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;;; follow.el --- synchronize windows showing the same buffer
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;; Copyright (C) 1995, 1996, 1997, 1999, 2001, 2005
;;           Free Software Foundation, Inc.
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;; Author: Anders Lindgren <andersl@andersl.com>
;; Maintainer: Anders Lindgren <andersl@andersl.com>
;; Created: 1995-05-25
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;; Keywords: display, window, minor-mode, convenience
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;; Last Changed: 1999-11-17
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;; This file is part of GNU Emacs.

;; GNU Emacs is free software; you can redistribute it and/or modify
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;; 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.

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;; GNU Emacs is distributed in the hope that it will be useful,
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;; 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
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;; along with GNU Emacs; see the file COPYING.  If not, write to the
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;; Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
;; Boston, MA 02110-1301, USA.
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;;; Commentary:

;;{{{ Documentation

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;; `Follow mode' is a minor mode for Emacs and XEmacs that
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;; combines windows into one tall virtual window.
;;
;; The feeling of a "virtual window" has been accomplished by the use
;; of two major techniques:
;;
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;; * The windows always displays adjacent sections of the buffer.
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;;   This means that whenever one window is moved, all the
;;   others will follow.  (Hence the name Follow Mode.)
;;
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;; * Should the point (cursor) end up outside a window, another
;;   window displaying that point is selected, if possible.  This
;;   makes it possible to walk between windows using normal cursor
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;;   movement commands.
;;
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;; Follow mode comes to its prime when a large screen and two
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;; side-by-side window are used. The user can, with the help of Follow
;; mode, use two full-height windows as though they would have been
;; one. Imagine yourself editing a large function, or section of text,
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;; and being able to use 144 lines instead of the normal 72... (your
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;; mileage may vary).

;; To test this package, make sure `follow' is loaded, or will be
;; autoloaded when activated (see below). Then do the following:
;;
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;; * Find your favorite file (preferably a long one).
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;;
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;; * Resize Emacs so that it will be wide enough for two full size
;;   columns.  Delete the other windows and split the window with
;;   the commands `C-x 1 C-x 3'.
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;;
;; * Give the command:
;;	M-x follow-mode <RETURN>
;;
;; * Now the display should look something like (assuming the text "71"
;;   is on line 71):
;;
;;		    +----------+----------+
;;		    |1         |73        |
;;		    |2         |74        |
;;		    |3         |75        |
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;;		         ...        ...
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;;		    |71        |143       |
;;		    |72        |144       |
;;		    +----------+----------+
;;
;;   As you can see, the right-hand window starts at line 73, the line
;;   immediately below the end of the left-hand window. As long as
;;   `follow-mode' is active, the two windows will follow eachother!
;;
;; * Play around and enjoy! Scroll one window and watch the other.
;;   Jump to the beginning or end. Press `Cursor down' at the last
;;   line of the left-hand window. Enter new lines into the
;;   text. Enter long lines spanning several lines, or several
;;   windows.
;;
;; * Should you find `Follow' mode annoying, just type
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;;	M-x follow-mode <RETURN>
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;;   to turn it off.


;; The command `follow-delete-other-windows-and-split' maximises the
;; visible area of the current buffer.
;;
;; I recommend adding it, and `follow-mode', to hotkeys in the global
;; key map.  To do so, add the following lines (replacing `[f7]' and
;; `[f8]' with your favorite keys) to the init file:
;;
;; (global-set-key [f8] 'follow-mode)
;; (global-set-key [f7] 'follow-delete-other-windows-and-split)


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;; There exists two system variables that controls the appearence of
;; lines that are wider than the window containing them.  The default
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;; is to truncate long lines whenever a window isn't as wide as the
;; frame.
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;;
;; To make sure lines are never truncated, please place the following
;; lines in your init file:
;;
;; (setq truncate-lines nil)
;; (setq truncate-partial-width-windows nil)


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;; Since the display of XEmacs is pixel-oriented, a line could be
;; clipped in half at the bottom of the window.
;;
;; To make XEmacs avoid clipping (normal) lines, please place the
;; following line in your init-file:
;;
;; (setq pixel-vertical-clip-threshold 30)


