Add cycling commands to outline

* lisp/outline.el (outline--cycle-state, outline-has-subheading-p)
(outline-cycle, outline-cycle-buffer): New functions.
(outline-mode-map): Add key bindings for the two new commands.
(outline--cycle-buffer-state): New variable.
* doc/emacs/text.text (Outline Visibility): Add 'outline-cycle' and
'outline-cycle-buffer'.
* etc/NEWS (Outline): Record the change (bug#41130).

doc/emacs/text.texi

@@ -1207,6 +1207,16 @@ everything except the top @var{n} levels of heading lines.  Note that
 it completely reveals all the @var{n} top levels and the body lines
 before the first heading.
 
+@findex outline-cycle
+@findex outline-cycle-buffer
+  Outline also provides two convenience commands to cycle the
+visibility of each section and the whole buffer.  Typing @kbd{TAB} on
+a heading invokes @code{outline-cycle}, which cycles the current
+section between "hide all", "subheadings", and "show all" state.
+Typing @kbd{S-TAB} invokes @code{outline-cycle-buffer}, which cycles
+the whole buffer between "only top-level headings", "all headings and
+subheadings", and "show all" states.
+
 @anchor{Outline Search}
 @findex reveal-mode
 @vindex search-invisible

etc/NEWS

@@ -233,6 +233,15 @@ preserving markers, properties and overlays.  The new variable
 number of seconds that 'revert-buffer-with-fine-grain' should spend
 trying to be non-destructive.
 
+** Outline
+
++++
+*** New commands to cycle heading visibility.
+Typing 'TAB' on a heading cycles the current section between "hide
+all", "subheadings", and "show all" state. Typing 'S-TAB' anywhere in
+the buffer cycles the whole buffer between "only top-level headings",
+"all headings and subheadings", and "show all" states.
+
 
 * Changes in Specialized Modes and Packages in Emacs 28.1
 

lisp/outline.el

@@ -179,6 +179,12 @@ in the file it applies to.")
   (let ((map (make-sparse-keymap)))
     (define-key map "\C-c" outline-mode-prefix-map)
     (define-key map [menu-bar] outline-mode-menu-bar-map)
+    ;; Only takes effect if the point is on a heading.
+    (define-key map (kbd "TAB")
+      `(menu-item "" outline-cycle
+                  :filter ,(lambda (cmd)
+                             (when (outline-on-heading-p) cmd))))
+    (define-key map (kbd "<backtab>") #'outline-cycle-buffer)
     map))
 
 (defvar outline-font-lock-keywords
@@ -1125,6 +1131,83 @@ convenient way to make a table of contents of the buffer."
                     (insert "\n\n"))))))
           (kill-new (buffer-string)))))))
 
+(defun outline--cycle-state ()
+  "Return the cycle state of current heading.
+Return either 'hide-all, 'headings-only, or 'show-all."
+  (save-excursion
+    (let (start end ov-list heading-end)
+      (outline-back-to-heading)
+      (setq start (point))
+      (outline-end-of-heading)
+      (setq heading-end (point))
+      (outline-end-of-subtree)
+      (setq end (point))
+      (setq ov-list (cl-remove-if-not
+                     (lambda (o) (eq (overlay-get o 'invisible) 'outline))
+                     (overlays-in start end)))
+      (cond ((eq ov-list nil) 'show-all)
+            ;; (eq (length ov-list) 1) wouldn’t work: what if there is
+            ;; one folded subheading?
+            ((and (eq (overlay-end (car ov-list)) end)
+                  (eq (overlay-start (car ov-list)) heading-end))
+             'hide-all)
+            (t 'headings-only)))))
+
+(defun outline-has-subheading-p ()
+  "Return t if this heading has subheadings, nil otherwise."
+  (save-excursion
+    (outline-back-to-heading)
+    (< (save-excursion (outline-next-heading) (point))
+       (save-excursion (outline-end-of-subtree) (point)))))
+
+(defun outline-cycle ()
+  "Cycle between `hide all', `headings only' and `show all'.
+
+`Hide all' means hide all subheadings and their bodies.
+`Headings only' means show sub headings but not their bodies.
+`Show all' means show all subheadings and their bodies."
+  (interactive)
+  (pcase (outline--cycle-state)
+    ('hide-all
+     (if (outline-has-subheading-p)
+         (progn (outline-show-children)
+                (message "Only headings"))
+       (outline-show-subtree)
+       (message "Show all")))
+    ('headings-only
+     (outline-show-subtree)
+     (message "Show all"))
+    ('show-all
+     (outline-hide-subtree)
+     (message "Hide all"))))
+
+(defvar-local outline--cycle-buffer-state 'show-all
+  "Internal variable used for tracking buffer cycle state.")
+
+(defun outline-cycle-buffer ()
+  "Cycle the whole buffer like in `outline-cycle'."
+  (interactive)
+  (pcase outline--cycle-buffer-state
+    ('show-all
+     (save-excursion
+       (let ((start-point (point)))
+         (while (not (eq (point) start-point))
+           (outline-up-heading 1))
+         (outline-hide-sublevels
+          (progn (outline-back-to-heading)
+                 (funcall 'outline-level)))))
+     (setq outline--cycle-buffer-state 'top-level)
+     (message "Top level headings"))
+    ('top-level
+     (outline-show-all)
+     (outline-hide-region-body (point-min) (point-max))
+     (setq outline--cycle-buffer-state 'all-heading)
+     (message "All headings"))
+    ('all-heading
+     (outline-show-all)
+     (setq outline--cycle-buffer-state 'show-all)
+     (message "Show all"))))
+
 (provide 'outline)
 (provide 'noutline)