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+;;; eudc-capf.el --- EUDC - completion-at-point bindings  -*- lexical-binding:t -*-
+
+;; Copyright (C) 2022 Free Software Foundation, Inc.
+;;
+;; Author: Alexander Adolf
+;;
+;; 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:
+
+;;    This library provides functions to deliver email addresses from
+;;    EUDC search results to `completion-at-point'.
+;;
+;;    Email address completion will likely be desirable only in
+;;    situations where designating email recipients plays a role, such
+;;    as when composing or replying to email messages, or when posting
+;;    to newsgroups, possibly with copies of the post being emailed.
+;;    Hence, modes relevant in such contexts, such as for example
+;;    `message-mode' and `mail-mode', often at least to some extent
+;;    provide infrastructure for different functions to be called when
+;;    completing in certain message header fields, or in the body of
+;;    the message.  In other modes for editing email messages or
+;;    newsgroup posts, which do not provide such infrastructure, any
+;;    completion function providing email addresses will need to check
+;;    whether the completion attempt occurs in an appropriate context
+;;    (that is, in a relevant message header field) before providing
+;;    completion candidates.  Two mechanisms are thus provided by this
+;;    library.
+;;
+;;    The first mechanism is intended for use by the modes listed in
+;;    `eudc-capf-modes', and relies on these modes adding
+;;    `eudc-capf-complete' to `completion-at-point-functions', as
+;;    would be usually done for any general-purpose completion
+;;    function.  In this mode of operation, and in order to offer
+;;    email addresses only in contexts where the user would expect
+;;    them, a check is performed whether point is on a line that is a
+;;    message header field suitable for email addresses, such as for
+;;    example "To:", "Cc:", etc.
+;;
+;;    The second mechanism is intended for when the user modifies
+;;    `message-completion-alist' to replace `message-expand-name' with
+;;    the function `eudc-capf-message-expand-name'.  As a result,
+;;    minibuffer completion (`completing-read') for email addresses
+;;    would no longer enabled in `message-mode', but
+;;    `completion-at-point' (in-buffer completion) only.
+
+;;; Usage:
+
+;;    In a major mode, or context where you want email address
+;;    completion, you would do something along the lines of:
+;;
+;;    (require 'eudc-capf)
+;;    (add-hook 'completion-at-point-functions #'eudc-capf-complete -1 t)
+;;
+;;    The minus one argument puts it at the front of the list so it is
+;;    called first, and the t value for the LOCAL parameter causes the
+;;    setting to be buffer local, so as to avoid modifying any global
+;;    setting.
+;;
+;;    The value of the variable `eudc-capf-modes' indicates which
+;;    major modes do such a setup as part of their initialisation
+;;    code.
+
+;;; Code:
+
+(require 'eudc)
+
+(defvar message-email-recipient-header-regexp)
+(defvar mail-abbrev-mode-regexp)
+(declare-function mail-abbrev-in-expansion-header-p "mailabbrev" ())
+
+(defconst eudc-capf-modes '(message-mode)
+  "List of modes in which email address completion is to be attempted.")
+
+;; completion functions
+
+;;;###autoload
+(defun eudc-capf-complete ()
+  "Email address completion function for `completion-at-point-functions'.
+
+This function checks whether the current major mode is one of the
+modes listed in `eudc-capf-modes', and whether point is on a line
+with a message header listing email recipients, that is, a line
+whose beginning matches `message-email-recipient-header-regexp',
+and, if the check succeeds, searches for records matching the
+words before point.
+
+The return value is either nil when no match is found, or a
+completion table as required for functions listed in
+`completion-at-point-functions'."
+  (if (and (seq-some #'derived-mode-p eudc-capf-modes)
+           (let ((mail-abbrev-mode-regexp message-email-recipient-header-regexp))
+             (mail-abbrev-in-expansion-header-p)))
+      (eudc-capf-message-expand-name)))
+
+;;;###autoload
+(defun eudc-capf-message-expand-name ()
+  "Email address completion function for `message-completion-alist'.
+
+When this function is added to `message-completion-alist',
+replacing any existing entry for `message-expand-name' there,
+with an appropriate regular expression such as for example
+`message-email-recipient-header-regexp', then EUDC will be
+queried for email addresses, and the results delivered to
+`completion-at-point'."
+  (if (or eudc-server eudc-server-hotlist)
+      (progn
+        (let* ((beg (save-excursion
+                      (re-search-backward "\\([:,]\\|^\\)[ \t]*")
+                      (match-end 0)))
+               (end (point))
+               (prefix (save-excursion (buffer-substring-no-properties beg end))))
+          (list beg end
+                (completion-table-with-cache
+                 (lambda (_)
+                   (eudc-query-with-words (split-string prefix "[ \t]+") t))
+                 t))))))
+
+(provide 'eudc-capf)
+;;; eudc-capf.el ends here