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;; The correct way to cofigurate Follow mode, or any other mode for
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;; that matter, is to create one (or more) function that does
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;; whatever you would like to do.  The function is then added to
;; a hook.
;;
;; When `Follow' mode is activated, functions stored in the hook
;; `follow-mode-hook' are called.  When it is deactivated
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;; `follow-mode-off-hook' is run.
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;;
;; The keymap `follow-key-map' contains key bindings activated by
;; `follow-mode'.
;;
;; Example:
;; (add-hook 'follow-mode-hook 'my-follow-mode-hook)
;;
;; (defun my-follow-mode-hook ()
;;    (define-key follow-mode-map "\C-ca" 'your-favorite-function)
;;    (define-key follow-mode-map "\C-cb" 'another-function))


;; Usage:
;;
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;; To activate issue the command "M-x follow-mode"
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;; and press return. To deactivate, do it again.
;;
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;; The following is a list of commands useful when follow-mode is active.
;;
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;;	follow-scroll-up			 C-c . C-v
;;		Scroll text in a Follow Mode window chain up.
;;
;;	follow-scroll-down			 C-c . v
;;		Like `follow-scroll-up', but in the other direction.
;;
;;	follow-delete-other-windows-and-split	 C-c . 1
;;		Maximise the visible area of the current buffer,
;;		and enter Follow Mode. 	This is a very convenient
;;		way to start Follow Mode, hence it is recomended
;;		that this command is added to the global keymap.
;;
;;	follow-recenter				 C-c . C-l
;;		Place the point in the center of the middle window,
;;		or a specified number of lines from either top or bottom.
;;
;;	follow-switch-to-buffer			 C-c . b
;;		Switch buffer in all windows displaying the current buffer
;;		in this frame.
;;
;;	follow-switch-to-buffer-all		 C-c . C-b
;;		Switch buffer in all windows in the active frame.
;;
;;	follow-switch-to-current-buffer-all
;;		Show the current buffer in all windows on the current
;;		frame and turn on `follow-mode'.
;;
;;	follow-first-window			 C-c . <
;;		Select the first window in the frame showing the same buffer.
;;
;;	follow-last-window			 C-c . >
;;		Select the last window in the frame showing the same buffer.
;;
;;	follow-next-window			 C-c . n
;;		Select the next window in the frame showing the same buffer.
;;
;;	follow-previous-window			 C-c . p
;;		Select the previous window showing the same buffer.


;; Well, it seems ok, but what if I really want to look at two different
;; positions in the text? Here are two simple methods to use:
;;
;; 1) Use multiple frames; `follow' mode only affects windows displayed
;;    in the same frame. (My apoligies to you who can't use frames.)
;;
;; 2) Bind `follow-mode' to key so you can turn it off whenever
;;    you want to view two locations. Of course, `follow' mode can
;;    be reactivated by hitting the same key again.
;;
;;    Example from my ~/.emacs:
;;	(global-set-key [f8] 'follow-mode)


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;; Implementation:
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;;
;; In an ideal world, follow mode would have been implemented in the
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;; kernel of the display routines, making sure that the windows (using
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;; follow mode) ALWAYS are aligned. On planet earth, however, we must
;; accept a solution where we ALMOST ALWAYS can make sure that the
;; windows are aligned.
;;
;; Follow mode does this in three places:
;; 1) After each user command.
;; 2) After a process output has been perfomed.
;; 3) When a scrollbar has been moved.
;;
;; This will cover most situations. (Let me know if there are other
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;; situations that should be covered.)
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;;
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;; Note that only the selected window is checked, for the reason of
;; efficiency and code complexity. (I.e. it is possible to make a
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;; non-selected windows unaligned. It will, however, pop right back
;; when it is selected.)

;;}}}

;;; Code:

;;{{{ Preliminaries

;; Make the compiler shut up!
;; There are two strategies:
;; 1) Shut warnings off completely.
;; 2) Handle each warning separately.
;;
;; Since I would like to see real errors, I've selected the latter
;; method.
;;
;; The problem with undefined variables and functions has been solved
;; by using `set', `symbol-value' and `symbol-function' rather than
;; `setq' and direct references to variables and functions.
;;
;; For example:
;;	(if (boundp 'foo)   ... (symbol-value 'foo) )
;;	(set 'foo ...)   <-- XEmacs doesn't fall for this one.
;;	(funcall (symbol-function 'set) 'bar ...)
;;
;; Note: When this file is interpreted, `eval-when-compile' is
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;; evaluted.  Since it doesn't hurt to evaluate it, but it is a bit
;; annoying, we test if the byte-compiler has been loaded.  This can,
;; of course, lead to some occasional unintended evaluation...
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;;
;; Should someone come up with a better solution, please let me
;; know.

