emacs/lisp/tabify.el

;;; tabify.el --- tab conversion commands for Emacs  -*- lexical-binding: t -*-

;; Copyright (C) 1985, 1994, 2001-2021 Free Software Foundation, Inc.

;; Maintainer: emacs-devel@gnu.org
;; Package: emacs

;; This file is part of GNU Emacs.

;; GNU Emacs is free software: you can redistribute it and/or modify
;; it under the terms of the GNU General Public License as published by
;; the Free Software Foundation, either version 3 of the License, or
;; (at your option) any later version.

;; GNU Emacs is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs.  If not, see <https://www.gnu.org/licenses/>.

;;; Commentary:

;; Commands to optimize spaces to tabs or expand tabs to spaces in a region
;; (`tabify' and `untabify').  The variable tab-width does the obvious.

;;; Code:

;;;###autoload
(defun untabify (start end &optional _arg)
  "Convert all tabs in region to multiple spaces, preserving columns.
If called interactively with prefix ARG, convert for the entire
buffer.

Called non-interactively, the region is specified by arguments
START and END, rather than by the position of point and mark.
The variable `tab-width' controls the spacing of tab stops."
  (interactive (if current-prefix-arg
		   (list (point-min) (point-max) current-prefix-arg)
		 (list (region-beginning) (region-end) nil)))
  (let ((c (current-column)))
    (save-excursion
      (save-restriction
        (narrow-to-region (point-min) end)
        (goto-char start)
        (while (search-forward "\t" nil t)      ; faster than re-search
          (forward-char -1)
          (let ((tab-beg (point))
                (indent-tabs-mode nil)
                column)
            (skip-chars-forward "\t")
            (setq column (current-column))
            (delete-region tab-beg (point))
            (indent-to column)))))
    (move-to-column c)))

(defvar tabify-regexp " [ \t]+"
  "Regexp matching whitespace that tabify should consider.
Usually this will be \" [ \\t]+\" to match a space followed by whitespace.
\"^\\t* [ \\t]+\" is also useful, for tabifying only initial whitespace.")

;;;###autoload
(defun tabify (start end &optional _arg)
  "Convert multiple spaces in region to tabs when possible.
A group of spaces is partially replaced by tabs
when this can be done without changing the column they end at.
If called interactively with prefix ARG, convert for the entire
buffer.

Called non-interactively, the region is specified by arguments
START and END, rather than by the position of point and mark.
The variable `tab-width' controls the spacing of tab stops."
  (interactive (if current-prefix-arg
		   (list (point-min) (point-max) current-prefix-arg)
		 (list (region-beginning) (region-end) nil)))
  (save-excursion
    (save-restriction
      ;; Include the beginning of the line in the narrowing
      ;; since otherwise it will throw off current-column.
      (goto-char start)
      (beginning-of-line)
      (narrow-to-region (point) end)
      (goto-char start)
      (let ((indent-tabs-mode t))
        (while (re-search-forward tabify-regexp nil t)
          ;; The region between (match-beginning 0) and (match-end 0) is just
          ;; spacing which we want to adjust to use TABs where possible.
          (let ((end-col (current-column))
                (beg-col (save-excursion (goto-char (match-beginning 0))
                                         (skip-chars-forward "\t")
                                         (current-column))))
            (if (= (/ end-col tab-width) (/ beg-col tab-width))
                ;; The spacing (after some leading TABs which we wouldn't
                ;; want to touch anyway) does not straddle a TAB boundary,
                ;; so it neither contains a TAB, nor will we be able to use
                ;; a TAB here anyway: there's nothing to do.
                nil
              (delete-region (match-beginning 0) (point))
              (indent-to end-col))))))))

(provide 'tabify)

;;; tabify.el ends here