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125 lines
4.4 KiB
EmacsLisp
125 lines
4.4 KiB
EmacsLisp
;;; rot13.el --- display a buffer in ROT13 -*- lexical-binding: t -*-
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;; Copyright (C) 1988, 2001-2024 Free Software Foundation, Inc.
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;; Author: Howard Gayle
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;; Simon Josefsson
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;; Maintainer: emacs-devel@gnu.org
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;; This file is part of GNU Emacs.
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;; GNU Emacs is free software: you can redistribute it and/or modify
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;; it under the terms of the GNU General Public License as published by
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;; the Free Software Foundation, either version 3 of the License, or
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;; (at your option) any later version.
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;; GNU Emacs is distributed in the hope that it will be useful,
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;; but WITHOUT ANY WARRANTY; without even the implied warranty of
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;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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;; GNU General Public License for more details.
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;; You should have received a copy of the GNU General Public License
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;; along with GNU Emacs. If not, see <https://www.gnu.org/licenses/>.
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;;; Commentary:
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;; "ROT13 ('rotate by 13 places') is a simple letter substitution
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;; cipher that replaces a letter with the 13th letter after it in
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;; the alphabet. ROT13 is a special case of the Caesar cipher
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;; which was developed in ancient Rome.
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;;
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;; Because there are 26 letters (2×13) in the basic Latin
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;; alphabet, ROT13 is its own inverse; that is, to undo ROT13, the
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;; same algorithm is applied, so the same action can be used for
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;; encoding and decoding. The algorithm provides virtually no
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;; cryptographic security, and is often cited as a canonical
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;; example of weak encryption.
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;;
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;; ROT13 is used in online forums as a means of hiding spoilers,
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;; punchlines, puzzle solutions, and offensive materials from the
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;; casual glance." - Wikipedia article on ROT13
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;;
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;; The entry points, `rot13', `rot13-string', and `rot13-region' performs ROT13
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;; encoding/decoding on buffers and strings. The entry point
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;; `rot13-other-window' performs a ROT13 encoding/decoding on the current
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;; buffer and displays the result in another window.
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;;; Code:
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(defconst rot13-display-table
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(let ((table (make-display-table)))
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(dotimes (i 26)
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(aset table (+ i ?a) (vector (+ (% (+ i 13) 26) ?a)))
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(aset table (+ i ?A) (vector (+ (% (+ i 13) 26) ?A))))
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table)
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"Char table for ROT13 display.")
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(put 'plain-char-table 'char-table-extra-slots 0)
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(defconst rot13-translate-table
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(let ((table (make-char-table 'translation-table)))
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(dotimes (i 26)
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(aset table (+ i ?a) (+ (% (+ i 13) 26) ?a))
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(aset table (+ i ?A) (+ (% (+ i 13) 26) ?A)))
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table)
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"Char table for ROT13 translation.")
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;;;###autoload
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(defun rot13 (object &optional start end)
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"ROT13 encrypt OBJECT, a buffer or string.
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If OBJECT is a buffer, encrypt the region between START and END.
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If OBJECT is a string, encrypt it in its entirety, ignoring START
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and END, and return the encrypted string."
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(if (bufferp object)
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(with-current-buffer object
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(rot13-region start end))
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(rot13-string object)))
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;;;###autoload
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(defun rot13-string (string)
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"Return ROT13 encryption of STRING."
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(with-temp-buffer
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(insert string)
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(rot13-region (point-min) (point-max))
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(buffer-string)))
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;;;###autoload
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(defun rot13-region (start end)
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"ROT13 encrypt the region between START and END in current buffer.
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If invoked interactively and the buffer is read-only, a message
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will be printed instead."
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(interactive "r")
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(condition-case nil
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(translate-region start end rot13-translate-table)
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(buffer-read-only
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(when (called-interactively-p 'interactive)
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(let ((dec (rot13-string (buffer-substring start end))))
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(message "Buffer is read-only:\n%s" (string-trim dec)))))))
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;;;###autoload
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(defun rot13-other-window ()
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"Display current buffer in ROT13 in another window.
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The text itself is not modified, only the way it is displayed is affected.
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To terminate the ROT13 display, delete that window. As long as that window
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is not deleted, any buffer displayed in it will become instantly encoded
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in ROT13.
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See also `toggle-rot13-mode'."
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(interactive)
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(let ((w (display-buffer (current-buffer) t)))
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(set-window-display-table w rot13-display-table)))
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;;;###autoload
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(defun toggle-rot13-mode ()
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"Toggle the use of ROT13 encoding for the current window."
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(interactive)
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(if (eq (window-display-table) rot13-display-table)
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(set-window-display-table (selected-window) nil)
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(if (null (window-display-table))
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(set-window-display-table (selected-window) rot13-display-table))))
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(provide 'rot13)
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;;; rot13.el ends here
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