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+;;; reveal.el --- Automatically reveal hidden text at point
+
+;; Copyright (C) 2000, 2001  Free Software Foundation, Inc.
+
+;; Author: Stefan Monnier <monnier@cs.yale.edu>
+;; Keywords: outlines
+
+;; This file is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation; either version 2, or (at your option)
+;; any later version.
+
+;; This file is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs; see the file COPYING.  If not, write to
+;; the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+;; Boston, MA 02111-1307, USA.
+
+;;; Commentary:
+
+;; Reveal mode is a minor mode that makes sure that text around point
+;; is always visible.  When point enters a region of hidden text,
+;; `reveal-mode' temporarily makes it visible.
+;;
+;; This is normally used in conjunction with `outline-minor-mode',
+;; `hs-minor-mode', `hide-ifdef-mode', ...
+;;
+;; It only works with packages that hide text using overlays.
+;; Packages can provide special support for it by placing
+;; a function in the `reveal-toggle-invisible' property on the symbol
+;; used as the value of the `invisible' overlay property.
+;; The function is called right after revealing (or re-hiding) the
+;; text with two arguments: the overlay and a boolean that's non-nil
+;; if we have just revealed the text.  When revealing, that function
+;; may re-hide some of the text.
+
+;;; Todo:
+
+;; - find other hysteresis features.
+
+;;; Code:
+
+(require 'pcvs-util)
+
+(defgroup reveal nil
+  "Reveal hidden text on the fly."
+  :group 'editing)
+
+(defcustom reveal-around-mark t
+  "Reveal text around the mark, if active."
+  :type 'boolean)
+
+(defvar reveal-open-spots nil)
+(make-variable-buffer-local 'reveal-open-spots)
+
+;; Actual code
+
+(defun reveal-post-command ()
+  ;; Refresh the spots that might have changed.
+  ;; `Refreshing' here means to try and re-hide the corresponding text.
+  ;; We don't refresh everything correctly:
+  ;; - we only refresh spots in the current window.
+  ;; FIXME: do we actually know that (current-buffer) = (window-buffer) ?
+  (with-local-quit
+   (let* ((spots (cvs-partition
+                  (lambda (x)
+                    ;; We refresh any spot in the current window as well
+                    ;; as any spots associated with a dead window or a window
+                    ;; which does not show this buffer any more.
+                    (or (eq (car x) (selected-window))
+                        (not (window-live-p (car x)))
+                        (not (eq (window-buffer (car x))
+                                 (current-buffer)))))
+                  reveal-open-spots))
+          (old-ols (mapcar 'cdr (car spots)))
+          (repeat t)
+          ;; `post-command-hook' binds it to t, but the user might want to
+          ;; interrupt our work if we somehow get stuck in an infinite loop.
+          (inhibit-quit nil)
+          inv open)
+     (setq reveal-open-spots (cdr spots))
+     ;; Open new overlays.
+     (while repeat
+       (setq repeat nil)
+       (dolist (ol (nconc (when (and reveal-around-mark mark-active)
+                            (overlays-at (mark)))
+                          (overlays-at (point))))
+         (push (cons (selected-window) ol) reveal-open-spots)
+         (setq old-ols (delq ol old-ols))
+         (when (setq inv (overlay-get ol 'invisible))
+           (when (or (overlay-get ol 'isearch-open-invisible)
+                     (and (consp buffer-invisibility-spec)
+                          (assq inv buffer-invisibility-spec)))
+             (overlay-put ol 'reveal-invisible inv)
+             (overlay-put ol 'invisible nil)
+             (when (setq open (get inv 'reveal-toggle-invisible))
+               ;; Use the provided opening function and repeat (since the
+               ;; opening function might have hidden a subpart around point).
+               (setq repeat t)
+               (condition-case err
+                   (funcall open ol t)
+                 (error (message "!!Reveal-show: %s !!" err))))))))
+     ;; Close old overlays.
+     (dolist (ol old-ols)
+       (when (and (setq inv (overlay-get ol 'reveal-invisible))
+                  (eq (current-buffer) (overlay-buffer ol))
+                  (not (rassq ol reveal-open-spots)))
+         (if (and (>= (point) (save-excursion
+                                (goto-char (overlay-start ol))
+                                (line-beginning-position 1)))
+                  (<= (point) (save-excursion
+                                (goto-char (overlay-end ol))
+                                (line-beginning-position 2))))
+             ;; Still near the overlay: keep it open.
+             (push (cons (selected-window) ol) reveal-open-spots)
+           ;; Really close it.
+           (overlay-put ol 'invisible inv)
+           (when (setq open (get inv 'reveal-toggle-invisible))
+             (condition-case err
+                 (funcall open ol nil)
+               (error (message "!!Reveal-hide: %s !!" err))))))))))
+
+;;;###autoload
+(define-minor-mode reveal-mode
+  "Toggle Reveal mode on or off.
+Reveal mode renders invisible text around point visible again.
+
+Interactively, with no prefix argument, toggle the mode.
+With universal prefix ARG (or if ARG is nil) turn mode on.
+With zero or negative ARG turn mode off."
+  :lighter "Reveal"
+  (if reveal-mode
+      (progn
+        (set (make-local-variable 'search-invisible) t)
+        (add-hook 'post-command-hook 'reveal-post-command nil t))
+    (kill-local-variable 'search-invisible)
+    (remove-hook 'post-command-hook 'reveal-post-command t)))
+
+;;;###autoload
+(define-minor-mode global-reveal-mode
+  "Toggle Reveal mode in all buffers on or off.
+Reveal mode renders invisible text around point visible again.
+
+Interactively, with no prefix argument, toggle the mode.
+With universal prefix ARG (or if ARG is nil) turn mode on.
+With zero or negative ARG turn mode off."
+  :global t
+  (setq-default reveal-mode global-reveal-mode)
+  (if global-reveal-mode
+      (progn
+        (setq search-invisible t)
+        (add-hook 'post-command-hook 'reveal-post-command))
+    (setq search-invisible 'open)       ;FIXME
+    (remove-hook 'post-command-hook 'reveal-post-command)))
+
+(provide 'reveal)
+;;; reveal.el ends here