;;; ---------------------------------------------------------------------
;;;  MIDALIGN-bgol.lsp
;;;  Command:  MIDL
;;;
;;;  PURPOSE
;;;  Derives a genuine centerline between two selected edge polylines
;;;  (e.g. the two edges of a surveyed road) by sampling both curves at
;;;  matching parametric positions and connecting the midpoints. Works
;;;  correctly even when the two edges are not perfectly parallel,
;;;  unlike the native OFFSET command.
;;;
;;;  USAGE
;;;    1. Load this file (APPLOAD or drag-drop onto the drawing window).
;;;    2. Type   MIDL   at the command line and press Enter.
;;;    3. Select the first edge polyline, then the second edge polyline.
;;;    4. A new centerline polyline is drawn between them on layer
;;;       "BGOL-MIDL". The original edge polylines are left untouched.
;;;
;;;  NOTES
;;;    - Sampling density defaults to *bgol-midl-samples* points; edit
;;;      the constant below for a smoother or coarser centerline.
;;;    - This is an original, independently written implementation.
;;;      It is INSPIRED by the general idea of edge-to-centerline
;;;      derivation utilities common in CAD/survey drafting workflows,
;;;      but the code, command name and file name here are new and are
;;;      NOT copied from, nor identical to, any specific third-party
;;;      product.
;;;
;;;  ---------------------------------------------------------------------
;;;  Developed and shared for the CAD community by:  BGol Community
;;;  Community website:                              https://bgol.in/
;;;  License:  Open-source & free to use, modify, and share for all.
;;;            Provided "as is", without warranty of any kind.
;;;  ---------------------------------------------------------------------

(vl-load-com)

(setq *bgol-midl-samples* 24)  ; number of midpoint samples along the pair

(defun BGOL:MIDL-EnsureLayer (lname / )
  (if (not (tblsearch "LAYER" lname))
    (entmake (list '(0 . "LAYER")
                   '(100 . "AcDbSymbolTableRecord")
                   '(100 . "AcDbLayerTableRecord")
                   (cons 2 lname)
                   '(70 . 0)
                   '(62 . 2)          ; yellow
                   '(6 . "Continuous")))
  )
)

(defun BGOL:MIDL-MakePolyline (pointlist lname / dxfdata pt)
  (setq dxfdata (list '(0 . "LWPOLYLINE")
                       '(100 . "AcDbEntity")
                       (cons 8 lname)
                       '(100 . "AcDbPolyline")
                       (cons 90 (length pointlist))
                       '(70 . 0)
                 )
  )
  (foreach pt pointlist
    (setq dxfdata (append dxfdata (list (cons 10 pt))))
  )
  (entmakex dxfdata)
)

(defun C:MIDL ( / e1 e2 obj1 obj2 sp1 ep1 sp2 ep2 n i frac t1 t2 p1 p2 mid pts)
  (BGOL:MIDL-EnsureLayer "BGOL-MIDL")
  (setq e1 (car (entsel "\nSelect first edge polyline: ")))
  (setq e2 (if e1 (car (entsel "\nSelect second edge polyline: "))))
  (if (and e1 e2)
    (progn
      (setq obj1 (vlax-ename->vla-object e1))
      (setq obj2 (vlax-ename->vla-object e2))
      (setq sp1 (vlax-curve-getStartParam obj1))
      (setq ep1 (vlax-curve-getEndParam obj1))
      (setq sp2 (vlax-curve-getStartParam obj2))
      (setq ep2 (vlax-curve-getEndParam obj2))
      (setq n (max 4 *bgol-midl-samples*))
      (setq pts '())
      (setq i 0)
      (while (<= i n)
        (setq frac (/ (float i) n))
        (setq t1 (+ sp1 (* frac (- ep1 sp1))))
        (setq t2 (+ sp2 (* frac (- ep2 sp2))))
        (setq p1 (vlax-curve-getPointAtParam obj1 t1))
        (setq p2 (vlax-curve-getPointAtParam obj2 t2))
        (if (and p1 p2)
          (progn
            (setq mid (mapcar '(lambda (a b) (/ (+ a b) 2.0)) p1 p2))
            (setq pts (append pts (list (list (car mid) (cadr mid)))))
          )
        )
        (setq i (1+ i))
      )
      (if (>= (length pts) 2)
        (progn
          (BGOL:MIDL-MakePolyline pts "BGOL-MIDL")
          (princ (strcat "\nMIDL: centerline created with " (itoa (length pts))
                          " vertices (layer BGOL-MIDL)."))
        )
        (princ "\nCould not compute enough midpoints; centerline not created.")
      )
    )
    (princ "\nBoth edges must be selected to compute a centerline.")
  )
  (princ)
)

(princ "\nMIDALIGN-bgol.lsp loaded. Type MIDL to derive a centerline between two edge polylines.")
(princ "\n  -- Free & open-source, courtesy of BGol Community (https://bgol.in/) --")
(princ)