(eval-when-compile
  (if (or (featurep 'bytecomp)
	  (featurep 'byte-compile))
      (cond ((string-match "XEmacs" emacs-version)
	     ;; Make XEmacs shut up!  I'm using standard Emacs
	     ;; functions, they are NOT obsolete!
	     (if (eq (get 'force-mode-line-update 'byte-compile)
		     'byte-compile-obsolete)
		 (put 'force-mode-line-update 'byte-compile 'nil))
	     (if (eq (get 'frame-first-window 'byte-compile)
		     'byte-compile-obsolete)
		 (put 'frame-first-window 'byte-compile 'nil))))))

;;}}}
;;{{{ Variables

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(defgroup follow nil
  "Synchronize windows showing the same buffer."
  :prefix "follow-"
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  :group 'windows
  :group 'convenience)
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(defvar follow-mode nil
  "Variable indicating if Follow mode is active.")

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(defcustom follow-mode-hook nil
  "*Hooks to run when follow-mode is turned on."
  :type 'hook
  :group 'follow)
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(defcustom follow-mode-off-hook nil
  "*Hooks to run when follow-mode is turned off."
  :type 'hook
  :group 'follow)
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(defvar follow-mode-map nil
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  "*Minor mode keymap for Follow mode.")
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(defcustom follow-mode-line-text " Follow"
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  "*Text shown in the mode line when Follow mode is active.
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Defaults to \" Follow\".  Examples of other values
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are \" Fw\", or simply \"\"."
  :type 'string
  :group 'follow)
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(defcustom follow-auto nil
  "*Non-nil activates Follow mode whenever a file is loaded."
  :type 'boolean
  :group 'follow)
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(defcustom follow-mode-prefix "\C-c."
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  "*Prefix key to use for follow commands in Follow mode.
The value of this variable is checked as part of loading Follow mode.
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After that, changing the prefix key requires manipulating keymaps."
  :type 'string
  :group 'follow)

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(defcustom follow-intercept-processes
  (fboundp 'start-process)
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  "*When non-nil, Follow Mode will monitor process output."
  :type 'boolean
  :group 'follow)
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(defvar follow-emacs-version-xemacs-p
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  (string-match "XEmacs" emacs-version)
  "Non-nil when running under XEmacs.")

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(defvar follow-avoid-tail-recenter-p
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  (not follow-emacs-version-xemacs-p)
  "*When non-nil, patch emacs so that tail windows won't be recentered.

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A \"tail window\" is a window that displays only the end of
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the buffer.  Normally it is practical for the user that empty
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windows are recentered automatically.  However, when using
Follow Mode it breaks the display when the end is displayed
in a window \"above\" the last window.  This is for
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example the case when displaying a short page in info.
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Must be set before Follow Mode is loaded.

Please note that it is not possible to fully prevent Emacs from
recentering empty windows.  Please report if you find a repeatable
situation in which Emacs recenters empty windows.

XEmacs, as of 19.12, does not recenter windows, good!")

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(defvar follow-cache-command-list
  '(next-line previous-line forward-char backward-char)
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  "List of commands that don't require recalculation.
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In order to be able to use the cache, a command should not change the
contents of the buffer, nor should it change selected window or current
buffer.

The commands in this list are checked at load time.

To mark other commands as suitable for caching, set the symbol
property `follow-mode-use-cache' to non-nil.")

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(defvar follow-debug nil
  "*Non-nil when debugging Follow mode.")


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;; Internal variables:
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(defvar follow-internal-force-redisplay nil
  "True when Follow mode should redisplay the windows.")

(defvar follow-process-filter-alist '()
  "The original filters for processes intercepted by Follow mode.")

(defvar follow-active-menu nil
  "The menu visible when Follow mode is active.")

(defvar follow-deactive-menu nil
  "The menu visible when Follow mode is deactivated.")

(defvar follow-inside-post-command-hook nil
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  "Non-nil when inside Follow modes `post-command-hook'.
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Used by `follow-window-size-change'.")

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(defvar follow-windows-start-end-cache nil
  "Cache used by `follow-window-start-end'.")

