From b176e9ede469e7187417e63cf8ae56c0372a12cc Mon Sep 17 00:00:00 2001
From: Felix Dietrich <felix.dietrich@sperrhaken.name>
Date: Sun, 6 Mar 2022 20:41:41 +0100
Subject: [PATCH] Restructure mailcap-add-mailcap-entry to improve readability

---
 lisp/net/mailcap.el | 56 +++++++++++++++++++++++++++++----------------
 1 file changed, 36 insertions(+), 20 deletions(-)

diff --git a/lisp/net/mailcap.el b/lisp/net/mailcap.el
index b65f7c25b8..efcf9d7134 100644
--- a/lisp/net/mailcap.el
+++ b/lisp/net/mailcap.el
@@ -718,27 +718,43 @@ to supply to the test."
 	   result))))
 
 (defun mailcap-add-mailcap-entry (major minor info &optional storage)
+  "Add handler INFO for mime type MAJOR/MINOR to STORAGE.
+
+MAJOR and MINOR should be strings.  MINOR is treated as a regexp
+in later lookups, and, therefore, you may need to escape it
+appropriately.
+
+The format of INFO is described in ‘mailcap-mime-data’.
+
+STORAGE should be a symbol refering to a variable.  The value of
+this variable should have the same format as ‘mailcap-mime-data’.
+STORAGE defaults to ‘mailcap--computed-mime-data’.
+
+None of this is enforced."
   (let* ((storage (or storage 'mailcap--computed-mime-data))
-         (old-major (assoc major (symbol-value storage))))
-    (if (null old-major)		; New major area
-        (set storage
-             (cons (cons major (list (cons minor info)))
-                   (symbol-value storage)))
-      (let ((cur-minor (assoc minor old-major)))
-	(cond
-	 ((or (null cur-minor)		; New minor area, or
-	      (assq 'test info))	; Has a test, insert at beginning
-	  (setcdr old-major
-                  (cons (cons minor info) (cdr old-major))))
-	 ((and (not (assq 'test info))	; No test info, replace completely
-	       (not (assq 'test cur-minor))
-	       (equal (assq 'viewer info)  ; Keep alternative viewer
-		      (assq 'viewer cur-minor)))
-	  (setcdr cur-minor info))
-	 (t
-	  (setcdr old-major
-                  (setcdr old-major
-                          (cons (cons minor info) (cdr old-major))))))))))
+	 (major-entry (assoc major (symbol-value storage)))
+	 (new-minor-entry (cons minor info))
+	 minor-entry)
+    (cond
+     ((null major-entry)
+      ;; Add a new major entry containing the new minor entry.
+      (setf major-entry (list major new-minor-entry))
+      (push major-entry (symbol-value storage)))
+     ((and (setf minor-entry (assoc minor major-entry))
+	   (not (assq 'test info))
+	   (not (assq 'test minor-entry))
+	   (equal (assq 'viewer info)
+		  (assq 'viewer minor-entry)))
+      ;; Replace a previous MINOR entry if it and the entry to be
+      ;; added both do *not* have a ‘test’ associated in their info
+      ;; alist and both use the same ‘viewer’ command.  This ignores
+      ;; other fields in the previous entryʼs info alist: they will be
+      ;; lost when the info alist in the cdr of the previous entry is
+      ;; replaced with the new INFO alist.
+      (setf (cdr minor-entry) info))
+     (t
+      ;; Add the new minor entry to the existing major entry.
+      (push new-minor-entry (cdr major-entry))))))
 
 (defun mailcap-add (type viewer &optional test)
   "Add VIEWER as a handler for TYPE.
-- 
2.35.1