From 070d8b972e5638ce90e7c2bb1608d16e53d516a4 Mon Sep 17 00:00:00 2001
From: Daniel Mendler <mail@daniel-mendler.de>
Date: Tue, 11 May 2021 09:08:05 +0200
Subject: [PATCH 5/5] (minibuffer-completion-help): Add group sorting

Allow sorting the groups as returned by the `group-function` of the
completion table with the `completions-group-sort-function`.

* lisp/minibuffer.el (completions-group-sort-function): New variable.
(minibuffer--group-by): Add SORT-FUN argument.
(minibuffer-completion-help): Pass `completions-group-sort-function`
to `minibuffer--group-by`.
---
 lisp/minibuffer.el | 26 +++++++++++++++++++++-----
 1 file changed, 21 insertions(+), 5 deletions(-)

diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index 73a38a8137..3399a02014 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -1151,6 +1151,14 @@ completions-group
   :type 'boolean
   :version "28.1")
 
+(defcustom completions-group-sort-function #'identity
+  "Sorting function for the groups.
+The function takes and returns an alist of groups, where the each
+element is a pair of a group title string and the candidate strings
+belonging to the group."
+  :type 'function
+  :version "28.1")
+
 (defcustom completions-group-format
   (concat
    (propertize "    " 'face 'completions-group-separator)
@@ -1432,16 +1440,21 @@ minibuffer--sort-preprocess-history
                      (substring c base-size)))
                  hist)))))
 
-(defun minibuffer--group-by (fun elems)
-  "Group ELEMS by FUN."
+(defun minibuffer--group-by (group-fun sort-fun elems)
+  "Group ELEMS by GROUP-FUN and sort using SORT-FUN."
   (let ((groups))
     (dolist (cand elems)
-      (let* ((key (funcall fun cand nil))
+      (let* ((key (funcall group-fun cand nil))
              (group (assoc key groups)))
         (if group
             (setcdr group (cons cand (cdr group)))
           (push (list key cand) groups))))
-    (mapcan (lambda (x) (nreverse (cdr x))) (nreverse groups))))
+    (setq groups (nreverse groups)
+          groups (mapc (lambda (x)
+                         (setcdr x (nreverse (cdr x))))
+                       groups)
+          groups (funcall sort-fun groups))
+    (mapcan #'cdr groups)))
 
 (defun completion-all-sorted-completions (&optional start end)
   (or completion-all-sorted-completions
@@ -2212,7 +2225,10 @@ minibuffer-completion-help
                       ;; `group-function'.
                       (when group-fun
                         (setq completions
-                              (minibuffer--group-by group-fun completions)))
+                              (minibuffer--group-by
+                               group-fun
+                               completions-group-sort-function
+                               completions)))
 
                       (cond
                        (aff-fun
-- 
2.20.1