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;;}}}
;;{{{ Bug report

(eval-when-compile (require 'reporter))

(defun follow-submit-feedback ()
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  "Submit feedback on Follow mode to the author: andersl@andersl.com"
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  (interactive)
  (require 'reporter)
  (and (y-or-n-p "Do you really want to submit a report on Follow mode? ")
       (reporter-submit-bug-report
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	"Anders Lindgren <andersl@andersl.com>"
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	"follow.el"
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	'(post-command-hook
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	  pre-command-hook
	  window-size-change-functions
	  window-scroll-functions
	  follow-mode-hook
	  follow-mode-off-hook
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	  follow-auto
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	  follow-intercept-processes
	  follow-avoid-tail-recenter-p
	  follow-process-filter-alist)
	nil
	nil
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	(concat
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	 "Hi Anders!\n\n"
	 "(I have read the section on how to report bugs in the "
	 "Emacs manual.)\n\n"
	 "Even though I know you are busy, I thought you might "
	 "want to know...\n\n"))))

;;}}}
;;{{{ Debug messages

;; This inline function must be as small as possible!
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;; Maybe we should define a macro that expands to nil if
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;; the variable is not set.
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(defsubst follow-debug-message (&rest args)
  "Like message, but only active when `follow-debug' is non-nil."
  (if (and (boundp 'follow-debug) follow-debug)
      (apply 'message args)))

;;}}}

;;{{{ Keymap/Menu

;;; Define keys for the follow-mode minor mode map and replace some
;;; functions in the global map.  All `follow' mode special functions
;;; can be found on (the somewhat cumbersome) "C-c . <key>"
;;; (Control-C dot <key>). (As of Emacs 19.29 the keys
;;; C-c <punctuation character> are reserved for minor modes.)
;;;
;;; To change the prefix, redefine `follow-mode-prefix' before
;;; `follow' is loaded, or see the section on `follow-mode-hook'
;;; above for an example of how to bind the keys the way you like.
;;;
;;; Please note that the keymap is defined the first time this file is
;;; loaded.  Also note that the only legal way to manipulate the
;;; keymap is to use `define-key'.  Don't change it using `setq' or
;;; similar!


(if follow-mode-map
    nil
  (setq follow-mode-map (make-sparse-keymap))
  (let ((map (make-sparse-keymap)))
    (define-key map "\C-v"	'follow-scroll-up)
    (define-key map "\M-v"	'follow-scroll-down)
    (define-key map "v"		'follow-scroll-down)
    (define-key map "1"		'follow-delete-other-windows-and-split)
    (define-key map "b"		'follow-switch-to-buffer)
    (define-key map "\C-b"	'follow-switch-to-buffer-all)
    (define-key map "\C-l"	'follow-recenter)
    (define-key map "<"		'follow-first-window)
    (define-key map ">"		'follow-last-window)
    (define-key map "n"		'follow-next-window)
    (define-key map "p"		'follow-previous-window)

    (define-key follow-mode-map follow-mode-prefix map)

    ;; Replace the standard `end-of-buffer', when in Follow Mode.  (I
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    ;; don't see the point in trying to replace every function that
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    ;; could be enhanced in Follow mode.  End-of-buffer is a special
    ;; case since it is very simple to define and it greatly enhances
    ;; the look and feel of Follow mode.)
    ;;
    ;; (The function `substitute-key-definition' does not work
    ;; in all versions of Emacs.)
    (mapcar
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     (function
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      (lambda (pair)
	(let ((old (car pair))
	      (new (cdr pair)))
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	  (mapcar (function (lambda (key)
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			      (define-key follow-mode-map key new)))
		  (where-is-internal old global-map)))))
     '((end-of-buffer      . follow-end-of-buffer)
       (fkey-end-of-buffer . follow-end-of-buffer)))

    ;;;
    ;;; The menu.
    ;;;

    (if (not follow-emacs-version-xemacs-p)

