;This lisp is developed as part of Survey Drawing project, which aims to help people prepare survey drawings easily.
;You may find more such lisps at www.surveydrawing.net and www.esurveying.net


(defun c:celn()
	(cre_lay "Cen_Line" 2)
	(setq selDetails(nentsel "\nSelect First Polyline:"))
	(setq plen(car selDetails))
	(setq plpt(cadr selDetails))
	(setq ptList1(FindNEntPList plen plpt))
	(setq selDetails(nentsel "\nSelect Second Polyline:"))
	(setq plen(car selDetails))
	(setq plpt(cadr selDetails))
	(setq ptList2(FindNEntPList plen plpt))
	(if (> (distance (nth 0 ptList1) (nth 0 ptList2)) (distance (nth 0 ptList1) (nth 1 ptList2)))
		(command "Line" (midp (nth 0 ptList1) (nth 1 ptList2)) (midp (nth 1 ptList1) (nth 0 ptList2)) "")
		(command "Line" (midp (nth 0 ptList1) (nth 0 ptList2)) (midp (nth 1 ptList1) (nth 1 ptList2)) "")
	)
	(cpl (entlast) "Cen_Line")
	(princ)
)

(defun c:altc()
	(command "erase" (entlast) "")
	(if (> (distance (nth 0 ptList1) (nth 0 ptList2)) (distance (nth 0 ptList1) (nth 1 ptList2)))
		(command "Line" (midp (nth 0 ptList1) (nth 0 ptList2)) (midp (nth 1 ptList1) (nth 1 ptList2)) "")
		(command "Line" (midp (nth 0 ptList1) (nth 1 ptList2)) (midp (nth 1 ptList1) (nth 0 ptList2)) "")
	)	
	(cpl (entlast) "Cen_Line")
)

(defun FindNEntPList(ent1 pntDetails)
	(setq entl1(Entget ent1))
	(setq entty1(Cdr (Assoc 0 entl1)))
	(If (= entty1 "VERTEX")
		(progn
			(setq fp(Cdr (Assoc 10 entl1)))
			(setq sp(Cdr (Assoc 10 (Entget (Entnext ent1)))))
		)
	)
	(If (= entty1 "LINE")
		(progn
			(setq fp(Cdr (Assoc 10 entl1)))
			(setq sp(Cdr (Assoc 11 entl1)))
		)
	) 
	(If (= entty1 "LWPOLYLINE")
		(progn
			(setq obj(vlax-ename->vla-object ent1))
			(setq ptOnLine(vlax-curve-getClosestPointTo obj pntDetails))
			(setq ptPara(vlax-curve-getParamAtPoint obj ptOnLine))
			(setq ptFirst(fix ptpara))
			(setq ptnext(1+ ptFirst))
			(setq fp(vlax-curve-getPointAtParam obj ptFirst))
			(setq sp(vlax-curve-getPointAtParam obj ptnext))
		)
	)	
	(setq return(List fp sp))	
)

;Function to Change the Layer of Specified Entity to Specified Layer
(defun CPL(ent entlay)
	(command "._change" ent "" "p" "layer" entlay "")
)

;Function to Create a Layer with given color
(defun Cre_Lay(lay_layn lay_laycol)
	(if (= (tblsearch "Layer" lay_layn) nil)
		(command "._Layer" "n" lay_layn "c" lay_laycol lay_layn "")
		(command "._Layer" "t" lay_layn "on" lay_layn "c" lay_laycol lay_layn "")
	)
	(princ)
)

;Function to find the midpoint of two points.
(Defun MidP(midp_fpo midp_spo)
	(setq midp_mpo(list (/ (+ (car midp_fpo) (car midp_spo)) 2) (/ (+ (cadr midp_fpo) (cadr midp_spo)) 2)))
)

(princ "\n1. \"CeLn\" to Create Centre Line\n2. AltC: Find Alternate") (princ)