visual-wrap.el

;;; visual-wrap.el --- Smart line-wrapping with wrap-prefix -*- lexical-binding: t -*-

;; Copyright (C) 2011-2021, 2024-2025 Free Software Foundation, Inc.

;; Author: Stephen Berman <stephen.berman@gmx.net>
;;         Stefan Monnier <monnier@iro.umontreal.ca>
;; Maintainer: emacs-devel@gnu.org
;; Keywords: convenience
;; Package: emacs

;; This file is part of GNU Emacs.

;; This program 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.

;; This program 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 this program.  If not, see <http://www.gnu.org/licenses/>.

;;; Commentary:

;; This package provides the `visual-wrap-prefix-mode' minor mode
;; which sets the wrap-prefix property on the fly so that
;; single-long-line paragraphs get word-wrapped in a way similar to
;; what you'd get with M-q using adaptive-fill-mode, but without
;; actually changing the buffer's text.

;;; Code:

(defcustom visual-wrap-extra-indent 0
  "Number of extra spaces to indent in `visual-wrap-prefix-mode'.

`visual-wrap-prefix-mode' indents the visual lines to the level
of the actual line plus `visual-wrap-extra-indent'.  A negative
value will do a relative de-indent.

Examples:

actual indent = 2
extra indent = -1

  Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed
 do eiusmod tempor incididunt ut labore et dolore magna
 aliqua. Ut enim ad minim veniam, quis nostrud exercitation
 ullamco laboris nisi ut aliquip ex ea commodo consequat.

actual indent = 2
extra indent = 2

  Lorem ipsum dolor sit amet, consectetur adipisicing elit, sed
    do eiusmod tempor incididunt ut labore et dolore magna
    aliqua. Ut enim ad minim veniam, quis nostrud exercitation
    ullamco laboris nisi ut aliquip ex ea commodo consequat."
  :type 'integer
  :safe 'integerp
  :version "30.1"
  :group 'visual-line)

(defun visual-wrap--face-extend-p (face)
  ;; Before Emacs 27, faces always extended beyond EOL, so we check
  ;; for a non-default background instead.
  (cond
   ((listp face)
    (plist-get face (if (fboundp 'face-extend-p) :extend :background)))
   ((symbolp face)
    (if (fboundp 'face-extend-p)
        (face-extend-p face nil t)
      (face-background face nil t)))))

(defun visual-wrap--prefix-face (fcp _beg end)
  ;; If the fill-context-prefix already specifies a face, just use that.
  (cond ((get-text-property 0 'face fcp))
        ;; Else, if the last character is a newline and has a face
        ;; that extends beyond EOL, assume that this face spans the
        ;; whole line and apply it to the prefix to preserve the
        ;; "block" visual effect.
        ;;
        ;; NB: the face might not actually span the whole line: see
        ;; for example removed lines in diff-mode, where the first
        ;; character has the diff-indicator-removed face, while the
        ;; rest of the line has the diff-removed face.
        ((= (char-before end) ?\n)
         (let ((eol-face (get-text-property (1- end) 'face)))
           ;; `eol-face' can be a face, a "face value"
           ;; (plist of face properties) or a list of one of those.
           (if (or (not (consp eol-face)) (keywordp (car eol-face)))
               ;; A single face.
               (if (visual-wrap--face-extend-p eol-face) eol-face)
             ;; A list of faces.  Keep the ones that extend beyond EOL.
             (delq nil (mapcar (lambda (f)
                                 (if (visual-wrap--face-extend-p f) f))
                               eol-face)))))))

(defun visual-wrap--adjust-prefix (prefix)
  "Adjust PREFIX with `visual-wrap-extra-indent'."
  (if (numberp prefix)
      (+ visual-wrap-extra-indent prefix)
    (let ((prefix-len (string-width prefix)))
      (cond
       ((= 0 visual-wrap-extra-indent)
        prefix)
       ((< 0 visual-wrap-extra-indent)
        (concat prefix (make-string visual-wrap-extra-indent ?\s)))
       ((< 0 (+ visual-wrap-extra-indent prefix-len))
        (substring prefix
                   0 (+ visual-wrap-extra-indent prefix-len)))
       (t
        "")))))

(defun visual-wrap--apply-to-line (position)
  "Apply visual-wrapping properties to the logical line starting at POSITION."
  (save-excursion
    (goto-char position)
    (when-let* ((first-line-prefix (fill-match-adaptive-prefix))
                (next-line-prefix (visual-wrap--content-prefix
                                   first-line-prefix position)))
      (when (numberp next-line-prefix)
        ;; Set a minimum width for the prefix so it lines up correctly
        ;; with subsequent lines.  Make sure not to do this past the end
        ;; of the line though!  (`fill-match-adaptive-prefix' could
        ;; potentially return a prefix longer than the current line in
        ;; the buffer.)
        (add-display-text-property
         position (min (+ position (length first-line-prefix))
                       (line-end-position))
         'min-width `((,next-line-prefix . width))))
      (setq next-line-prefix (visual-wrap--adjust-prefix next-line-prefix))
      (put-text-property
       position (line-end-position) 'wrap-prefix
       (if (numberp next-line-prefix)
           `(space :align-to (,next-line-prefix . width))
         next-line-prefix)))))

(defun visual-wrap--content-prefix (prefix position)
  "Get the next-line prefix for the specified first-line PREFIX.
POSITION is the position in the buffer where PREFIX is located.