	;;
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	;; Emacs
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	;;
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	(let ((menumap (funcall (symbol-function 'make-sparse-keymap)
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			"Follow"))
	      (count 0)
	      id)
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	  (mapcar
	   (function
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	    (lambda (item)
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	      (setq id
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		    (or (cdr item)
			(progn
			  (setq count (+ count 1))
			  (intern (format "separator-%d" count)))))
	      (define-key menumap (vector id) item)
	      (or (eq id 'follow-mode)
		  (put id 'menu-enable 'follow-mode))))
	   ;; In reverse order:
	   '(("Toggle Follow mode" . follow-mode)
	     ("--")
	     ("Recenter"           . follow-recenter)
	     ("--")
	     ("Previous Window"    . follow-previous-window)
	     ("Next Windows"       . follow-next-window)
	     ("Last Window"        . follow-last-window)
	     ("First Window"       . follow-first-window)
	     ("--")
	     ("Switch To Buffer (all windows)"
	                           . follow-switch-to-buffer-all)
	     ("Switch To Buffer"   . follow-switch-to-buffer)
	     ("--")
	     ("Delete Other Windows and Split"
	                           . follow-delete-other-windows-and-split)
	     ("--")
	     ("Scroll Down"        . follow-scroll-down)
	     ("Scroll Up"          . follow-scroll-up)))
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	  ;; If there is a `tools' meny, we use it. However, we can't add a
	  ;; minor-mode specific item to it (it's broken), so we make the
	  ;; contents ghosted when not in use, and add ourselves to the
	  ;; global map.  If no `tools' menu is present, just make a
	  ;; top-level menu visible when the mode is activated.
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	  (let ((tools-map (lookup-key (current-global-map) [menu-bar tools]))
		(last nil))
	    (if (sequencep tools-map)
		(progn
		  ;; Find the last entry in the menu and store it in `last'.
		  (mapcar (function
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			   (lambda (x)
			     (setq last (or (cdr-safe
					     (cdr-safe
					      (cdr-safe x)))
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					    last))))
			  tools-map)
		  (if last
		      (progn
			(funcall (symbol-function 'define-key-after)
				  tools-map [separator-follow] '("--") last)
			(funcall (symbol-function 'define-key-after)
				 tools-map [follow] (cons "Follow" menumap)
				 'separator-follow))
		    ;; Didn't find the last item, Adding to the top of
		    ;; tools.  (This will probably never happend...)
		    (define-key (current-global-map) [menu-bar tools follow]
		      (cons "Follow" menumap))))
	      ;; No tools menu, add "Follow" to the menubar.
	      (define-key follow-mode-map [menu-bar follow]
		(cons "Follow" menumap)))))

      ;;
      ;; XEmacs.
      ;;

      ;; place the menu in the `Tools' menu.
      (let ((menu '("Follow"
		    :filter follow-menu-filter
		    ["Scroll Up" follow-scroll-up t]
		    ["Scroll Down" follow-scroll-down t]
		    ["Delete Other Windows and Split"
		     follow-delete-other-windows-and-split t]
		    ["Switch To Buffer" follow-switch-to-buffer t]
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		    ["Switch To Buffer (all windows)"
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		     follow-switch-to-buffer-all t]
		    ["First Window" follow-first-window t]
		    ["Last Window" follow-last-window t]
		    ["Next Windows" follow-next-window t]
		    ["Previous Window" follow-previous-window t]
		    ["Recenter" follow-recenter t]
		    ["Deactivate" follow-mode t])))

	;; Why not just `(set-buffer-menubar current-menubar)'?  The
	;; question is a very good question.  The reason is that under
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	;; Emacs, neither `set-buffer-menubar' nor
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	;; `current-menubar' is defined, hence the byte-compiler will
	;; warn.
	(funcall (symbol-function 'set-buffer-menubar)
		 (symbol-value 'current-menubar))
	(funcall (symbol-function 'add-submenu) '("Tools") menu))

      ;; When the mode is not activated, only one item is visible:
      ;; "Activate".
      (defun follow-menu-filter (menu)
	(if follow-mode
	    menu
	  '(["Activate          " follow-mode t]))))))


;;; Register the follow mode keymap.
(or (assq 'follow-mode minor-mode-map-alist)
    (setq minor-mode-map-alist
	  (cons (cons 'follow-mode follow-mode-map) minor-mode-map-alist)))

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;;}}}
;;{{{ Cache

(let ((cmds follow-cache-command-list))
  (while cmds
    (put (car cmds) 'follow-mode-use-cache t)
    (setq cmds (cdr cmds))))

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;;}}}

;;{{{ The mode

;;;###autoload
(defun turn-on-follow-mode ()
  "Turn on Follow mode. Please see the function `follow-mode'."
  (interactive)
  (follow-mode 1))


;;;###autoload
(defun turn-off-follow-mode ()
  "Turn off Follow mode. Please see the function `follow-mode'."
  (interactive)
  (follow-mode -1))


;;;###autoload
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(defun follow-mode (arg)
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  "Minor mode that combines windows into one tall virtual window.
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The feeling of a \"virtual window\" has been accomplished by the use
of two major techniques:

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* The windows always displays adjacent sections of the buffer.
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  This means that whenever one window is moved, all the
  others will follow.  (Hence the name Follow Mode.)

