bug#64202: bug#66188: 29.1; Include Atom feed reader

[Daniel Semyonov via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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>>>>> Daniel Semyonov writes:

    > I pushed a few more commits (an updated patch is attached) which fix
    > some edge cases with ephemeral groups and simplify some functions.

And a few more which fix a typo which caused duplicate groups being
created after renaming a group and relaunching Gnus, and fix the "Web
Feed Back Ends" subsection in the Gnus manual by making it a
subsubsection.  Updated patch attached.

Daniel


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From ca54f563b9dd03664225639c42b193a29947d391 Mon Sep 17 00:00:00 2001
From: Daniel Semyonov <daniel@dsemy.com>
Date: Wed, 21 Jun 2023 10:05:04 +0300
Subject: [PATCH] Gnus: Add back end for Atom feeds (nnatom)

* lisp/gnus/gnus.el (gnus-valid-select-methods): Add entry for nnatom.
* lisp/gnus/nnfeed.el: New file implementing an abstract web feed back
end.
* lisp/gnus/nnatom.el: New file implementing a back end for Atom feeds.
* doc/misc/gnus.texi (Browsing the Web, Back End Interface):
* etc/NEWS (Gnus): Document nnatom and nnfeed.
---
 doc/misc/gnus.texi  |  75 +++++
 etc/NEWS            |  10 +
 lisp/gnus/gnus.el   |   1 +
 lisp/gnus/nnatom.el | 276 ++++++++++++++++++
 lisp/gnus/nnfeed.el | 683 ++++++++++++++++++++++++++++++++++++++++++++
 5 files changed, 1045 insertions(+)
 create mode 100644 lisp/gnus/nnatom.el
 create mode 100644 lisp/gnus/nnfeed.el

diff --git a/doc/misc/gnus.texi b/doc/misc/gnus.texi
index 419a5390374..7a0e6dd79a3 100644
--- a/doc/misc/gnus.texi
+++ b/doc/misc/gnus.texi
@@ -715,6 +715,7 @@ Top
 
 * Web Searches::                Creating groups from articles that match a string.
 * RSS::                         Reading RDF site summary.
+* Atom::                        Reading Atom Syndication Format feeds.
 
 Other Sources
 
@@ -975,6 +976,7 @@ Top
 * Writing New Back Ends::       Extending old back ends.
 * Hooking New Back Ends Into Gnus::  What has to be done on the Gnus end.
 * Mail-like Back Ends::         Some tips on mail back ends.
+* Web Feed Back Ends::          Easily defining back ends for web feeds.
 
 Various File Formats
 
@@ -17252,6 +17254,7 @@ Browsing the Web
 @menu
 * Web Searches::                Creating groups from articles that match a string.
 * RSS::                         Reading RDF site summary.
+* Atom::                        Reading Atom Syndication Format feeds.
 @end menu
 
 The main caveat with all these web sources is that they probably won't
@@ -17496,6 +17499,40 @@ RSS
 @end lisp
 
 
+@node Atom
+@subsection Atom
+@cindex nnatom
+@cindex Atom
+
+Some web sites provide an Atom Syndication Format feed.  Atom is a web
+feed format similar in function to RDF Site Summary (@pxref{RSS}).
+
+The @code{nnatom} back end allows you to add HTTP or local Atom feeds
+as Gnus servers (with a single group), as you would with any other
+method, by supplying the location of the feed as the server address.
+Note, however, that the server address shouldn't be prefixed with
+@url{http://} or @url{https://}.
+
+Features of @code{nnatom} include:
+
+@itemize @bullet
+
+@item
+Server data is saved per-server in the @file{atom} sub-directory of
+@file{gnus-directory}.
+
+@item
+An article part is generated for both the summary and the content for
+each entry in the feed.  Content of all MIME types should be displayed
+correctly through Gnus (as long as they are supported and the feed
+specifies a MIME type).
+
+@item
+Article modification and publish dates are tracked, and articles are
+updated if changed.
+
+@end itemize
+
 @node Other Sources
 @section Other Sources
 
@@ -29997,6 +30034,7 @@ Back End Interface
 * Writing New Back Ends::       Extending old back ends.
 * Hooking New Back Ends Into Gnus::  What has to be done on the Gnus end.
 * Mail-like Back Ends::         Some tips on mail back ends.
+* Web Feed Back Ends::          Easily defining back ends for web feeds.
 @end menu
 
 
@@ -30770,6 +30808,43 @@ Mail-like Back Ends
 @end example
 
 
+@node Web Feed Back Ends
+@subsubsection Web Feed Back Ends
+
+If you want to write a back end for a new type of web feed (RSS,
+Atom), or some other type of feed, an ``abstract'' back end
+(@code{nnfeed}) exists to enable the creation of such back ends with
+minimal knowledge of Gnus.
+
+@code{nnfeed} defines a generic parser, which uses functions stored in
+server variables to parse information from a feed (@code{nnfeed}
+itself doesn't actually define any such functions though).
+
+The data parsed from the feed is stored in server variables (and
+stored per-feed in a sub-directory of @option{gnus-directory} whose name
+corresponds to the name of the back end).
+
+A Gnus back end interface is also defined, which uses the data parsed
+from the feed.
+
+Therefore, a new back end only needs to inherit from @code{nnfeed},
+define (fairly) generic parsing functions for the feed type and setup
+the required server variables.
+
+@code{nnfeed} was originally created to support Atom Syndication
+Format feeds (@pxref{Atom}), but is very generic (as of writing this,
+no standard web feed exists which can meaningfully use all the
+features supported): it supports multiple groups contained in a single
+feed, it allows for situations when the entire feed can't (or
+shouldn't) be read ahead of time and it allows for very advanced
+customization of the actual printing of articles from parsed data
+(while providing a reasonably powerful default method).
+
+Further implementation details are available in the documentation
+strings of the various @code{nnfeed-*} server variables and
+the commentary and other comments of @file{nnfeed.el}.
+
+
 @node Score File Syntax
 @subsection Score File Syntax
 
diff --git a/etc/NEWS b/etc/NEWS
index 19588fe8eeb..552f814ed34 100644
--- a/etc/NEWS
+++ b/etc/NEWS
@@ -1220,6 +1220,16 @@ This keyword enables the user to install packages using 'package-vc'.
 
