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;;; rfc1843.el --- HZ (RFC 1843) decoding -*- lexical-binding:t -*-
;; Copyright (C) 1998-2025 Free Software Foundation, Inc.
;; Author: Shenghuo Zhu <zsh@cs.rochester.edu>
;; Keywords: news HZ HZ+ mail i18n
;; 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:
;; HZ (RFC 1843) decoding. HZ is a data format for exchanging files
;; of arbitrarily mixed Chinese and ASCII characters.
;;
;; Test:
;; (rfc1843-decode-string "~{<:Ky2;S{#,NpJ)l6HK!#~}")
;;; Code:
(eval-when-compile (require 'cl-lib))
(defvar rfc1843-word-regexp
"~\\({\\([\041-\167][\041-\176]\\| \\)+\\)\\(~}\\|$\\)")
(defvar rfc1843-word-regexp-strictly
"~\\({\\([\041-\167][\041-\176]\\)+\\)\\(~}\\|$\\)")
(defvar rfc1843-hzp-word-regexp
"~\\({\\([\041-\167][\041-\176]\\| \\)+\\|\
[<>]\\([\041-\175][\041-\176]\\| \\)+\\)\\(~}\\|$\\)")
(defvar rfc1843-hzp-word-regexp-strictly
"~\\({\\([\041-\167][\041-\176]\\)+\\|\
[<>]\\([\041-\175][\041-\176]\\)+\\)\\(~}\\|$\\)")
(defcustom rfc1843-decode-loosely nil
"Loosely check HZ encoding if non-nil.
When it is set non-nil, only buffers or strings with strictly
HZ-encoded are decoded."
:type 'boolean
:group 'mime)
(defcustom rfc1843-decode-hzp t
"HZ+ decoding support if non-nil.
HZ+ specification (also known as HZP) is to provide a standardized
7-bit representation of mixed Big5, GB, and ASCII text for convenient
e-mail transmission, news posting, etc."
:type 'boolean
:group 'mime)
(defcustom rfc1843-newsgroups-regexp "chinese\\|hz"
"Regexp of newsgroups in which might be HZ encoded."
:type 'regexp
:group 'mime)
(defun rfc1843-decode-region (from to)
"Decode HZ in the region between FROM and TO."
(interactive "r")
(let (str firstc)
(save-excursion
(goto-char from)
(if (or rfc1843-decode-loosely
(re-search-forward (if rfc1843-decode-hzp
rfc1843-hzp-word-regexp-strictly
rfc1843-word-regexp-strictly) to t))
(save-restriction
(narrow-to-region from to)
(goto-char (point-min))
(while (re-search-forward (if rfc1843-decode-hzp
rfc1843-hzp-word-regexp
rfc1843-word-regexp) (point-max) t)
(setq str (buffer-substring-no-properties
(match-beginning 1)
(match-end 1)))
(setq firstc (aref str 0))
(insert (decode-coding-string
(rfc1843-decode
(prog1
(substring str 1)
(delete-region (match-beginning 0) (match-end 0)))
firstc)
(if (eq firstc ?{) 'cn-gb-2312 'cn-big5))))
(goto-char (point-min))
(while (search-forward "~" (point-max) t)
(cond ((eq (char-after) ?\n)
(delete-char -1)
(delete-char 1))
((eq (char-after) ?~)
(delete-char 1)))))))))
(defun rfc1843-decode-string (string)
"Decode HZ STRING and return the results."
(let ((m enable-multibyte-characters))
(with-temp-buffer
(when m
(set-buffer-multibyte 'to))
(insert string)
(inline
(rfc1843-decode-region (point-min) (point-max)))
(buffer-string))))
(defun rfc1843-decode (word &optional firstc)
"Decode HZ WORD and return it."
(let ((i -1) (s (substring word 0)) v)
(if (or (not firstc) (eq firstc ?{))
(while (< (cl-incf i) (length s))
(if (eq (setq v (aref s i)) ? ) nil
(aset s i (+ 128 v))))
(while (< (cl-incf i) (length s))
(if (eq (setq v (aref s i)) ? ) nil
(setq v (+ (* 94 v) (aref s (1+ i)) -3135))
(aset s i (+ (/ v 157) (if (eq firstc ?<) 201 161)))
(setq v (% v 157))
(aset s (cl-incf i) (+ v (if (< v 63) 64 98))))))
s))
(provide 'rfc1843)
;;; rfc1843.el ends here
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