--- /dev/null
+;;; utf7.el --- UTF-7 encoding/decoding for Emacs -*-coding: utf-8;-*-
+
+;; Copyright (C) 1999-2016 Free Software Foundation, Inc.
+
+;; Author: Jon K Hellan <hellan@acm.org>
+;; Maintainer: bugs@gnus.org
+;; Keywords: mail
+
+;; 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 <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; UTF-7 - A Mail-Safe Transformation Format of Unicode - RFC 2152
+;; This is a transformation format of Unicode that contains only 7-bit
+;; ASCII octets and is intended to be readable by humans in the limiting
+;; case that the document consists of characters from the US-ASCII
+;; repertoire.
+;; In short, runs of characters outside US-ASCII are encoded as base64
+;; inside delimiters.
+;; A variation of UTF-7 is specified in IMAP 4rev1 (RFC 2060) as the way
+;; to represent characters outside US-ASCII in mailbox names in IMAP.
+;; This library supports both variants, but the IMAP variation was the
+;; reason I wrote it.
+;; The routines convert UTF-7 -> UTF-16 (16 bit encoding of Unicode)
+;; -> current character set, and vice versa.
+;; However, until Emacs supports Unicode, the only Emacs character set
+;; supported here is ISO-8859.1, which can trivially be converted to/from
+;; Unicode.
+;; When decoding results in a character outside the Emacs character set,
+;; an error is thrown. It is up to the application to recover.
+
+;; UTF-7 should be done by providing a coding system. Mule-UCS does
+;; already, but I don't know if it does the IMAP version and it's not
+;; clear whether that should really be a coding system. The UTF-16
+;; part of the conversion can be done with coding systems available
+;; with Mule-UCS or some versions of Emacs. Unfortunately these were
+;; done wrongly (regarding handling of byte-order marks and how the
+;; variants were named), so we don't have a consistent name for the
+;; necessary coding system. The code below doesn't seem to DTRT
+;; generally. E.g.:
+;;
+;; (utf7-encode "a+£")
+;; => "a+ACsAow-"
+;;
+;; $ echo "a+£"|iconv -f utf-8 -t utf-7
+;; a+-+AKM
+;;
+;; -- fx
+
+
+;;; Code:
+
+(require 'base64)
+(eval-when-compile (require 'cl))
+(require 'mm-util)
+
+(defconst utf7-direct-encoding-chars " -%'-*,-[]-}"
+ "Character ranges which do not need escaping in UTF-7.")
+
+(defconst utf7-imap-direct-encoding-chars
+ (concat utf7-direct-encoding-chars "+\\~")
+ "Character ranges which do not need escaping in the IMAP variant of UTF-7.")
+
+(defconst utf7-utf-16-coding-system
+ (cond ((mm-coding-system-p 'utf-16-be-no-signature) ; Mule-UCS
+ 'utf-16-be-no-signature)
+ ((and (mm-coding-system-p 'utf-16-be) ; Emacs
+ ;; Avoid versions with BOM.
+ (= 2 (length (encode-coding-string "a" 'utf-16-be))))
+ 'utf-16-be)
+ ((mm-coding-system-p 'utf-16-be-nosig) ; ?
+ 'utf-16-be-nosig))
+ "Coding system which encodes big endian UTF-16 without a BOM signature.")
+
+(defsubst utf7-imap-get-pad-length (len modulus)
+ "Return required length of padding for IMAP modified base64 fragment."
+ (mod (- len) modulus))
+
+(defun utf7-encode-internal (&optional for-imap)
+ "Encode text in (temporary) buffer as UTF-7.
+Use IMAP modification if FOR-IMAP is non-nil."
+ (let ((start (point-min))
+ (end (point-max)))
+ (narrow-to-region start end)
+ (goto-char start)
+ (let* ((esc-char (if for-imap ?& ?+))
+ (direct-encoding-chars
+ (if for-imap utf7-imap-direct-encoding-chars
+ utf7-direct-encoding-chars))
+ (not-direct-encoding-chars (concat "^" direct-encoding-chars)))
+ (while (not (eobp))
+ (skip-chars-forward direct-encoding-chars)
+ (unless (eobp)
+ (insert esc-char)
+ (let ((p (point))
+ (fc (following-char))
+ (run-length
+ (skip-chars-forward not-direct-encoding-chars)))
+ (if (and (= fc esc-char)
+ (= run-length 1)) ; Lone esc-char?
+ (delete-char -1) ; Now there's one too many
+ (utf7-fragment-encode p (point) for-imap))
+ (insert "-")))))))
+
+(defun utf7-fragment-encode (start end &optional for-imap)
+ "Encode text from START to END in buffer as UTF-7 escape fragment.
+Use IMAP modification if FOR-IMAP is non-nil."