 ** Gnus
 
++++
+*** New back end 'nnfeed'.
+This allows back end developers to easily create new back ends for web
+feeds, as inheriting back ends of 'nnfeed'.
+
++++
+*** New back end 'nnatom'.
+This allow users to add Atom Syndication Format feeds to Gnus as
+servers.
+
 *** The 'nnweb-type' option 'gmane' has been removed.
 The gmane.org website is, sadly, down since a number of years with no
 prospect of it coming back.  Therefore, it is no longer valid to set
diff --git a/lisp/gnus/gnus.el b/lisp/gnus/gnus.el
index dab66b60205..bc8819dc967 100644
--- a/lisp/gnus/gnus.el
+++ b/lisp/gnus/gnus.el
@@ -1360,6 +1360,7 @@ gnus-valid-select-methods
     ("nnimap" post-mail address prompt-address physical-address respool
      server-marks cloud)
     ("nnmaildir" mail respool address server-marks)
+    ("nnatom" address)
     ("nnnil" none))
   "An alist of valid select methods.
 The first element of each list lists should be a string with the name
diff --git a/lisp/gnus/nnatom.el b/lisp/gnus/nnatom.el
new file mode 100644
index 00000000000..e8dfa12aff5
--- /dev/null
+++ b/lisp/gnus/nnatom.el
@@ -0,0 +1,276 @@
+;;; nnatom.el --- Atom backend for Gnus -*- lexical-binding: t -*-
+
+;; Copyright (C) 2023 Free Software Foundation, Inc.
+;; Author: Daniel Semyonov <daniel@dsemy.com>
+
+;; This file is part of GNU Emacs.
+
+;; nnatom 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.
+
+;; nnatom 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 nnatom.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Gnus backend for HTTP or local feeds following the
+;; Atom Syndication Format <https://www.ietf.org/rfc/rfc4287>.
+
+;;; Code:
+
+(eval-when-compile
+  (require 'cl-lib)
+  (require 'subr-x))
+
+(require 'nnfeed)
+(require 'mm-url)
+(require 'dom)
+
+(defgroup nnatom nil
+  "Atom backend for Gnus."
+  :group 'nnfeed)
+
+(nnoo-declare nnatom nnfeed)
+
+(nnfeed-define-basic-backend-interface nnatom)
+
+;;;; Atom feed parser:
+
+(defun nnatom--read-feed (feed _)
+  "Return a list structure representing FEED, or nil."
+  (if (string-match-p "\\`https?://" feed)
+      (nnheader-report
+       nnatom-backend
+       "Address shouldn't start with \"http://\" or \"https://\"")
+    (with-temp-buffer
+      (condition-case e
+          (if (file-name-absolute-p feed)
+              (insert-file-contents feed)
+            (mm-url-insert-file-contents (concat "https://" feed)))
+        (file-error (nnheader-report nnatom-backend (cdr e)))
+        (:success (when-let ((data (if (libxml-available-p)
+                                       (libxml-parse-xml-region
+                                        (point-min) (point-max))
+                                     (car (xml-parse-region
+                                           (point-min) (point-max)))))
+                             (authors (list 'authors)))
+                    (when (eq (car data) 'top)
+                      (setq data (assq 'feed data)))
+                    (dom-add-child-before data authors)
+                    (let ((all (dom-children data)))
+                      (while-let ((rest (cdr all))
+                                  (child (car-safe rest))
+                                  (type (car-safe child))
+                                  ((not (eq type 'entry))))
+                        (and (or (eq type 'author)
+                                 (eq type 'contributor))
+                             (dom-add-child-before authors child))
+                        (setq all rest))
+                      ;; Order of entries is reversed as most Atom feeds
+                      ;; list only the "most recent" entries, in reverse
+                      ;; chronological order.
+                      (setcdr all (nreverse (cdr all))))
+                    data))))))
+(defvoo nnatom-read-feed-function #'nnatom--read-feed
+  nil nnfeed-read-feed-function)
+
+(defun nnatom--read-group (data)
+  "Return the next group and the remaining DATA in a cons cell, or nil."
+  `(,data))
+(defvoo nnatom-read-group-function #'nnatom--read-group
+  nil nnfeed-read-group-function)
+
+(defun nnatom--read-article (data _)
+  "Return the next article and the remaining DATA in a cons cell, or nil."
+  (when (eq (car data) 'feed) (setq data (dom-children data)))
+  ;; Discard any children between/after entries.
+  (while (and data (not (eq (car-safe (car data)) 'entry))) (pop data))
+  (when-let ((article (car data))
+             (auths (list 'authors)) (links (list 'links)))
+    (dom-add-child-before article links)
+    (dom-add-child-before article auths)
+    (dolist (child (cddddr article) `(,article . ,(cdr data)))
+      (pcase (car-safe child)                ; Authors and links can appear
+        ((or 'author 'contributor)           ; anywhere in the entry so we
+         (dom-add-child-before auths child)  ; collect them all here to
+         (dom-add-child-before links child)) ; avoid looping over the
+        ((or 'link                           ; entry multiple times later.
+             (and 'content (guard (assq 'src (dom-attributes child)))))
+         (dom-add-child-before links child))))))
+(defvoo nnatom-read-article-function #'nnatom--read-article
+  nil nnfeed-read-article-function)
+
+(defun nnatom--read-title (group)
+  "Return the title of GROUP, or nil."
+  (dom-text (dom-child-by-tag group 'title)))
+(defvoo nnatom-read-title-function #'nnatom--read-title
+  nil nnfeed-read-title-function)
+
+(defun nnatom--read-description (group)
+  "Return the description of GROUP, or nil."
+  (dom-text (dom-child-by-tag group 'subtitle)))
+(defvoo nnatom-read-description-function #'nnatom--read-description
+  nil nnfeed-read-description-function)
+
+(defun nnatom--read-article-or-group-authors (article-or-group)
+  "Return the authors of ARTICLE-OR-GROUP, or nil."
+  (when-let
+      ((a (mapconcat
+           (lambda (author)
+             (let* ((name (dom-text (dom-child-by-tag author 'name)))
+                    (name (unless (string-blank-p name) name))
+                    (email (dom-text (dom-child-by-tag author 'email)))
+                    (email (unless (string-blank-p email) email)))
