(completing-read): Add `group-function` to the completion metadata

A completion table can specify a `group-function` in its metadata.
The group function takes two arguments, a completion candidate and a
transform argument.  The group function is used to group the
candidates after sorting and to enhance the completion UI with group
titles.

If the transform argument is nil, the function must return the title
of the group to which the completion candidate belongs.  The function
may also return nil if the candidate does not belong to a group.

If the transform argument is non-nil, the function must return the
transformed candidate.  For example, the transformation allows to
remove a redundant part of the candidate, which is then displayed in
the title.

The grouping functionality is guarded by the customizable variable
`completions-group` and turned off by default for the *Completions*
buffer.

The specific form of the `group-function` has been chosen in order to
allow allocation-free grouping.  This is important for completion UIs,
which continously update the displayed set of candidates (Icomplete,
Vertico, Ivy, etc.).  Only when the transform argument is non-nil the
candidate transformation is performed, which may involve a string
allocation as done in the function `xref--completing-read-group`.

The function `xref-show-definitions-completing-read` makes use of the
`group-function`, by moving the file name prefix to the title.  If
grouping is enabled, the *Completions* are displayed as
"linenum:summary" instead of "file:linenum:summary".  This way the
*Completions* buffer resembles the *Occur* buffer.

* doc/lispref/minibuf.texi: Add documentation.

* lisp/minibuffer.el (completion-metadata): Describe the
`group-function` in the docstring.
(completions-group): Add guard variable, off by default.
(completions-group-format): Add variable defining the format string
for the group titles.
(completions-group-title): Add face used by `completions-group-format`
for the group titles.
(completions-group-separator): Add face used by
`completions-group-format` for the group separator lines.
(minibuffer--group-by): New grouping function.
(minibuffer-completion-help): Use it.
(display-completion-list): Add optional GROUP-FUN argument.
(completion--insert-strings): Add optional GROUP-FUN argument.  Insert
group titles if `completions-format` is `one-column`.  Transform each
candidate with the GROUP-FUN.  Attach the untransformed candidate to
the property `completion--string`.

* lisp/simple.el (choose-completion): Retrieve the untransformed
completion candidate from the property `completion--string`.

* lisp/progmodes/xref.el:
(xref--completing-read-group): New grouping function.
(xref-show-definitions-completing-read): Use it.

1 files changed, 105 insertions, 21 deletions

diff --git a/lisp/minibuffer.el b/lisp/minibuffer.el
index d6a6f9aa375..ba9de7df049 100644
--- a/lisp/minibuffer.el
+++ b/lisp/minibuffer.el
@@ -126,6 +126,13 @@ This metadata is an alist.  Currently understood keys are:
    three-element lists: completion, its prefix and suffix.  This
    function takes priority over `annotation-function' when both are
    provided, so only this function is used.
+- `group-function': function for grouping the completion candidates.
+   Takes two arguments: a completion candidate (COMPLETION) and a
+   boolean flag (TRANSFORM).  If TRANSFORM is nil, the function
+   returns the group title of the group to which the candidate
+   belongs.  The returned title may be nil.  Otherwise the function
+   returns the transformed candidate.  The transformation can remove a
+   redundant prefix, which is displayed in the group title.
 - `display-sort-function': function to sort entries in *Completions*.
    Takes one argument (COMPLETIONS) and should return a new list
    of completions.  Can operate destructively.
@@ -1138,6 +1145,32 @@ completion candidates than this number."
   :version "24.1"
   :type completion--cycling-threshold-type)
 
