[PATCH] Make rectangle-select able to skip lines(empty one, for example)

[Constantin Kulikov <zxnotdead@gmail.com>]

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Code to try:

(with-eval-after-load "rect.el"
  (defvar *rectangle-skip-lines* nil
"If `t' -- skip empty lines, if `function' -- skip line if it returns `nil'.
The function gets all arguments of `apply-to-rectangle' as input.")
  (defun apply-on-rectangle (function start end &rest args)
"Call FUNCTION for each line of rectangle with corners at START, END.
FUNCTION is called with two arguments: the start and end columns of the
rectangle, plus ARGS extra arguments.  Point is at the beginning of line
when
the function is called.
Application of the FUNCTION is affected by the `*rectangle-skip-lines*'.
The final point after the last operation will be returned."
(save-excursion
 (let* ((cols (rectangle--pos-cols start end))
(startcol (car cols))
(endcol (cdr cols))
(startpt (progn (goto-char start) (line-beginning-position)))
(endpt (progn (goto-char end)
  (copy-marker (line-end-position))))
final-point)
;; Ensure the start column is the left one.
(if (< endcol startcol)
(let ((col startcol))
 (setq startcol endcol endcol col)))
;; Start looping over lines.
(goto-char startpt)
(while
(progn
 (when (cond
((null *rectangle-skip-lines*)
 t)
((functionp *rectangle-skip-lines*)
 (apply *rectangle-skip-lines* function start end args))
(t
 (/= (line-beginning-position) (line-end-position))))
(apply function startcol endcol args))
 (setq final-point (point))
 (and (zerop (forward-line 1)) (bolp)
  (<= (point) endpt))))
final-point))))

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--- rect.el	2023-01-30 14:18:22.542176710 +0300
+++ rect-patched.el	2023-01-30 15:44:14.953579566 +0300
@@ -143,12 +143,16 @@
   (rectangle--reset-point-crutches))
 
 ;;; Rectangle operations.
+(defvar *rectangle-skip-lines* nil
+  "If `t' -- skip empty lines, if `function' -- skip line if it returns `nil'.
+The function gets all arguments of `apply-to-rectangle' as input.")
 
 (defun apply-on-rectangle (function start end &rest args)
   "Call FUNCTION for each line of rectangle with corners at START, END.
 FUNCTION is called with two arguments: the start and end columns of the
 rectangle, plus ARGS extra arguments.  Point is at the beginning of line when
 the function is called.
+Application of the FUNCTION is affected by the `*rectangle-skip-lines*'.
 The final point after the last operation will be returned."
   (save-excursion
     (let* ((cols (rectangle--pos-cols start end))
@@ -166,7 +170,14 @@
       (goto-char startpt)
       (while
           (progn
-            (apply function startcol endcol args)
+            (when (cond
+				   ((null *rectangle-skip-lines*)
+					t)
+				   ((functionp *rectangle-skip-lines*)
+					(apply *rectangle-skip-lines* function start end args))
+				   (t
+					(/= (line-beginning-position) (line-end-position))))
+			  (apply function startcol endcol args))
             (setq final-point (point))
             (and (zerop (forward-line 1)) (bolp)
                  (<= (point) endpt))))