spacemacs/core/libs/page-break-lines.el

;;; page-break-lines.el --- Display ^L page breaks as tidy horizontal lines

;; Copyright (C) 2012-2015 Steve Purcell

;; Author: Steve Purcell <steve@sanityinc.com>
;; URL: https://github.com/purcell/page-break-lines
;; Package-Version: 0
;; Package-Requires: ((emacs "24.4"))
;; Keywords: convenience, faces

;; 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 library provides a global mode which displays form feed
;; characters as horizontal rules.

;; Install from Melpa or Marmalade, or add to `load-path' and use
;; (require 'page-break-lines).

;; Use `page-break-lines-mode' to enable the mode in specific buffers,
;; or customize `page-break-lines-modes' and enable the mode globally with
;; `global-page-break-lines-mode'.

;; Issues and limitations:

;; If `page-break-lines-char' is displayed at a different width to
;; regular characters, the rule may be either too short or too long:
;; rules may then wrap if `truncate-lines' is nil.  On some systems,
;; Emacs may erroneously choose a different font for the page break
;; symbol, which choice can be overridden using code such as:

;; (set-fontset-font "fontset-default"
;;                   (cons page-break-lines-char page-break-lines-char)
;;                   (face-attribute 'default :family))

;; Use `describe-char' on a page break char to determine whether this
;; is the case.

;; Additionally, the use of `text-scale-increase' or
;; `text-scale-decrease' will cause the rule width to be incorrect,
;; because the reported window width (in characters) will continue to
;; be the width in the frame's default font, not the scaled font used to
;; display the rule.

;; Adapted from code http://www.emacswiki.org/emacs/PageBreaks

;;; Code:

(defgroup page-break-lines nil
  "Display ugly ^L page breaks as tidy horizontal lines."
  :prefix "page-break-lines-"
  :group 'faces)

(defcustom page-break-lines-char ?─
  "Character used to render page break lines."
  :type 'character
  :group 'page-break-lines)

(defcustom page-break-lines-lighter " PgLn"
  "Mode-line indicator for `page-break-lines-mode'."
  :type '(choice (const :tag "No lighter" "") string)
  :group 'page-break-lines)

(defcustom page-break-lines-max-width nil
  "If non-nil, maximum width (in characters) of page break indicator.
If nil, indicator will span the width of the frame."
  :type '(choice integer (const :tag "Full width" nil))
  :group 'page-break-lines)

(defcustom page-break-lines-modes
  '(emacs-lisp-mode lisp-mode scheme-mode compilation-mode outline-mode help-mode)
  "Modes in which to enable `page-break-lines-mode'."
  :type '(repeat symbol)
  :group 'page-break-lines)

(defface page-break-lines
  '((t :inherit font-lock-comment-face :bold nil :italic nil))
  "Face used to colorize page break lines.
If using :bold or :italic, please ensure `page-break-lines-char'
is available in that variant of your font, otherwise it may be
displayed as a junk character."
  :group 'page-break-lines)



;;;###autoload
(define-minor-mode page-break-lines-mode
  "Toggle Page Break Lines mode.

In Page Break mode, page breaks (^L characters) are displayed as a
horizontal line of `page-break-lines-char' characters."
  :lighter page-break-lines-lighter
  :group 'page-break-lines
  (page-break-lines--update-display-tables))

;;;###autoload
(define-obsolete-function-alias 'turn-on-page-break-lines-mode 'page-break-lines-mode "2018-07-24")

(dolist (hook '(window-configuration-change-hook
                window-size-change-functions
                after-setting-font-hook
                display-line-numbers-mode-hook))
  (add-hook hook 'page-break-lines--update-display-tables))



(defun page-break-lines--update-display-table (window)
  "Modify a display-table that displays page-breaks prettily.
If the buffer inside WINDOW has `page-break-lines-mode' enabled,
its display table will be modified as necessary."
  (with-current-buffer (window-buffer window)
    (with-selected-window window
      (if page-break-lines-mode
          (progn
            (unless buffer-display-table
              (setq buffer-display-table (make-display-table)))
            (let ((default-height (face-attribute 'default :height nil 'default)))
              (set-face-attribute 'page-break-lines nil :height default-height)
              (let* ((cwidth (char-width page-break-lines-char))
                     (wwidth-pix (- (window-width nil t)
                                    (if (bound-and-true-p display-line-numbers)
                                        (line-number-display-width t)
                                      0)))
                     (width (- (/ wwidth-pix (frame-char-width) cwidth)
                               (if (display-graphic-p) 0 1)))
                     (width (if page-break-lines-max-width
                                (min width page-break-lines-max-width)
                              width))
                     (glyph (make-glyph-code page-break-lines-char 'page-break-lines))
                     (new-display-entry (vconcat (make-list width glyph))))
                (unless (equal new-display-entry (elt buffer-display-table ?\^L))
                  (aset buffer-display-table ?\^L new-display-entry)))))
        (when (and (apply 'derived-mode-p page-break-lines-modes)
                   buffer-display-table)
          (aset buffer-display-table ?\^L nil))))))

(defun page-break-lines--update-display-tables  (&optional frame)
  "Function called for updating display table in windows of FRAME."
  (unless (minibufferp)
    (mapc 'page-break-lines--update-display-table (window-list frame 'no-minibuffer))))



;;;###autoload
(defun page-break-lines-mode-maybe ()
  "Enable `page-break-lines-mode' in the current buffer if desired.
When `major-mode' is listed in `page-break-lines-modes', then
`page-break-lines-mode' will be enabled."
  (if (and (not (minibufferp))
           (apply 'derived-mode-p page-break-lines-modes))
      (page-break-lines-mode 1)))

;;;###autoload
(define-global-minor-mode global-page-break-lines-mode
  page-break-lines-mode page-break-lines-mode-maybe
  :require 'page-break-lines
  :group 'page-break-lines)


(provide 'page-break-lines)
;; End:
;;; page-break-lines.el ends here