+(defcustom completions-group nil
+  "Enable grouping of completion candidates in the *Completions* buffer.
+See also `completions-group-format'."
+  :type 'boolean
+  :version "28.1")
+
+(defcustom completions-group-format
+  (concat
+   (propertize "    " 'face 'completions-group-separator)
+   (propertize " %s " 'face 'completions-group-title)
+   (propertize " " 'face 'completions-group-separator
+               'display '(space :align-to right)))
+  "Format string used for the group title."
+  :type 'string
+  :version "28.1")
+
+(defface completions-group-title
+  '((t :inherit shadow :slant italic))
+  "Face used for the title text of the candidate group headlines."
+  :version "28.1")
+
+(defface completions-group-separator
+  '((t :inherit shadow :strike-through t))
+  "Face used for the separator lines between the candidate groups."
+  :version "28.1")
+
 (defun completion--cycle-threshold (metadata)
   (let* ((cat (completion-metadata-get metadata 'category))
          (over (completion--category-override cat 'cycle)))
@@ -1401,6 +1434,17 @@ Remove completion BASE prefix string from history elements."
                      (substring c base-size)))
                  hist)))))
 
+(defun minibuffer--group-by (fun elems)
+  "Group ELEMS by FUN."
+  (let ((groups))
+    (dolist (cand elems)
+      (let* ((key (funcall 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))))
+
 (defun completion-all-sorted-completions (&optional start end)
   (or completion-all-sorted-completions
       (let* ((start (or start (minibuffer-prompt-end)))
@@ -1747,11 +1791,17 @@ or appended to completions."
   :type 'boolean
   :version "28.1")
 
-(defun completion--insert-strings (strings)
+;; TODO: Split up this function in one function per `completions-format'.
+;; TODO: Add group title support for horizontal and vertical format.
+(defun completion--insert-strings (strings &optional group-fun)
   "Insert a list of STRINGS into the current buffer.
-Uses columns to keep the listing readable but compact.
-It also eliminates runs of equal strings."
+Uses columns to keep the listing readable but compact.  It also
+eliminates runs of equal strings.  GROUP-FUN is a `group-function'
+used for grouping the completion."
   (when (consp strings)
+    ;; FIXME: Currently grouping is enabled only for the 'one-column format.
+    (unless (eq completions-format 'one-column)
+      (setq group-fun nil))
     (let* ((length (apply #'max
 			  (mapcar (lambda (s)
 				    (if (consp s)
@@ -1768,6 +1818,7 @@ It also eliminates runs of equal strings."
 		     (max 1 (/ (length strings) 2))))
 	   (colwidth (/ wwidth columns))
            (column 0)
+           (last-title nil)
 	   (rows (/ (length strings) columns))
 	   (row 0)
            (first t)
@@ -1780,6 +1831,13 @@ It also eliminates runs of equal strings."
       ;; The insertion should be "sensible" no matter what choices were made
       ;; for the parameters above.
       (dolist (str strings)
+        ;; Add group titles.
+        (when group-fun
+          (let ((title (funcall group-fun (if (consp str) (car str) str) nil)))
+            (unless (equal title last-title)
+              (when title
+                (insert (format completions-group-format title) "\n"))
+              (setq last-title title))))
 	(unless (equal laststring str) ; Remove (consecutive) duplicates.
 	  (setq laststring str)
           ;; FIXME: `string-width' doesn't pay attention to
@@ -1825,8 +1883,15 @@ It also eliminates runs of equal strings."
 		  nil))))
             (setq first nil)
             (if (not (consp str))
-                (put-text-property (point) (progn (insert str) (point))
-                                   'mouse-face 'highlight)
+                (add-text-properties
+                 (point)
+                 (progn
+                   (insert
+                    (if group-fun
+                        (funcall group-fun str 'transform)
+                      str))
+                   (point))
+                 `(mouse-face highlight completion--string ,str))
               ;; If `str' is a list that has 2 elements,
               ;; then the second element is a suffix annotation.
               ;; If `str' has 3 elements, then the second element
@@ -1837,8 +1902,15 @@ It also eliminates runs of equal strings."
                   (let ((beg (point))
                         (end (progn (insert prefix) (point))))
                     (put-text-property beg end 'mouse-face nil)))
-                (put-text-property (point) (progn (insert (car str)) (point))
-                                   'mouse-face 'highlight)
+                (add-text-properties
+                 (point)
+                 (progn
+                   (insert
+                    (if group-fun
+                         (funcall group-fun (car str) 'transform)
+                      (car str)))
+                   (point))
+                 `(mouse-face highlight completion--string ,(car str)))
                 (let ((beg (point))
                       (end (progn (insert suffix) (point))))
                   (put-text-property beg end 'mouse-face nil)
@@ -1923,7 +1995,7 @@ and with BASE-SIZE appended as the last element."
         completions)
        base-size))))
 