+               (or (and name email (format "%s <%s>" name email)) name email)))
+           (dom-children (dom-child-by-tag article-or-group 'authors))
+           ", "))
+       ((not (string-blank-p a))))
+    a))
+(defvoo nnatom-read-author-function #'nnatom--read-article-or-group-authors
+  nil nnfeed-read-author-function)
+(defvoo nnatom-read-group-author-function
+    #'nnatom--read-article-or-group-authors
+  nil nnfeed-read-group-author-function)
+
+(defun nnatom--read-subject (article)
+  "Return the subject of ARTICLE, or nil."
+  (dom-text (dom-child-by-tag article 'title)))
+(defvoo nnatom-read-subject-function #'nnatom--read-subject
+  nil nnfeed-read-subject-function)
+
+(defun nnatom--read-id (article)
+  "Return the ID of ARTICLE.
+If the ARTICLE doesn't contain an ID but it does contain a subject,
+return the subject.  Otherwise, return nil."
+  (or (dom-text (dom-child-by-tag article 'id))
+      (nnatom--read-subject article)))
+(defvoo nnatom-read-id-function #'nnatom--read-id
+  nil nnfeed-read-id-function)
+
+(defun nnatom--read-publish (article)
+  "Return the date and time ARTICLE was published, or nil."
+  (when-let (d (dom-child-by-tag article 'published))
+    (date-to-time (dom-text d))))
+(defvoo nnatom-read-publish-date-function #'nnatom--read-publish
+  nil nnfeed-read-publish-date-function)
+
+(defun nnatom--read-update (article)
+  "Return the date and time of the last update to ARTICLE, or nil."
+  (when-let (d (dom-child-by-tag article 'updated))
+    (date-to-time (dom-text d))))
+(defvoo nnatom-read-update-date-function #'nnatom--read-update
+  nil nnfeed-read-update-date-function)
+
+(defun nnatom--read-links (article)
+  "Return all links contained in ARTICLE, or nil."
+  (let ((alt 0) (rel 0) (sel 0) (enc 0) (via 0) (aut 0))
+    (mapcan
+     (lambda (link)
+       (when-let ((l (car-safe link)))
+         (or
+          (when-let (((eq l 'content))
+                     (src (dom-attr link 'src))
+                     (label (concat "Link"
+                                    (and (< 1 (cl-incf alt))
+                                         (format " %s" alt)))))
+            `(((("text/plain") . ,(format "%s: %s\n" label src))
+               (("text/html") . ,(format "<a href=\"%s\">[%s]</a> "
+                                         src label)))))
+          (when-let (((or (eq l 'author) (eq l 'contributor)))
+                     (name (dom-text (dom-child-by-tag link 'name)))
+                     (name (if (string-blank-p name)
+                               (concat "Author"
+                                       (and (< 1 (cl-incf aut))
+                                            (format " %s" aut)))
+                             name))
+                     (uri (dom-text (dom-child-by-tag link 'uri)))
+                     ((not (string-blank-p uri))))
+            `(((("text/plain") . ,(format "%s: %s\n" name uri))
+               (("text/html") . ,(format "<a href=\"%s\">[%s]</a> "
+                                         uri name)))))
+          (when-let (((eq l 'link))
+                     (attrs (dom-attributes link))
+                     (label (or (cdr (assq 'title attrs))
+                                (pcase (cdr (assq 'rel attrs))
+                                  ("related"
+                                   (concat "Related"
+                                           (and (< 1 (cl-incf rel))
+                                                (format " %s" rel))))
+                                  ("self"
+                                   (concat "More"
+                                           (and (< 1 (cl-incf sel))
+                                                (format " %s" sel))))
+                                  ("enclosure"
+                                   (concat "Enclosure"
+                                           (and (< 1 (cl-incf enc))
+                                                (format " %s" enc))))
+                                  ("via"
+                                   (concat "Source"
+                                           (and (< 1 (cl-incf via))
+                                                (format " %s" via))))
+                                  (_ (if-let
+                                         ((lang (cdr (assq 'hreflang link))))
+                                         (format "Link (%s)" lang)
+                                       (concat
+                                        "Link"
+                                        (and (< 1 (cl-incf alt))
+                                             (format " %s" alt))))))))
+                     (link (cdr (assq 'href attrs))))
+            `(((("text/plain") . ,(format "%s: %s\n" label link))
+               (("text/html") . ,(format "<a href=\"%s\">[%s]</a> "
+                                         link label))))))))
+     (dom-children (dom-child-by-tag article 'links)))))
+(defvoo nnatom-read-links-function #'nnatom--read-links
+  nil nnfeed-read-links-function)
+
+(defun nnatom--read-part (part type)
+  (let* ((atypes '("html" "plain"))
+         (mtypes '(("xhtml" . "text/html") ("text" . "text/plain")))
+         (xsuff (concat "[+/]xml\\(-\\(dtd\\|external-parsed-entity\\)\\)?\\'"
+                        "\\|^text"))
+         (part (if (string= type "xhtml")
+                   (with-temp-buffer
+                     (dom-print (dom-child-by-tag part 'div) nil t)
+                     (buffer-substring-no-properties
+                      (point-min) (point-max)))
+                 (dom-text part)))
+         (type (if (member type atypes) (concat "text/" type) type))
+         (type (or (cdr (assoc type mtypes)) type)))
+    (unless (string-blank-p part)
+      `(,part (Content-Type . ,(or type (setq type "text/plain")))
+              ,(and (not (string-match-p xsuff type))
+                    '(Content-Transfer-Encoding . "base64"))))))
+
+(defun nnatom--read-parts (article)
+  "Return all parts contained in ARTICLE, or an empty HTML part with links."
+  (let* ((summary (dom-child-by-tag article 'summary))
+         (stype (cdr (assq 'type (dom-attributes summary))))
+         (summary (nnatom--read-part summary stype))
