From d8fee4c307178fc2e0e7c206b8b8a42b2acda719 Mon Sep 17 00:00:00 2001
From: Matthias Meulien <orontee@gmail.com>
Date: Wed, 8 Dec 2021 22:35:42 +0100
Subject: [PATCH] Extend Outline mode with default visibility state

* lisp/outline.el (outline-mode, outline-minor-mode): Ensure default
visibility state is applied
(outline-hide-sublevels): Add optional argument for function to call
on each heading
(outline-default-state): Define the default visibility state
(outline-apply-default-state): Apply default visibility state
---
 lisp/outline.el | 50 ++++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 45 insertions(+), 5 deletions(-)

diff --git a/lisp/outline.el b/lisp/outline.el
index 2ede4e23ea..a3e5da4f5b 100644
--- a/lisp/outline.el
+++ b/lisp/outline.el
@@ -353,7 +353,8 @@ outline-mode
               '(outline-font-lock-keywords t nil nil backward-paragraph))
   (setq-local imenu-generic-expression
 	      (list (list nil (concat "^\\(?:" outline-regexp "\\).*$") 0)))
-  (add-hook 'change-major-mode-hook #'outline-show-all nil t))
+  (add-hook 'change-major-mode-hook #'outline-show-all nil t)
+  (outline-apply-default-state))
 
 (defvar outline-minor-mode-map)
 
@@ -436,7 +437,8 @@ outline-minor-mode
 		  nil t)
         (setq-local line-move-ignore-invisible t)
 	;; Cause use of ellipses for invisible text.
-	(add-to-invisibility-spec '(outline . t)))
+	(add-to-invisibility-spec '(outline . t))
+        (outline-apply-default-state))
     (when (or outline-minor-mode-cycle outline-minor-mode-highlight)
       (if font-lock-fontified
           (font-lock-remove-keywords nil outline-font-lock-keywords))
@@ -1058,13 +1060,16 @@ outline-show-heading
 		       (progn (outline-end-of-heading) (point))
 		       nil))
 
-(defun outline-hide-sublevels (levels)
+(defun outline-hide-sublevels (levels &optional fun)
   "Hide everything but the top LEVELS levels of headers, in whole buffer.
 This also unhides the top heading-less body, if any.
 
 Interactively, the prefix argument supplies the value of LEVELS.
 When invoked without a prefix argument, LEVELS defaults to the level
-of the current heading, or to 1 if the current line is not a heading."
+of the current heading, or to 1 if the current line is not a heading.
+
+When FUN is defined, sublevels aren't hidden but FUN is called
+for each of them."
   (interactive (list
 		(cond
 		 (current-prefix-arg (prefix-numeric-value current-prefix-arg))
@@ -1093,7 +1098,9 @@ outline-hide-sublevels
       (outline-map-region
        (lambda ()
 	 (if (<= (funcall outline-level) levels)
-	     (outline-show-heading)))
+	     (if fun
+                 (funcall fun)
+               (outline-show-heading))))
        beg end)
       ;; Finally unhide any trailing newline.
       (goto-char (point-max))
@@ -1307,6 +1314,39 @@ outline-headers-as-kill
                     (insert "\n\n"))))))
           (kill-new (buffer-string)))))))
 
+(defcustom outline-default-state nil
+  "If non-nil, some headings are initially outlined.
+
+If equal to `only-headings', only heading are shown.
+
+If equal to a number, hide everything but the headings at that
+level.
+
+If equal to a lambda function or function name, this function is
+expected to toggle headings visibility, and will be called after
+the mode is enabled."
+  :version "29.1"
+  :type '(choice (const :tag "Show all" nil)
+                 (const :tag "Only headings" only-headings)
+                 (natnum :tag "Outline level")
+                 (function :tag "Custom function"))
+  :local t
+  :safe t)
+;; TODO fix variable being set through file local variable
+
+(defun outline-apply-default-state ()
+  "Apply the outline state defined by `outline-default-state'."
+  (interactive)
+  (cond
+   ((not outline-default-state) (outline-show-all))
+   ((eq outline-default-state 'only-headings)
+    (outline-show-all)
+    (outline-hide-region-body (point-min) (point-max)))
+   ((integerp outline-default-state)
+    (outline-hide-sublevels outline-default-state))
+   ((when (functionp outline-default-state)
+      (funcall outline-default-state)))))
+
 (defun outline--cycle-state ()
   "Return the cycle state of current heading.
 Return either 'hide-all, 'headings-only, or 'show-all."
-- 
2.30.2