bug#49731: 28.0.50; Filter xref results by filename

[Juri Linkov]

>> Now pushed to master in the commit 3573ebfa6d (it seems this is
>> backward-compatible since it only sets buffer-local variables).
>
> Nice.
>
> Do you plan on adding an outline-[minor-]mode command to hide/show by
> regexp?

So now here are these two commands:

  / s   outline-show-by-heading-regexp
  / h   outline-hide-by-heading-regexp

Also there is an additional helper function that is needed
to keep hidden outlines and restore them after reverting the
xref buffer with 'g' (xref-revert-buffer).
This is an example of advice that does this.
Later when xref will use revert-buffer-function,
this advice could be replaced by a simple hook call:

#+begin_src emacs-lisp
(define-advice xref-revert-buffer (:around (ofun &rest args) keep-outlines)
  "Keep hidden outlines after xref revert."
  (let ((regexp (outline-hidden-headings-regexp))
        (value (apply ofun args)))
    (outline-hide-by-heading-regexp regexp)
    value))
#+end_src

diff --git a/lisp/outline.el b/lisp/outline.el
index 5ac0f0707f1..d933bd4a444 100644
--- a/lisp/outline.el
+++ b/lisp/outline.el
@@ -92,6 +92,8 @@ outline-mode-prefix-map
     (define-key map "\C-o" 'outline-hide-other)
     (define-key map "\C-^" 'outline-move-subtree-up)
     (define-key map "\C-v" 'outline-move-subtree-down)
+    (keymap-set map "/ s" #'outline-show-by-heading-regexp)
+    (keymap-set map "/ h" #'outline-hide-by-heading-regexp)
     (define-key map [(control ?<)] 'outline-promote)
     (define-key map [(control ?>)] 'outline-demote)
     (define-key map "\C-m" 'outline-insert-heading)
@@ -1661,6 +1663,42 @@ outline--show-headings-up-to-level
          beg end)))
     (run-hooks 'outline-view-change-hook)))
 
+(defun outline-show-by-heading-regexp (regexp)
+  (interactive (list (read-regexp "Regexp to show outlines")))
+  (let (outline-view-change-hook)
+    (outline-map-region
+     (lambda ()
+       (when (string-match-p regexp (buffer-substring (pos-bol) (pos-eol)))
+         (outline-show-branches) ;; To reveal all parent headings
+         (outline-show-entry)))
+     (point-min) (point-max)))
+  (run-hooks 'outline-view-change-hook))
+
+(defun outline-hide-by-heading-regexp (regexp)
+  (interactive (list (read-regexp "Regexp to hide outlines")))
+  (let (outline-view-change-hook)
+    (outline-map-region
+     (lambda ()
+       (when (string-match-p regexp (buffer-substring (pos-bol) (pos-eol)))
+         (outline-hide-subtree)))
+     (point-min) (point-max)))
+  (run-hooks 'outline-view-change-hook))
+
+(defun outline-hidden-headings-regexp ()
+  (let ((headings))
+    (outline-map-region
+     (lambda ()
+       (when (save-excursion
+               (outline-end-of-heading)
+               (seq-some (lambda (o) (eq (overlay-get o 'invisible)
+                                         'outline))
+                         (overlays-at (point))))
+         (push (buffer-substring (pos-bol) (pos-eol)) headings)))
+     (point-min) (point-max))
+    (mapconcat (lambda (heading)
+                 (regexp-quote heading))
+               (nreverse headings) "\\|")))
+
 
 ;;; Visibility cycling