+         (content (dom-child-by-tag article 'content))
+         (ctype (cdr (assq 'type (dom-attributes content))))
+         (content (nnatom--read-part content ctype))
+         (st (string= stype ctype))
+         parts)
+    (cond ((and summary content)
+           (and st (push summary parts))
+           (push (append content '(links)) parts)
+           (or st (push summary parts)))
+          ((setq content (or summary content))
+           (push (append content '(links)) parts))
+          (t (push '((nil (Content-Type . "text/html") links)) parts)))
+    parts))
+(defvoo nnatom-read-parts-function #'nnatom--read-parts
+  nil nnfeed-read-parts-function)
+
+(gnus-declare-backend (symbol-name nnatom-backend) 'address)
+
+(provide 'nnatom)
+
+;;; nnatom.el ends here
diff --git a/lisp/gnus/nnfeed.el b/lisp/gnus/nnfeed.el
new file mode 100644
index 00000000000..0bf599553e4
--- /dev/null
+++ b/lisp/gnus/nnfeed.el
@@ -0,0 +1,683 @@
+;;; nnfeed.el --- Generic feed backend for Gnus -*- lexical-binding: t -*-
+
+;; Copyright (C) 2023 Free Software Foundation, Inc.
+;; Author: Daniel Semyonov <daniel@dsemy.com>
+
+;; This file is part of GNU Emacs.
+
+;; nnfeed 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.
+
+;; nnfeed 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 nnfeed.  If not, see <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; Generic Gnus backend (intended) for implementing backends for web
+;; feeds (Atom, RSS).
+;;
+;; This backend is abstract - it doesn't implement a parser for any
+;; specific web feed type, and so can't be used independently.
+;;
+;; Instead, it implements a generic parser, feed data storage and most
+;; of the Gnus backend interface; the intended use for this backend is
+;; to be a source of inheritance for backends supporting new web feed
+;; types.
+;;
+;; To implement new backends, use `nnfeed-define-basic-backend-interface':
+;;
+;;    ...
+;;    (require 'nnfeed)
+;;
+;;    (nnoo-declare nnfoo nnfeed)
+;;
+;;    (nnfeed-define-basic-backend-interface nnfoo)
+;;    ...
+;;    [  definitions of parsing functions, see the "Feed parser interface"
+;;       section for more information.  ]
+;;
+;;    (defvoo nnfoo-read-feed-function #'nnfoo--read-feed
+;;     nil nnfeed-read-feed-function)
+;;    ...
+;;    (gnus-declare-backend (symbol-name nnfeed-backend) 'address)
+;;
+;;    (provide 'nnfoo)
+;;
+;;; Code:
+
+(eval-when-compile
+  (require 'subr-x))
+
+(require 'gnus)
+(require 'nnoo)
+
+(defgroup nnfeed nil
+  "Generic feed backend for Gnus."
+  :group 'gnus)
+
+(defcustom nnfeed-date-format "%F %X%p"
+  "Format of displayed dates (see function `format-time-string')."
+  :type 'string)
+
+(nnoo-declare nnfeed)
+
+(defvoo nnfeed-backend nil
+  "Symbol which identifies this backend.")
+
+(defvoo nnfeed-status-string nil
+  "Last status message reported by this backend.")
+
+;; This macro should be used to define inheriting backends.
+
+(defmacro nnfeed-define-basic-backend-interface (backend)
+  "Define a basic set of functions and variables for BACKEND."
+  `(progn
+     (defvoo ,(nnoo-symbol backend 'backend) ',backend nil nnfeed-backend)
+     (defvoo ,(nnoo-symbol backend 'status-string)
+         nil nil nnfeed-status-string)
+     (defvoo ,(nnoo-symbol backend 'group) nil nil nnfeed-group)
+     (defvoo ,(nnoo-symbol backend 'servers)
+         (make-hash-table :test 'equal) nil nnfeed-servers)
+     (defvoo ,(nnoo-symbol backend 'group-article-ids)
+         (make-hash-table :test 'equal) nil nnfeed-group-article-ids)
+     (defvoo ,(nnoo-symbol backend 'group-articles)
+         (make-hash-table :test 'eql) nil nnfeed-group-articles)
+     (defvoo ,(nnoo-symbol backend 'group-article-max-num) 0 nil
+             nnfeed-group-article-max-num)
+     (defvoo ,(nnoo-symbol backend 'group-article-mix-num) 1 nil
+             nnfeed-group-article-min-num)
+     ,@(mapcar (lambda (fun)
+                 `(deffoo ,(nnoo-symbol backend fun) (&optional server)
+	            (,(nnoo-symbol 'nnoo fun) ',backend server)))
+	       '(server-opened status-message))
+     (deffoo ,(nnoo-symbol backend 'open-server) (server &optional defs)
+       (nnfeed-open-server server defs ',backend))
+     (nnoo-import ,backend (nnfeed))))
+
+;;;; Feed parser interface:
+
+;; The following set of server variables define a parser for a
+;; specific web feed type.
+;; An inheriting backend doesn't necessarily have to define all of
+;; these functions (see the comments below for more information).
+;; Apart from this set of variables there is also
+;; `nnfeed-print-content-function' which can optionally be defined
+;; by an inheriting backend to allow more advanced control over the
+;; printing of articles.
+
+(defvoo nnfeed-read-feed-function #'ignore
+  "Function returning a Lisp object representing a feed (or part of it).
+
+It should accept two arguments, the address of a feed and the name of
+a group (or nil).
+If a group name is supplied, it should return a representation of only
+the group (as if it was extracted from the feed with
+`nnfeed-read-group-function').")
+
+(defvoo nnfeed-read-group-function #'ignore
+  "Function returning a cons cell of a group and remaining data from a feed.
+
+The returned group can be represented by any Lisp object.
+It should accept a single argument, a Lisp object representing a feed
+\(as can be returned by this function or `nnfeed-read-feed-function').")
+
+(defvoo nnfeed-read-article-function #'ignore
+  "Function returning a cons cell of an article and remaining data from a group.
+
+The returned article can be represented by any Lisp object.
+It should accept two arguments, a Lisp object representing a group
+\(as can be returned by this function or `nnfeed-read-group-function'),
+and a flag indicating whether the last article was not new or updated.")
+
+(defvoo nnfeed-read-title-function #'ignore
+  "Function returning the title of a group (a string).
+
+It should accept a single argument, a Lisp object representing a group
+\(as returned by `nnfeed-read-group-function').")
+
+;; Optional.
+(defvoo nnfeed-read-description-function #'ignore
+  "Function returning the description of a group (a string), or nil.
+
