Help with the code : 2d stair

[mhy3sx]

Hi I am writting a lisp to draw 2d stair. The idea is to select the fist step --> move it to layer STAIR then pick two points for the length of the stair and give the number of the steps. Then the code draw the stair with direction arrow in the middle. The code work perfect if i select the two points only from the one side of the stair. Is not all the time the left or the right side depends of the rotation. Can any one help me to update the code to work for bouth sides and don't misalign the steps. Here is the code . By the way i use ZWCAD.

(defun c:Stair2D (/ p1 p2 ent ent_data step_start step_end step_vec stair_vec num_steps i base_pt new_p1 new_p2 mid1 mid2 last_base last_p1 last_p2 arrow_p1 arrow_p2 arrow_p3)
  (vl-load-com)
  (setvar "CMDECHO" 0)
  (command "._UNDO" "_BEGIN")

  (if (not (tblsearch "LAYER" "STAIR"))
    (command "_layer" "_m" "STAIR" "_c" "90" "" "")
  )
  (setvar "CLAYER" "STAIR")

  (princ "\nSelect the first step of the staircase: ")
  (while (not (setq ent (ssget ":S" '((0 . "LINE")))))
    (princ "\nTry again!! ")
  )
  (command "._CHANGE" ent "" "_P" "_LA" "STAIR" "")
  
  (setq ent_data (entget (ssname ent 0)))
  (setq step_start (cdr (assoc 10 ent_data)))
  (setq step_end   (cdr (assoc 11 ent_data)))
  (setq step_vec   (mapcar '- step_end step_start))

  (setvar "OSMODE" 1)
  (setq p1 (getpoint "\nSelect the start of the staircase: "))
  (setq p2 (getpoint p1 "\nSelect the end of the staircase: "))
  (setvar "OSMODE" 0)

  (initget 7)
  (setq num_steps (getint "\nEnter the number of steps: "))
  (setq stair_vec (mapcar '(lambda (a b) (/ (- b a) num_steps)) p1 p2))

  (setq i 1)
  (while (<= i num_steps)
    (setq base_pt (mapcar '+ p1 (mapcar '(lambda (x) (* x i)) stair_vec)))
    (setq new_p1 base_pt)
    (setq new_p2 (mapcar '+ base_pt step_vec))
    (command "._LINE" new_p1 new_p2 "")
    (setq i (1+ i))
  )

  (setq mid1 (mapcar '(lambda (a b) (/ (+ a b) 2.0)) step_start step_end))
  (setq last_base (mapcar '+ p1 (mapcar '(lambda (x) (* x num_steps)) stair_vec)))
  (setq last_p1 last_base)
  (setq last_p2 (mapcar '+ last_base step_vec))
  (setq mid2 (mapcar '(lambda (a b) (/ (+ a b) 2.0)) last_p1 last_p2))
  (command "._line" mid1 mid2 "" "" "_N")
  (command "._CIRCLE" mid1 0.06)

  (setq mid1mid2_vec (mapcar '- mid2 mid1))
  (setq mid1mid2_length (distance mid1 mid2))
  (setq unit_vec (mapcar '(lambda (x) (/ x mid1mid2_length)) mid1mid2_vec))
  (setq perp_vec (list (- (cadr unit_vec)) (car unit_vec) 0.0))
  (setq arrow_p1 (mapcar '+ mid2 (mapcar '(lambda (x) (* x 0.10)) unit_vec)))
  (setq arrow_p2 (mapcar '+ mid2 (mapcar '(lambda (x) (* x -0.15)) perp_vec)))
  (setq arrow_p3 (mapcar '+ mid2 (mapcar '(lambda (x) (* x 0.15)) perp_vec)))
  (command "._LINE" arrow_p1 arrow_p2 "")
  (command "._LINE" arrow_p1 arrow_p3 "")

  (command "._UNDO" "_END")
  (setvar "CMDECHO" 1)
  (setvar "CLAYER" "0")
  (princ)
)

 

Thanks

Drawing2.dwg