+ (save-restriction
+ (narrow-to-region start end)
+ (funcall (utf7-get-u16char-converter 'to-utf-16))
+ (mm-with-unibyte-current-buffer
+ (base64-encode-region start (point-max)))
+ (goto-char start)
+ (let ((pm (point-max)))
+ (when for-imap
+ (while (search-forward "/" nil t)
+ (replace-match ",")))
+ (skip-chars-forward "^= \t\n" pm)
+ (delete-region (point) pm))))
+
+(defun utf7-decode-internal (&optional for-imap)
+ "Decode UTF-7 text in (temporary) buffer.
+Use IMAP modification if FOR-IMAP is non-nil."
+ (let ((start (point-min))
+ (end (point-max)))
+ (goto-char start)
+ (let* ((esc-pattern (concat "^" (char-to-string (if for-imap ?& ?+))))
+ (base64-chars (concat "A-Za-z0-9+"
+ (char-to-string (if for-imap ?, ?/)))))
+ (while (not (eobp))
+ (skip-chars-forward esc-pattern)
+ (unless (eobp)
+ (forward-char)
+ (let ((p (point))
+ (run-length (skip-chars-forward base64-chars)))
+ (when (and (not (eobp)) (= (following-char) ?-))
+ (delete-char 1))
+ (unless (= run-length 0) ; Encoded lone esc-char?
+ (save-excursion
+ (utf7-fragment-decode p (point) for-imap)
+ (goto-char p)
+ (delete-char -1)))))))))
+
+(defun utf7-fragment-decode (start end &optional for-imap)
+ "Decode base64 encoded fragment from START to END of UTF-7 text in buffer.
+Use IMAP modification if FOR-IMAP is non-nil."
+ (save-restriction
+ (narrow-to-region start end)
+ (when for-imap
+ (goto-char start)
+ (while (search-forward "," nil 'move-to-end) (replace-match "/")))
+ (let ((pl (utf7-imap-get-pad-length (- end start) 4)))
+ (insert (make-string pl ?=))
+ (base64-decode-region start (+ end pl)))
+ (funcall (utf7-get-u16char-converter 'from-utf-16))))
+
+(defun utf7-get-u16char-converter (which-way)
+ "Return a function to convert between UTF-16 and current character set."
+ (if utf7-utf-16-coding-system
+ (if (eq which-way 'to-utf-16)
+ (lambda ()
+ (encode-coding-region (point-min) (point-max)
+ utf7-utf-16-coding-system))
+ (lambda ()
+ (decode-coding-region (point-min) (point-max)
+ utf7-utf-16-coding-system)))
+ ;; Add test to check if we are really Latin-1.
+ (if (eq which-way 'to-utf-16)
+ 'utf7-latin1-u16-char-converter
+ 'utf7-u16-latin1-char-converter)))
+
+(defun utf7-latin1-u16-char-converter ()
+ "Convert latin 1 (ISO-8859.1) characters to 16 bit Unicode.
+Characters are converted to raw byte pairs in narrowed buffer."
+ (mm-encode-coding-region (point-min) (point-max) 'iso-8859-1)
+ (mm-disable-multibyte)
+ (goto-char (point-min))
+ (while (not (eobp))
+ (insert 0)
+ (forward-char)))
+
+(defun utf7-u16-latin1-char-converter ()
+ "Convert 16 bit Unicode characters to latin 1 (ISO-8859.1).
+Characters are in raw byte pairs in narrowed buffer."
+ (goto-char (point-min))
+ (while (not (eobp))
+ (if (= 0 (following-char))
+ (delete-char 1)
+ (error "Unable to convert from Unicode"))
+ (forward-char))
+ (mm-decode-coding-region (point-min) (point-max) 'iso-8859-1)
+ (mm-enable-multibyte))
+
+;;;###autoload
+(defun utf7-encode (string &optional for-imap)
+ "Encode UTF-7 STRING. Use IMAP modification if FOR-IMAP is non-nil."
+ (if (and (coding-system-p 'utf-7) (coding-system-p 'utf-7-imap))
+ ;; Emacs 23 with proper support for IMAP
+ (encode-coding-string string (if for-imap 'utf-7-imap 'utf-7))
+ (mm-with-multibyte-buffer
+ (insert string)
+ (utf7-encode-internal for-imap)
+ (buffer-string))))
+
+(defun utf7-decode (string &optional for-imap)
+ "Decode UTF-7 STRING. Use IMAP modification if FOR-IMAP is non-nil."
+ (if (and (coding-system-p 'utf-7) (coding-system-p 'utf-7-imap))
+ ;; Emacs 23 with proper support for IMAP
+ (decode-coding-string string (if for-imap 'utf-7-imap 'utf-7))
+ (mm-with-unibyte-buffer
+ (insert string)
+ (utf7-decode-internal for-imap)
+ (mm-enable-multibyte)
+ (buffer-string))))
+
+(provide 'utf7)
+
+;;; utf7.el ends here
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