+It should accept a single argument, a Lisp object representing a group
+\(as returned by `nnfeed-read-group-function').")
+
+;; Either this function or `nnfeed-read-author-function' is required.
+(defvoo nnfeed-read-group-author-function #'ignore
+  "Function returning the author of a group (a string), or nil.
+
+It should accept a single argument, a Lisp object representing a group
+\(as returned by `nnfeed-read-group-function')..")
+
+(defvoo nnfeed-read-id-function #'ignore
+  "Function returning the ID of an article.
+
+It should accept a single argument, a Lisp object representing an article
+\(as returned by `nnfeed-read-article-function').")
+
+(defvoo nnfeed-read-subject-function #'ignore
+  "Function returning the subject of an article (a string), or nil.
+
+It should accept a single argument, a Lisp object representing an article
+\(as returned by `nnfeed-read-article-function').")
+
+;; Either this function or `nnfeed-read-update-date-function' is required.
+(defvoo nnfeed-read-publish-date-function #'ignore
+  "Function returning the publish date of an article (a time value), or nil.
+
+It should accept a single argument, a Lisp object representing an article
+\(as returned by `nnfeed-read-article-function').")
+
+;; Either this function or `nnfeed-read-publish-date-function' is required.
+(defvoo nnfeed-read-update-date-function #'ignore
+  "Function returning the update date of an article (a time value), or nil.
+
+It should accept a single argument, a Lisp object representing an article
+\(as returned by `nnfeed-read-article-function').")
+
+;; Either this function or `nnfeed-read-group-author-function' is required.
+(defvoo nnfeed-read-author-function #'ignore
+  "Function returning the author of an article (a string), or nil.
+
+It should accept a single argument, a Lisp object representing an article
+\(as returned by `nnfeed-read-article-function').")
+
+(defvoo nnfeed-read-headers-function #'ignore
+  "Function returning an alist of article-wide MIME headers.
+
+Each element of this alist maps a MIME header (a symbol,
+i.e. `Content-Type') to its value.  As a special case, `:boundary'
+maps to a string which will serve as the boundary between article
+parts.  This must be supplied if a custom boundary is used in a
+multipart content type header.  The default boundary is \"-_nnfeed_-\",
+and is automatically modified to match the name of the back end.
+It should accept a single argument, a Lisp object representing an article
+\(as returned by `nnfeed-read-article-function').")
+
+;; As mentioned in their docstrings, the last two parsing functions
+;; can optionally return any Lisp representation they want, provided
+;; an appropriate `nnfeed-print-content-function' is defined.  This
+;; means they are also not _strictly_ required.
+
+(defvoo nnfeed-read-links-function #'ignore
+  "Function returning all links contained in an article.
+
+With the default `nnfeed-print-content-function', it should return a
+list of links, where each link is an alist mapping MIME content types
+to links formatted for display in a part of that type.  Each content
+type may also be a list of content types.
+Otherwise, it could return any Lisp object.
+It should accept a single argument, a Lisp object representing an article
+\(as returned by `nnfeed-read-article-function').")
+
+(defvoo nnfeed-read-parts-function #'ignore
+  "Function returning an alist associating parts of an article to their headers.
+
+With the default `nnfeed-print-content-function', each part should be a
+string.  Otherwise, it can be any Lisp object.  The \"headers\" of
+each part should be a list where each element is either a cons of a
+MIME header (a symbol, i.e. `Content-Type') and its value (a string),
+or any other Lisp object.  MIME headers will be printed, the rest will
+be passed on to `nnfeed-print-content-function', which recognizes the
+following extra data by default:
+- `links', if present, will cause links to be printed in the part.
+It should accept a single argument, a Lisp object representing an article
+\(as returned by `nnfeed-read-article-function').")
+
+;;;; Feed data storage:
+
+;; This section defines the data types used to store feed data, and
+;; functions to read and write it.
+;; All variables in this section are automatically defined by
+;; `nnfeed-define-basic-backend-interface'.
+
+(defvoo nnfeed-servers (make-hash-table :test 'equal)
+  "Hash table mapping known servers to their groups.
+
+Each value in this table should itself be a hash table mapping known
+group names to their data, which should be a vector of the form
+[GROUP IDS ARTICLES MAX MIN DESCRIPTION], where:
+- GROUP is the \"real\" group name (the name known to the server).
+- IDS is a hash table mapping article IDs to their numbers.
+- ARTICLES is a hash table mapping article numbers to articles and
+  their attributes (see `nnfeed-group-articles').
+- MAX is the maximum article number.
+- MIN is the minimum article number.
+- DESCRIPTION is the group description.")
+
+(defvoo nnfeed-group-names (make-hash-table :test 'equal)
+  "Hash table mapping real group names to their custom name.")
+
+(defun nnfeed--server-address (server)
+  "Return SERVER's real address."
+  (if (string-suffix-p "-ephemeral" server)
+      (setq server (or (cadr (assq (nnoo-symbol nnfeed-backend 'address)
+                                   (cddr (gnus-server-to-method
+                                          (concat
+                                           (symbol-name nnfeed-backend) ":"
+                                           server)))))
+                       server))
+    server))
+
+(defun nnfeed--server-file (server)
+  "Return the file containing data for SERVER."
+  (expand-file-name (format "%s/%s.eld"
+                            (string-trim (symbol-name nnfeed-backend)
+                                         "nn")
+                            (gnus-newsgroup-savable-name
+                             (nnfeed--server-address server)))
+                    gnus-directory))
+
+(defun nnfeed--read-server (server)
+  "Read SERVER's information from storage."
+  (if-let ((f (nnfeed--server-file server))
+           ((file-readable-p f)))
+      (with-temp-buffer
