 (cdr set2)))
        (T (MY-UNION (cons (car set2) set1) (cdr set2)))
  )
)

; MY-SUBST returns a tree with one value replaced by another.
(defun MY-SUBST (new1 old1 tree1)
  (cond ((null tree1) nil)
        ((not (listp tree1)) (if (eq tree1 old1) new1 tree1))
        ((eq (car tree1) old1) (cons new1 (MY-SUBST new1 old1 (cdr tree1))))
        ((eq (cdr tree1) old1) (cons (MY-SUBST new1 old1 (car tree1)) new1))
        (T (cons (MY-SUBST new1 old1 (car tree1)) (MY-SUBST new1 old1 (cdr tree1))))
  )
)

; NUMBERS-FIRST returns a list or tree with all the numbers at the front.
(defun NUMBERS-FIRST (list1)
  (cond ((null list1) nil)
        ((numberp (car list1)) (cons (car list1) (NUMBERS-FIRST (cdr list1))))
        (T (append (NUMBERS-FIRST (cdr list1)) (cons (car list1) nil)))
  )
)

; FLAT-NUMBERS-FIRST returns a list with all the numbers at the front.
(defun FLAT-NUMBERS-FIRST (list1)
  (cond ((null list1) nil)
        ((listp (car list1)) (FLAT-NUMBERS-FIRST (append (car list1) (cdr list1))))
        ((numberp (car list1)) (cons (car list1) (FLAT-NUMBERS-FIRST (cdr list1))))
        (T (append (FLAT-NUMBERS-FIRST (cdr list1)) (cons (car list1) nil)))
  )
)