-(defun display-completion-list (completions &optional common-substring)
+(defun display-completion-list (completions &optional common-substring group-fun)
   "Display the list of completions, COMPLETIONS, using `standard-output'.
 Each element may be just a symbol or string
 or may be a list of two strings to be printed as if concatenated.
@@ -1933,7 +2005,9 @@ alternative, the second serves as annotation.
 The actual completion alternatives, as inserted, are given `mouse-face'
 properties of `highlight'.
 At the end, this runs the normal hook `completion-setup-hook'.
-It can find the completion buffer in `standard-output'."
+It can find the completion buffer in `standard-output'.
+GROUP-FUN is a `group-function' used for grouping the completion
+candidates."
   (declare (advertised-calling-convention (completions) "24.4"))
   (if common-substring
       (setq completions (completion-hilit-commonality
@@ -1946,7 +2020,7 @@ It can find the completion buffer in `standard-output'."
 	(let ((standard-output (current-buffer))
 	      (completion-setup-hook nil))
           (with-suppressed-warnings ((callargs display-completion-list))
-	    (display-completion-list completions common-substring)))
+	    (display-completion-list completions common-substring group-fun)))
 	(princ (buffer-string)))
 
     (with-current-buffer standard-output
@@ -1954,7 +2028,7 @@ It can find the completion buffer in `standard-output'."
       (if (null completions)
           (insert "There are no possible completions of what you have typed.")
         (insert "Possible completions are:\n")
-        (completion--insert-strings completions))))
+        (completion--insert-strings completions group-fun))))
 
   (run-hooks 'completion-setup-hook)
   nil)
@@ -2067,6 +2141,9 @@ variables.")
              (aff-fun (or (completion-metadata-get all-md 'affixation-function)
                           (plist-get completion-extra-properties
                                      :affixation-function)))
+             (sort-fun (completion-metadata-get all-md 'display-sort-function))
+             (group-fun (and completions-group
+                             (completion-metadata-get all-md 'group-function)))
              (mainbuf (current-buffer))
              ;; If the *Completions* buffer is shown in a new
              ;; window, mark it as softly-dedicated, so bury-buffer in
@@ -2098,15 +2175,22 @@ variables.")
                       ;; Remove the base-size tail because `sort' requires a properly
                       ;; nil-terminated list.
                       (when last (setcdr last nil))
-                      (setq completions
-                            ;; FIXME: This function is for the output of all-completions,
-                            ;; not completion-all-completions.  Often it's the same, but
-                            ;; not always.
-                            (let ((sort-fun (completion-metadata-get
-                                             all-md 'display-sort-function)))
-                              (if sort-fun
-                                  (funcall sort-fun completions)
-                                (sort completions 'string-lessp))))
+
+                      ;; Sort first using the `display-sort-function'.
+                      ;; FIXME: This function is for the output of
+                      ;; all-completions, not
+                      ;; completion-all-completions.  Often it's the
+                      ;; same, but not always.
+                      (setq completions (if sort-fun
+                                            (funcall sort-fun completions)
+                                          (sort completions 'string-lessp)))
+
+                      ;; After sorting, group the candidates using the
+                      ;; `group-function'.
+                      (when group-fun
+                        (setq completions
+                              (minibuffer--group-by group-fun completions)))
+
                       (cond
                        (aff-fun
                         (setq completions
@@ -2152,7 +2236,7 @@ variables.")
                                                      (if (eq (car bounds) (length result))
                                                          'exact 'finished)))))))
 
-                      (display-completion-list completions)))))
+                      (display-completion-list completions nil group-fun)))))
           nil)))
     nil))