+        (insert-file-contents f)
+        (goto-char (point-min))
+        (puthash server (read (current-buffer)) nnfeed-servers))
+    (nnheader-report nnfeed-backend "Can't read %s" server)))
+
+(defun nnfeed--write-server (server)
+  "Write SERVER's information to storage."
+  (if-let ((f (nnfeed--server-file server))
+           ((file-writable-p f)))
+      (if-let ((s (gethash server nnfeed-servers))
+               ((hash-table-p s)))
+          (with-temp-file f
+            (insert ";;;; -*- mode: lisp-data -*- DO NOT EDIT\n")
+            (prin1 s (current-buffer))
+            (insert "\n")
+            t)
+        t)
+    (nnheader-report nnfeed-backend "Can't write %s" f)))
+
+;; The following function uses the parsing functions defined in the last
+;; section to parse a feed (or just one group from it).
+;; This is the only place where these parsing functions are used; the Gnus
+;; backend interface extracts all required information from the parsed feed.
+
+(defun nnfeed--parse-feed (feed &optional group)
+  "Parse GROUP from FEED into a new or existing server.
+If GROUP is omitted or nil, parse the entire FEED."
+  (let* ((feed (nnfeed--server-address feed))
+         (s (or (gethash feed nnfeed-servers) (nnfeed--read-server feed)
+                (make-hash-table :test 'equal)))
+         (name group) ; (Maybe) fake name (or nil)
+         (group (aref (gethash group s `[,group]) 0)) ; Real name (or nil)
+         data)
+    (when (setq data (funcall nnfeed-read-feed-function feed group))
+      (while-let ((cg (or (and name (cons data)) ; `data' is a single group
+                          (funcall nnfeed-read-group-function data)))
+                  (cg (prog1 (car cg) (setq data (cdr cg)))))
+        (let* ((name (funcall nnfeed-read-title-function cg)) ; Real name
+               (group (gethash name nnfeed-group-names name)) ; (Maybe) fake name
+               (info (gnus-get-info
+                      (concat (symbol-name nnfeed-backend) "+" feed ":" group)))
+               (g (or (gethash group s)
+                      `[ ,name ,(make-hash-table :test 'equal)
+                         ,(make-hash-table :test 'eql) nil 1 ""]))
+               (desc (funcall nnfeed-read-description-function cg))
+               (ids (aref g 1))
+               (articles (aref g 2))
+               (max (aref g 3))
+               (max (if max max
+                      (setq max 0) ; Find max article number
+                      (dolist      ; remembered by Gnus.
+                          ( r (cons (gnus-info-read info)
+                                    (gnus-info-marks info))
+                            max)
+                        (mapc (lambda (x)
+                                (let ((x (if (consp x)
+                                             (if (< (car x) (cdr x))
+                                                 (cdr x) (car x))
+                                           x)))
+                                  (when (< max x) (setq max x))))
+                              (if (symbolp (car r)) (cdr r) r)))))
+               (group-author (funcall nnfeed-read-group-author-function cg))
+               stale)
+          (and desc (aset g 5 desc))
+          (while-let ((article (funcall nnfeed-read-article-function cg stale))
+                      (article (prog1 (car article) (setq cg (cdr article)))))
+            (when-let ((id (funcall nnfeed-read-id-function article))
+                       (id (format "<%s@%s.%s>" id name nnfeed-backend)))
+              (let* ((num (gethash id ids))
+                     (update (funcall nnfeed-read-update-date-function article))
+                     (prev-update (aref (gethash num articles
+                                                 '[nil nil nil nil nil])
+                                        4)))
+                (if (or (null num) ; New article ID.
+                        (and (null prev-update) update)
+                        (and prev-update update
+                             (time-less-p prev-update update)))
+                    (let* ((num (or num (aset g 3 (setq max (1+ max)))))
+                           (publish (funcall nnfeed-read-publish-date-function
+                                             article)))
+                      (setf
+                       (gethash id (aref g 1)) num
+                       (gethash num (aref g 2))
+                       `[ ,id
+                          ,(or (funcall nnfeed-read-author-function article)
+                               group-author group)
+                          ,(or (funcall nnfeed-read-subject-function article)
+                               "no subject")
+                          ,(or publish update '(0 0)) ; published
+                          ,(or update publish '(0 0)) ; updated
+                          ,(funcall nnfeed-read-links-function article)
+                          ,(funcall nnfeed-read-parts-function article)
+                          ,(funcall nnfeed-read-headers-function article)]
+                       stale nil))
+                  (setq stale t)))))
+          (puthash group g s)))
+      (puthash feed s nnfeed-servers))))
+
+;;;; Gnus backend functions:
+
+;; The following two sections define a Gnus backend interface based on
+;; the parsed data from the last section.
+;; All server variables in this section are automatically defined by
+;; `nnfeed-define-basic-backend-interface'.
+;; For more information about these functions see the "Back End
+;; Interface" section of the Gnus manual.
+
+(defvoo nnfeed-group nil
+  "Name of the current group.")
+
+(defvoo nnfeed-group-article-ids (make-hash-table :test 'equal)
+  "Hash table mapping article IDs to their article number.")
+
+(defvoo nnfeed-group-articles (make-hash-table :test 'eql)
+  "Hash table mapping article numbers to articles and their attributes.
+
+Each value in this table should be a vector of the form
+[ID FROM SUBJECT DATE UPDATED LINKS PARTS HEADERS], where:
+- ID is the ID of the article.
+- FROM is the author of the article or group.
+- SUBJECT is the subject of the article.
+- DATE is the date the article was published, or last updated (time value).
