From c9228b90ceed55502cfcbf70d5c9332028860100 Mon Sep 17 00:00:00 2001
From: Xinglu Chen <public@yoctocell.xyz>
Date: Wed, 20 Jan 2021 09:56:23 +0100
Subject: gnus: Add piem-gnus-mid-to-thread

Inserts a string of the whole thread in mbox format.

Message-Id: <796fb84d852f2a5adea1502db144dae1bdc4fe0c.1611132172.git.public@yoctocell.xyz>
---
 piem-gnus.el | 41 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 37 insertions(+), 4 deletions(-)

diff --git a/piem-gnus.el b/piem-gnus.el
index 5f10be8..867b45f 100644
--- a/piem-gnus.el
+++ b/piem-gnus.el
@@ -28,6 +28,7 @@
 ;;; Code:
 
 (require 'gnus)
+(require 'gnus-art)
 (require 'gnus-sum)
 (require 'message)
 (require 'piem)
@@ -55,8 +56,38 @@
             (match-string 1 mid)
           mid)))))
 
-;; If there is an easy way to generate an mbox for a thread in Gnus, a
-;; function for `piem-mid-to-thread-functions' should be defined.
+(defun piem-gnus-mid-to-thread (mid)
+  (when (and (derived-mode-p 'gnus-summary-mode)
+             (string-equal (substring
+                            (mail-header-id (gnus-summary-article-header))
+                            1 -1)       ; Remove "<" and ">"
+                           mid))
+    (save-excursion
+      ;; Cursor has to be at the root of the thread
+      (gnus-summary-refer-parent-article most-positive-fixnum)
+      (let ((articles (gnus-summary-articles-in-thread))
+            messages
+            ;; Just show raw message
+            (gnus-have-all-headers t)
+            gnus-article-prepare-hook
+            gnus-article-decode-hook
+            gnus-display-mime-function
+            gnus-break-pages)
+        (mapc (lambda (article)
+                (gnus-summary-display-article article)
+                (push (format
+                       "From mboxrd@z Thu Jan  1 00:00:00 1970\n%s\n"
+                       (replace-regexp-in-string ; From-munge
+                        "^>*From "
+                        ">\\&"
+                        (with-current-buffer gnus-article-buffer
+                          (buffer-substring-no-properties
+                           (point-min)
+                           (point-max)))))
+                      messages))
+              articles)
+        (lambda ()
+          (insert (apply #'concat (nreverse messages))))))))
 
 (defun piem-gnus-am-ready-mbox ()
   "Return a function that inserts an am-ready mbox.
@@ -97,10 +128,12 @@ the mode if ARG is omitted or nil."
       (progn
         (add-hook 'piem-am-ready-mbox-functions #'piem-gnus-am-ready-mbox)
         (add-hook 'piem-get-inbox-functions #'piem-gnus-get-inbox)
-        (add-hook 'piem-get-mid-functions #'piem-gnus-get-mid))
+        (add-hook 'piem-get-mid-functions #'piem-gnus-get-mid)
+        (add-hook 'piem-mid-to-thread-functions #'piem-gnus-mid-to-thread))
     (remove-hook 'piem-am-ready-mbox-functions #'piem-gnus-am-ready-mbox)
     (remove-hook 'piem-get-inbox-functions #'piem-gnus-get-inbox)
-    (remove-hook 'piem-get-mid-functions #'piem-gnus-get-mid)))
+    (remove-hook 'piem-get-mid-functions #'piem-gnus-get-mid)
+    (remove-hook 'piem-mid-to-thread-functions #'piem-gnus-mid-to-thread)))
 
 ;;; piem-gnus.el ends here
 (provide 'piem-gnus)
-- 
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