234 lines
8.5 KiB
EmacsLisp
234 lines
8.5 KiB
EmacsLisp
;;; utf7.el --- UTF-7 encoding/decoding for Emacs -*- lexical-binding:t -*-
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;; Copyright (C) 1999-2024 Free Software Foundation, Inc.
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;; Author: Jon K Hellan <hellan@acm.org>
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;; Maintainer: emacs-devel@gnu.org
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;; Keywords: mail
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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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;; UTF-7 - A Mail-Safe Transformation Format of Unicode - RFC 2152
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;; This is a transformation format of Unicode that contains only 7-bit
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;; ASCII octets and is intended to be readable by humans in the limiting
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;; case that the document consists of characters from the US-ASCII
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;; repertoire.
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;; In short, runs of characters outside US-ASCII are encoded as base64
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;; inside delimiters.
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;; A variation of UTF-7 is specified in IMAP 4rev1 (RFC 2060) as the way
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;; to represent characters outside US-ASCII in mailbox names in IMAP.
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;; This library supports both variants, but the IMAP variation was the
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;; reason I wrote it.
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;; The routines convert UTF-7 -> UTF-16 (16 bit encoding of Unicode)
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;; -> current character set, and vice versa.
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;; However, until Emacs supports Unicode, the only Emacs character set
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;; supported here is ISO-8859.1, which can trivially be converted to/from
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;; Unicode.
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;; When decoding results in a character outside the Emacs character set,
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;; an error is thrown. It is up to the application to recover.
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;; UTF-7 should be done by providing a coding system. Mule-UCS does
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;; already, but I don't know if it does the IMAP version and it's not
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;; clear whether that should really be a coding system. The UTF-16
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;; part of the conversion can be done with coding systems available
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;; with Mule-UCS or some versions of Emacs. Unfortunately these were
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;; done wrongly (regarding handling of byte-order marks and how the
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;; variants were named), so we don't have a consistent name for the
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;; necessary coding system. The code below doesn't seem to DTRT
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;; generally. E.g.:
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;;
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;; (utf7-encode "a+£")
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;; => "a+ACsAow-"
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;;
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;; $ echo "a+£"|iconv -f utf-8 -t utf-7
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;; a+-+AKM
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;;
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;; -- fx
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;;; Code:
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(require 'mm-util)
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(defconst utf7-direct-encoding-chars " -%'-*,-[]-}"
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"Character ranges which do not need escaping in UTF-7.")
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(defconst utf7-imap-direct-encoding-chars
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(concat utf7-direct-encoding-chars "+\\~")
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"Character ranges which do not need escaping in the IMAP variant of UTF-7.")
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(defconst utf7-utf-16-coding-system
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(cond ((mm-coding-system-p 'utf-16-be-no-signature) ; Mule-UCS
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'utf-16-be-no-signature)
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((and (mm-coding-system-p 'utf-16-be) ; Emacs
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;; Avoid versions with BOM.
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(= 2 (length (encode-coding-string "a" 'utf-16-be))))
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'utf-16-be)
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((mm-coding-system-p 'utf-16-be-nosig) ; ?
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'utf-16-be-nosig))
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"Coding system which encodes big endian UTF-16 without a BOM signature.")
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(defsubst utf7-imap-get-pad-length (len modulus)
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"Return required length of padding for IMAP modified base64 fragment."
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(mod (- len) modulus))
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(defun utf7-encode-internal (&optional for-imap)
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"Encode text in (temporary) buffer as UTF-7.
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Use IMAP modification if FOR-IMAP is non-nil."
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(let ((start (point-min))
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(end (point-max)))
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(narrow-to-region start end)
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(goto-char start)
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(let* ((esc-char (if for-imap ?& ?+))
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(direct-encoding-chars
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(if for-imap utf7-imap-direct-encoding-chars
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utf7-direct-encoding-chars))
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(not-direct-encoding-chars (concat "^" direct-encoding-chars)))
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(while (not (eobp))
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(skip-chars-forward direct-encoding-chars)
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(unless (eobp)
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(insert esc-char)
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(let ((p (point))
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(fc (following-char))
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(run-length
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(skip-chars-forward not-direct-encoding-chars)))
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(if (and (= fc esc-char)
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(= run-length 1)) ; Lone esc-char?
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(delete-char -1) ; Now there's one too many
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(utf7-fragment-encode p (point) for-imap))
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(insert "-")))))))
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(defun utf7-fragment-encode (start end &optional for-imap)
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"Encode text from START to END in buffer as UTF-7 escape fragment.
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Use IMAP modification if FOR-IMAP is non-nil."