+- UPDATE is the date the article was last updated, or published (time value).
+- LINKS is a collection of links (any Lisp object).
+- PARTS is an alist associating the content of each part of the
+  article to its headers.
+- HEADERS is an alist associating article-wide MIME headers to their value.")
+
+(defvoo nnfeed-group-article-max-num 0
+  "Maximum article number for the current group.")
+
+(defvoo nnfeed-group-article-min-num 1
+  "Minimum article number for the current group.")
+
+(nnoo-define-basics nnfeed)
+
+(defun nnfeed--current-server-no-prefix ()
+  "Remove the \"<backend>+\" prefix from the current server."
+  (string-remove-prefix (concat (symbol-name nnfeed-backend) "+")
+                        (nnoo-current-server nnfeed-backend)))
+
+(defun nnfeed--group-data (group server)
+  "Get parsed data for GROUP from SERVER."
+  (when-let ((server (nnfeed--server-address server))
+             (s (gethash server nnfeed-servers))
+             ((hash-table-p s)))
+    (gethash group s)))
+
+(defun nnfeed-retrieve-article (article group)
+  "Retrieve headers for ARTICLE from GROUP."
+  (if-let ((a (gethash article (aref group 2))))
+      (insert (format "221 %s Article retrieved.
+From: %s\nSubject: %s\nDate: %s\nMessage-ID: %s\n.\n"
+                      article
+                      (aref a 1)
+                      (aref a 2)
+                      (format-time-string "%F %H:%M" (aref a 3))
+                      (aref a 0)))
+    (insert "404 Article not found.\n.\n")))
+
+(deffoo nnfeed-retrieve-headers (articles &optional group server _fetch-old)
+  (if-let ((server (or server (nnfeed--current-server-no-prefix)))
+           (g (or (nnfeed--group-data group server)
+                  `[ nil ,nnfeed-group-article-ids ,nnfeed-group-articles
+                     nil nil nil])))
+      (with-current-buffer nntp-server-buffer
+        (erase-buffer)
+        (or (and (stringp (car articles))
+                 (mapc (lambda (a)
+                         (nnfeed-retrieve-article
+                          (gethash a (aref g 2)) g))
+                       articles))
+            (and (numberp (car articles))
+                 (range-map (lambda (a) (nnfeed-retrieve-article a g))
+                            articles)))
+        'headers)
+    (nnheader-report nnfeed-backend "Group %s not found" (or group ""))))
+
+(deffoo nnfeed-open-server (server &optional defs backend)
+  (let ((backend (or backend 'nnfeed))
+        (a (nnfeed--server-address server))
+        s)
+    (nnoo-change-server backend server defs)
+    (when (setq s (or (gethash a nnfeed-servers) (nnfeed--read-server server)))
+      (maphash (lambda (group g)
+                 (setq g (aref g 0))
+                 (unless (string= group g)
+                   (puthash g group nnfeed-group-names)))
+               s))
+    (setq a (nnfeed--server-file server))
+    (or s (condition-case _ (make-directory (file-name-parent-directory a) t)
+            (:success (file-writable-p a))
+            (t nil))
+        (and (nnoo-close-server nnfeed-backend server)
+             (nnheader-report
+              nnfeed-backend "Server file %s not readable or writable"
+              server)))))
+
+(deffoo nnfeed-request-close ()
+  (when (hash-table-p nnfeed-servers)
+    (maphash (lambda (server _) (nnfeed--write-server server)) nnfeed-servers)
+    (setq nnfeed-servers (make-hash-table :test 'equal)))
+  (setq nnfeed-status-string nil)
+  t)
+
+;; The default content printing function, which should be suitable for
+;; most inheriting backends.
+
+(defun nnfeed--print-content (content attributes links)
+  "Return CONTENT formatted according to ATTRIBUTES, with LINKS possibly added."
+  (let ((links (and (memq 'links attributes) links)))
+    (when (or content links)
+      (concat
+       (and content (format "%s\n\n" content))
+       (mapconcat (lambda (link)
+                    (cdr (assoc (cdr (assq 'Content-Type attributes)) link
+                                (lambda (types type)
+                                  (if (stringp types) (string= types type)
+                                    (member type types))))))
+                  links)))))
+
+(defvoo nnfeed-print-content-function #'nnfeed--print-content
+  "Function returning a single piece of content for an article (a string).
+
+It should accept three arguments, a part and its attributes (as returned
+by `nnfeed-read-parts-function'), and links (as returned by
+`nnfeed-read-links-function').")
+
+(defun nnfeed--print-part (content headers mime links)
+  "Print part of an article using its CONTENT, HEADERS, and LINKS.
+Only HEADERS of a type included in MIME are considered."
+  (concat
+   (mapconcat (lambda (header)
+                (when-let ((m (car-safe header))
+                           ((member m mime)))
+                  (format "%s: %s\n" m (cdr header))))
+              headers)
+   "\n"
+   (funcall nnfeed-print-content-function content headers links)))
+
+(deffoo nnfeed-request-article (article &optional group server to-buffer)
+  (if-let ((server (or server (nnfeed--current-server-no-prefix)))
+           (g (or (nnfeed--group-data group server)
+                  (and (setq group nnfeed-group)
+                       `[ nil ,nnfeed-group-article-ids
+                          ,nnfeed-group-articles
+                          ,nnfeed-group-article-max-num
+                          ,nnfeed-group-article-min-num nil])))
+           (num (or (and (stringp article)
+                         (gethash article (aref g 1)))
+                    (and (numberp article) article)))
+           ((and (<= num (aref g 3))
+                 (>= num (aref g 4))))
+           (a (gethash num (aref g 2))))
+      (with-current-buffer (or to-buffer nntp-server-buffer)
+        (erase-buffer)
+        (let* ((links (aref a 5))
+               (parts (aref a 6))
+               (headers (aref a 7))
+               (boundary (or (cdr (assq :boundary headers))
+                             (format "-_%s_-" nnfeed-backend)))
+               (multi (length> parts 1))
+               (mime '( Content-Type Content-Disposition
+                        Content-Transfer-Encoding)))
