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+;;; rfc6068.el --- support for rfc6068  -*- lexical-binding: t -*-
+
+;; Copyright (C) 2021 Free Software Foundation, Inc.
+
+;; 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 <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+;;; Code:
+
+(defun rfc6068-unhexify-string (string)
+  "Unhexify STRING -- e.g. `hello%20there' -> `hello there'."
+  (decode-coding-string
+   (with-temp-buffer
+     (set-buffer-multibyte nil)
+     (insert string)
+     (goto-char (point-min))
+     (while (re-search-forward "%\\([[:xdigit:]]\\{2\\}\\)" nil t)
+       (replace-match (string (string-to-number (match-string 1) 16)) t t))
+     (buffer-string))
+   'utf-8))
+
+(defun rfc6068-parse-mailto-url (mailto-url)
+  "Parse MAILTO-URL, and return an alist of header-name, header-value pairs.
+MAILTO-URL should be a RFC 6068 (mailto) compliant url.  A cons cell w/ a
+key of `Body' is a special case and is considered a header for this purpose.
+The returned alist is intended for use w/ the `compose-mail' interface.
+Note: make sure MAILTO-URL has been \"unhtmlized\" (e.g., &amp; -> &), before
+calling this function."
+  (let ((case-fold-search t)
+	headers-alist)
+    (setq mailto-url (string-replace "\n" " " mailto-url))
+    (when (string-match "^\\(mailto:\\)\\([^?]+\\)?\\(\\?\\(.*\\)\\)*"
+                        mailto-url)
+      (let ((address (match-string 2 mailto-url))
+            (query (match-string 4 mailto-url)))
+	;; Build alist of header name-value pairs.
+	(when query
+	  (setq headers-alist
+		(mapcar
+		 (lambda (x)
+		   (let* ((pair (split-string x "="))
+			  (name (car pair))
+			  (value (cadr pair)))
+		     ;; Return ("Header-Name" . "header-value").
+		     (cons
+		      (capitalize (rfc6068-unhexify-string name))
+		      (rfc6068-unhexify-string value))))
+		 (split-string query "&"))))
+
+	(when address
+	  (setq address (rfc6068-unhexify-string address))
+	  ;; Deal with multiple 'To' recipients.
+	  (if-let ((elem (assoc "To" headers-alist)))
+	      (setcdr elem (concat address ", " (cdr elem)))
+            (push (cons "To" address) headers-alist)))
+
+	headers-alist))))
+
+(provide 'rfc6068)
+
+;;; rfc6068.el ends here