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(save-restriction
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(let* ((buf (current-buffer))
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(base (with-temp-buffer
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(set-buffer-multibyte nil)
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(insert-buffer-substring buf start end)
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(funcall (utf7-get-u16char-converter 'to-utf-16))
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(base64-encode-region (point-min) (point-max))
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(buffer-string))))
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(narrow-to-region start end)
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(delete-region (point-min) (point-max))
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(insert base))
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(goto-char (point-min))
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(let ((pm (point-max)))
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(when for-imap
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(while (search-forward "/" nil t)
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(replace-match ",")))
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(skip-chars-forward "^= \t\n" pm)
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(delete-region (point) pm))))
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(defun utf7-decode-internal (&optional for-imap)
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"Decode UTF-7 text in (temporary) buffer.
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Use IMAP modification if FOR-IMAP is non-nil."
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(let ((start (point-min)))
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(goto-char start)
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(let* ((esc-pattern (concat "^" (char-to-string (if for-imap ?& ?+))))
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(base64-chars (concat "A-Za-z0-9+"
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(char-to-string (if for-imap ?, ?/)))))
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(while (not (eobp))
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(skip-chars-forward esc-pattern)
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(unless (eobp)
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(forward-char)
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(let ((p (point))
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(run-length (skip-chars-forward base64-chars)))
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(when (and (not (eobp)) (= (following-char) ?-))
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(delete-char 1))
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(unless (= run-length 0) ; Encoded lone esc-char?
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(save-excursion
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(utf7-fragment-decode p (point) for-imap)
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(goto-char p)
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(delete-char -1)))))))))
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(defun utf7-fragment-decode (start end &optional for-imap)
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"Decode base64 encoded fragment from START to END of UTF-7 text in buffer.
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Use IMAP modification if FOR-IMAP is non-nil."
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(save-restriction
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(narrow-to-region start end)
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(when for-imap
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(goto-char start)
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(while (search-forward "," nil 'move-to-end) (replace-match "/")))
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(let ((pl (utf7-imap-get-pad-length (- end start) 4)))
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(insert (make-string pl ?=))
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(base64-decode-region start (+ end pl)))
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(funcall (utf7-get-u16char-converter 'from-utf-16))))
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(defun utf7-get-u16char-converter (which-way)
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"Return a function to convert between UTF-16 and current character set."
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(if utf7-utf-16-coding-system
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(if (eq which-way 'to-utf-16)
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(lambda ()
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(encode-coding-region (point-min) (point-max)
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utf7-utf-16-coding-system))
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(lambda ()
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(decode-coding-region (point-min) (point-max)
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utf7-utf-16-coding-system)))
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;; Add test to check if we are really Latin-1.
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(if (eq which-way 'to-utf-16)
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'utf7-latin1-u16-char-converter
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'utf7-u16-latin1-char-converter)))
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(defun utf7-latin1-u16-char-converter ()
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"Convert latin 1 (ISO-8859.1) characters to 16 bit Unicode.
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Characters are converted to raw byte pairs in narrowed buffer."
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(encode-coding-region (point-min) (point-max) 'iso-8859-1)
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(goto-char (point-min))
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(while (not (eobp))
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(insert 0)
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(forward-char)))
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(defun utf7-u16-latin1-char-converter ()
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"Convert 16 bit Unicode characters to latin 1 (ISO-8859.1).
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Characters are in raw byte pairs in narrowed buffer."
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(goto-char (point-min))
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(while (not (eobp))
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(if (= 0 (following-char))
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(delete-char 1)
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(error "Unable to convert from Unicode"))
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(forward-char))
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(decode-coding-region (point-min) (point-max) 'iso-8859-1)
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(mm-enable-multibyte))
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;;;###autoload
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(defun utf7-encode (string &optional for-imap)
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"Encode UTF-7 STRING. Use IMAP modification if FOR-IMAP is non-nil."
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(if (and (coding-system-p 'utf-7) (coding-system-p 'utf-7-imap))
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;; Emacs 23 with proper support for IMAP
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(encode-coding-string string (if for-imap 'utf-7-imap 'utf-7))
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(mm-with-multibyte-buffer
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(insert string)
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(utf7-encode-internal for-imap)
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(buffer-string))))
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(defun utf7-decode (string &optional for-imap)
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"Decode UTF-7 STRING. Use IMAP modification if FOR-IMAP is non-nil."
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(if (and (coding-system-p 'utf-7) (coding-system-p 'utf-7-imap))
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;; Emacs 23 with proper support for IMAP
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(decode-coding-string string (if for-imap 'utf-7-imap 'utf-7))
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(mm-with-unibyte-buffer
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(insert string)
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(utf7-decode-internal for-imap)
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(mm-enable-multibyte)
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(buffer-string))))
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(provide 'utf7)
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;;; utf7.el ends here
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