+          (insert (format
+                   "Subject: %s\nFrom: %s\nDate: %s\nMessage-ID: %s\n"
+                   (aref a 2) (aref a 1)
+                   (format-time-string
+                    nnfeed-date-format (or (aref a 3) '(0 0)))
+                   (aref a 0))
+                  (if (assq 'MIME-Version headers) "" "MIME-Version: 1.0\n")
+                  (mapconcat (lambda (header)
+                               (unless (keywordp (car header))
+                                 (format "%s: %s\n" (car header) (cdr header))))
+                             headers)
+                  (if multi
+                      (if (assq 'Content-Type headers) ""
+                        (format
+                         "Content-Type: multipart/alternative; boundary=%s\n"
+                         boundary))
+                    (prog1 (nnfeed--print-part
+                            (caar parts) (cdar parts) mime links)
+                      (setq parts nil)))
+                  (mapconcat (lambda (part)
+                               (format "--%s\n%s\n" boundary
+                                       (nnfeed--print-part
+                                        (car part) (cdr part) mime links)))
+                             parts)
+                  (if multi (format "--%s--" boundary) "\n")))
+        `(,group . ,num))
+    (nnheader-report nnfeed-backend "No such article")))
+
+(deffoo nnfeed-request-group (group &optional server fast _info)
+  (with-current-buffer nntp-server-buffer
+    (erase-buffer)
+    (if-let ((server (or server (nnfeed--current-server-no-prefix)))
+             (g (or (if fast (nnfeed--group-data group server)
+                      (setq server (nnfeed--parse-feed server group))
+                      (and (hash-table-p server) (gethash group server)))
+                    `[ ,group ,(make-hash-table :test 'equal)
+                       ,(make-hash-table :test 'eql) 0 1 ""])))
+        (progn
+          (setq nnfeed-group group
+                nnfeed-group-article-ids (aref g 1)
+                nnfeed-group-articles (aref g 2)
+                nnfeed-group-article-max-num (aref g 3)
+                nnfeed-group-article-min-num (aref g 4))
+          (insert (format "211 %s %s %s \"%s\""
+                          (hash-table-count nnfeed-group-article-ids)
+                          nnfeed-group-article-min-num
+                          nnfeed-group-article-max-num group))
+          t)
+      (insert "404 group not found")
+      (nnheader-report nnfeed-backend "Group %s not found" group))))
+
+(deffoo nnfeed-close-group (group &optional server)
+  (and (string= group nnfeed-group)
+       (setq nnfeed-group nil
+             nnfeed-group-article-ids (make-hash-table :test 'equal)
+             nnfeed-group-articles (make-hash-table :test 'eql)
+             nnfeed-group-article-max-num 0
+             nnfeed-group-article-min-num 1))
+  (setq server (or server (nnfeed--current-server-no-prefix)))
+  (nnfeed--write-server server))
+
+(deffoo nnfeed-request-list (&optional server)
+  (with-current-buffer nntp-server-buffer
+    (erase-buffer)
+    (when-let ((p (point))
+               (s (nnfeed--parse-feed
+                   (or server (nnfeed--current-server-no-prefix))))
+               ((hash-table-p s)))
+      (maphash (lambda (group g)
+                 (insert (format "\"%s\" %s %s y\n"
+                                 group (aref g 3) (aref g 4))))
+               s)
+      (not (= (point) p)))))
+
+(deffoo nnfeed-request-post (&optional _server)
+  (nnheader-report nnfeed-backend "%s is a read only backend" nnfeed-backend))
+
+;;;; Optional back end functions:
+
+(deffoo nnfeed-retrieve-groups (_groups &optional server)
+  (nnfeed-request-list server)
+  'active)
+
+(deffoo nnfeed-request-type (_group &optional _article)
+  'unknown)
+
+(deffoo nnfeed-request-group-description (group &optional server)
+  (when-let ((server (or server (nnfeed--current-server-no-prefix)))
+             (g (nnfeed--group-data group server)))
+    (with-current-buffer nntp-server-buffer
+      (erase-buffer)
+      (insert group "	" (aref g 5) "\n"))))
+
+(deffoo nnfeed-request-list-newsgroups (&optional server)
+  (when-let ((server (or server (nnfeed--current-server-no-prefix)))
+             (s (gethash (nnfeed--server-address server) nnfeed-servers))
+             ((hash-table-p s)))
+    (with-current-buffer nntp-server-buffer
+      (erase-buffer)
+      (maphash (lambda (group g)
+                 (insert group "	" (aref g 5) "\n"))
+               s))))
+
+(deffoo nnfeed-request-rename-group (group new-name &optional server)
+  (when-let ((server (or server (nnfeed--current-server-no-prefix)))
+             (a (nnfeed--server-address server))
+             (s (or (gethash a nnfeed-servers)
+                    (and ; Open the server to add it to `nnfeed-servers'
+                     (save-match-data
+                       (nnfeed-open-server
+                        server
+                        (cdr ; Get defs and backend.
+                         (assoc a (cdr (assq nnfeed-backend nnoo-state-alist))
+                                (lambda (car key)
+                                  (and (stringp car)
+                                       (string-match
+                                        (concat
+                                         "\\`\\(\\(nn[[:alpha:]]+\\)\\+\\)?"
+                                         (regexp-quote key) "\\'")
+                                        car)
+                                       (setq server car)))))
+                        (if (match-string 1 server)
+                            (intern (match-string 2 server)) 'nnfeed)))
+                     (gethash a nnfeed-servers))))
+             (g (or (nnfeed--group-data group a)
+                    `[ ,group ,(make-hash-table :test 'equal)
+                       ,(make-hash-table :test 'eql) nil 1 ""])))
+    (puthash new-name g s)
+    (puthash group new-name nnfeed-group-names)
+    (remhash group s)
+    (and (string= group nnfeed-group)
+         (setq nnfeed-group new-name))
+    t))
+
+(provide 'nnfeed)
+
+;;; nnfeed.el ends here
-- 
2.44.0