This returns a string prefix to use for subsequent lines; an integer,
indicating the number of canonical-width spaces to use; or nil, if
PREFIX was empty."
  (cond
   ((string= prefix "")
    nil)
   ((or (and adaptive-fill-first-line-regexp
             (string-match adaptive-fill-first-line-regexp prefix))
        (and comment-start-skip
             (string-match comment-start-skip prefix)))
    ;; If we want to repeat the first-line prefix on subsequent lines,
    ;; return its string value.  However, we remove any `wrap-prefix'
    ;; property that might have been added earlier.  Otherwise, we end
    ;; up with a string containing a `wrap-prefix' string containing a
    ;; `wrap-prefix' string...
    (remove-text-properties 0 (length prefix) '(wrap-prefix) prefix)
    prefix)
   (t
    ;; Otherwise, we want the prefix to be whitespace of the same width
    ;; as the first-line prefix.  We want to return an integer width (in
    ;; units of the font's average-width) large enough to fit the
    ;; first-line prefix.
    (let ((avg-space (propertize (buffer-substring position (1+ position))
                                 'display '(space :width (1 . width)))))
      ;; Remove any `min-width' display specs since we'll replace with
      ;; our own later in `visual-wrap--apply-to-line' (bug#73882).
      (add-display-text-property 0 (length prefix) 'min-width nil prefix)
      (max (string-width prefix)
           (ceiling (string-pixel-width prefix (current-buffer))
                    (string-pixel-width avg-space (current-buffer))))))))

(defun visual-wrap-fill-context-prefix (beg end)
  "Compute visual wrap prefix from text between BEG and END.
This is like `fill-context-prefix', but with prefix length adjusted
by `visual-wrap-extra-indent'."
  (declare (obsolete nil "31.1"))
  (let* ((fcp
          ;; `fill-context-prefix' ignores prefixes that look like
          ;; paragraph starts, in order to avoid inadvertently
          ;; creating a new paragraph while filling, but here we're
          ;; only dealing with single-line "paragraphs" and we don't
          ;; actually modify the buffer, so this restriction doesn't
          ;; make much sense (and is positively harmful in
          ;; taskpaper-mode where paragraph-start matches everything).
          (or (let ((paragraph-start regexp-unmatchable))
                (fill-context-prefix beg end))
                  ;; Note: fill-context-prefix may return nil; See:
                  ;; http://article.gmane.org/gmane.emacs.devel/156285
              ""))
         (prefix (visual-wrap--adjust-prefix fcp))
         (face (visual-wrap--prefix-face fcp beg end)))
    (if face
        (propertize prefix 'face face)
      prefix)))

(defun visual-wrap-prefix-function (beg end)
  "Indent the region between BEG and END with visual filling."
  ;; Any change at the beginning of a line might change its wrap
  ;; prefix, which affects the whole line.  So we need to "round-up"
  ;; `end' to the nearest end of line.  We do the same with `beg'
  ;; although it's probably not needed.
  (goto-char end)
  (unless (bolp) (forward-line 1))
  (setq end (point))
  (goto-char beg)
  (forward-line 0)
  (setq beg (point))
  (while (< (point) end)
    (visual-wrap--apply-to-line (point))
    (forward-line))
  `(jit-lock-bounds ,beg . ,end))

;;;###autoload
(define-minor-mode visual-wrap-prefix-mode
  "Display continuation lines with prefixes from surrounding context.
To enable this minor mode across all buffers, enable
`global-visual-wrap-prefix-mode'."
  :lighter ""
  :group 'visual-line
  (if visual-wrap-prefix-mode
      (progn
        ;; HACK ATTACK!  We want to run after font-lock (so our
        ;; wrap-prefix includes the faces applied by font-lock), but
        ;; jit-lock-register doesn't accept an `append' argument, so
        ;; we add ourselves beforehand, to make sure we're at the end
        ;; of the hook (bug#15155).
        (add-hook 'jit-lock-functions
                  #'visual-wrap-prefix-function 'append t)
        (jit-lock-register #'visual-wrap-prefix-function))
    (jit-lock-unregister #'visual-wrap-prefix-function)
    (with-silent-modifications
      (save-restriction
        (widen)
        (remove-text-properties (point-min) (point-max) '(wrap-prefix nil))))))

;;;###autoload
(define-globalized-minor-mode global-visual-wrap-prefix-mode
  visual-wrap-prefix-mode visual-wrap-prefix-mode
  :init-value nil
  :group 'visual-line)

(provide 'visual-wrap)
;;; visual-wrap.el ends here