(defun c:ml()
  (princ "\n**** 二姊的第四個心願: 中央平行線 **** (測試階段)")

  ;Memorizing osnap status into the variable ostatus before we clear osnap
  (setq ostatus (getvar "osmode") )

  (setvar "osmode" 512)    ;setting osnap to capture nearest 512
  (setq p1 (getpoint "\n1st point: _NEArest"))  ; capture points
  (setq p2 (getpoint "\n2nd point: _NEArest"))
; (princ "\nInput point 1    : p1 ") (princ p1) ; just shows the data
; (princ "\nInput point 2    : p2 ") (princ p2)

  (Rectify)   ; force perpendicular by rectifying the two input points

  (setq d (distance p1 p2))          ; distance between p1 and p2
  (setq slope (angle p1 p2))         ; gradient of line p1p2

  (setq x1 (car p1)) (setq y1 (cadr p1))   ; extract abcessae and ordinates
  (setq x2 (car p2)) (setq y2 (cadr p2))   ; ie. (x1,y1) and (x2,y2) are the
                                           ; two user input points.

  (setq midx (/ (+ x1 x2) 2))   ; calculate midpoint
  (setq midy (/ (+ y1 y2) 2))



  ;Under the condition that the output line will
  ;     (1) be half the distance p1p2
  ;     (2) be perpendicular to p1p2
  ;     (3) has its midpoint coincide with the midpoint of p1p2
  ;the following calculation applies.

  ;The slope of p1 to one end of the output line
   (setq slopePlus (+ (atan 0.5) slope))
  ;The slope of p1 to the other end of the output line
   (setq slopeMinus (- slope (atan 0.5)))
  ;Distance between p1 and either end of output line
  (setq hypotinuse (* d (/ (sqrt 5) 4)))

  ;Locate the coordinates of the two ends of the output line.
  ;A and B are the two ends of the output line.
   (setq A (polar p1 slopePlus hypotinuse))
   (setq B (polar p1 slopeMinus hypotinuse))

  (setvar "osmode" 0)    ;shut off osnap to avoid interference
  (princ "\nDrawing mid parallel line into layer ") (princ (getvar "clayer")) (princ "\n")
  ;Finally, drawing the output line
  (command "line" A B "")


  ;Print out the data
;  (princ "\nRectified point 1: p1 ") (princ p1)
;  (princ "\nRectified point 2: p2 ") (princ p2)
;  (princ "\nOutput line end 1:  A ") (princ A)
;  (princ "\nOutput line end 2:  B ") (princ B)
;  (princ "\np1p2 gradient = ") (princ slope)
;  (princ "\np1A  gradient = ") (princ slopePlus)
;  (princ "\np2A  gradient = ") (princ slopeMinus)

  (setvar "osmode" ostatus)      ;Returning initial osnap settings
  (princ)   ; to get rid of end nil
) ; end of defun m






(defun Rectify()

        (setq layerStat (getvar "clayer"))      ; stores user current layer

       ;Search the Layer section in the table for the layer _temp
        (setq checkExist (tblsearch "LAYER" "_temp"))
       ;Creates _temp layer if it doesn't already exist
        (if (= checkExist nil) (command "layer" "n" "_temp" ""))

        (command "layer" "ON" "_temp" "")   ; open up temp layer
        (setvar "clayer" "_temp")      ; set temp as current


        (command "line" "nea" p1 "per" p2 "")  ; temp construction line
        (setq cLs (ssget "c" p1 p2))  ; construction line included in selection

       ; Screen for that line we have drawn into temp layer _temp
        (setq i 0)
        (repeat (sslength cLs) (progn
         (setq cL (entget (ssname cLs i) ))       ; extract entity by entity
         (setq Lay (cdr (assoc 8 cL) )) ; obtain the layer of individual entity

         ; please note, although we created layer _temp in lower
         ; case, autoCAD automatically converts it into upper case, and so
         ; it is important that in our if statement to use the upper case.
         ; If we use the same small case _temp, autoCAD won't pick it up.
         (if (= Lay "_TEMP") (setq j i))  ; trap entity number of _TEMP line
         (setq i (+ i 1))
         ) ;end progn
        ) ;end repeat
        (setq cL (entget (ssname cLs j) ))     ; extract the _temp line entity



        (setq x1 (cadr (assoc 10 cL)))         ; starting x
        (setq y1 (cadr (cdr (assoc 10 cL))))   ; starting y
        (setq x2 (cadr (assoc 11 cL)))         ; ending x
        (setq y2 (cadr (cdr (assoc 11 cL))))   ; ending y

        (setq p1 (list x1 y1 0.0))  ; feed rectified values back into p1 and p2
        (setq p2 (list x2 y2 0.0))



        ;Search the Layer section in the table for the layer _pleasekillme
        ;(setq checkExist (tblsearch "LAYER" "_pleasekillme"))
        ;Creates _pleasekillme temp layer if it doesn't already exist
        ;(if (= checkExist nil) (command "layer" "n" "_pleasekillme" ""))

;Erase the temp line to prevent interference with future temp construction line.
;I tried (command "erase" cL "") at first but it didn't work. Apparently
;erase wanted the name of the entity rather than the content of the entity, so
;when I then tried (command "erase" (ssname cLs j) "") , it worked.
        (command "erase" (ssname cLs j) "")

        (setvar "clayer" layerStat)     ; returns user layer

; Closing off temp layer
;(if (= layerStat (getvar "clayer"))  (command "layer" "OFF" "_temp" "y" "")  )
;(if (/= layerStat (getvar "clayer"))  (command "layer" "OFF" "_temp" "")  )
(command "layer" "OFF" "_temp" "")

) ;end rectify








; M.LSP
;
; Input:
; User selects two points on two parallel walls (one point on each wall)
;
; Output:
; A line of half the length between the two input points will be drawn.
; Provided that the two walls are parallel, the output line will be
; parallel to both walls and runs through the mid point between these two walls.
; The mid point of the output line coincides with the mid point between
; input p1 p2.
;
; Note in the case when wall1 is not parallel to wall2 to start off with,
; the output line will be parallel to wall2
;
; Future development:
; (1) Make the line fall between p1 and p2 rather than originating from p1
; (2) Fix up the two ifs in the last closing temp section