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* Should the point (cursor) end up outside a window, another
  window displaying that point is selected, if possible.  This
  makes it possible to walk between windows using normal cursor
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  movement commands.

Follow mode comes to its prime when used on a large screen and two
side-by-side window are used. The user can, with the help of Follow
mode, use two full-height windows as though they would have been
one. Imagine yourself editing a large function, or section of text,
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and being able to use 144 lines instead of the normal 72... (your
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mileage may vary).

To split one large window into two side-by-side windows, the commands
`\\[split-window-horizontally]' or \
`M-x follow-delete-other-windows-and-split' can be used.

Only windows displayed in the same frame follow each-other.

If the variable `follow-intercept-processes' is non-nil, Follow mode
will listen to the output of processes and redisplay accordingly.
\(This is the default.)

When Follow mode is switched on, the hook `follow-mode-hook'
is called.  When turned off, `follow-mode-off-hook' is called.

Keys specific to Follow mode:
\\{follow-mode-map}"
  (interactive "P")
  (make-local-variable 'follow-mode)
  (put 'follow-mode 'permanent-local t)
  (let ((follow-mode-orig follow-mode))
    (setq follow-mode
	  (if (null arg)
	      (not follow-mode)
	    (> (prefix-numeric-value arg) 0)))
    (if (and follow-mode follow-intercept-processes)
	(follow-intercept-process-output))
    (cond ((and follow-mode (not follow-mode-orig)) ; On
	   ;; XEmacs: If this is non-nil, the window will scroll before
	   ;; the point will have a chance to get into the next window.
	   (if (boundp 'scroll-on-clipped-lines)
	       (set 'scroll-on-clipped-lines nil))
	   (force-mode-line-update)
	   (add-hook 'post-command-hook 'follow-post-command-hook t)
	   (run-hooks 'follow-mode-hook))

	  ((and (not follow-mode) follow-mode-orig) ; Off
	   (force-mode-line-update)
	   (run-hooks 'follow-mode-off-hook)))))


;; Register follow-mode as a minor mode.

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(if (fboundp 'add-minor-mode)
    ;; XEmacs
    (funcall (symbol-function 'add-minor-mode)
	     'follow-mode 'follow-mode-line-text)
  (or (assq 'follow-mode minor-mode-alist)
      (setq minor-mode-alist
	    (cons '(follow-mode follow-mode-line-text) minor-mode-alist))))
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;;}}}
;;{{{ Find file hook

;; This will start follow-mode whenever a new file is loaded, if
;; the variable `follow-auto' is non-nil.

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(add-hook 'find-file-hook 'follow-find-file-hook t)
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(defun follow-find-file-hook ()
  "Find-file hook for Follow Mode.  See the variable `follow-auto'."
  (if follow-auto (follow-mode t)))

;;}}}

;;{{{ User functions

;;;
;;; User functions usable when in Follow mode.
;;;

;;{{{ Scroll

;; `scroll-up' and `-down', but for windows in Follow Mode.
;;
;; Almost like the real thing, excpet when the cursor ends up outside
;; the top or bottom...  In our case however, we end up outside the
;; window and hence we are recenterd.  Should we let `recenter' handle
;; the point position we would never leave the selected window.  To do
;; it ourselves we would need to do our own redisplay, which is easier
;; said than done.  (Why didn't I do a real display abstraction from
;; the beginning?)
;;
;; We must sometimes set `follow-internal-force-redisplay', otherwise
;; our post-command-hook will move our windows back into the old
;; position...  (This would also be corrected if we would have had a
;; good redisplay abstraction.)

(defun follow-scroll-up (&optional arg)
  "Scroll text in a Follow Mode window chain up.

If called with no ARG, the `next-screen-context-lines' last lines of
the bottom window in the chain will be visible in the top window.

If called with an argument, scroll ARG lines up.
Negative ARG means scroll downward.

Works like `scroll-up' when not in Follow Mode."
  (interactive "P")
  (cond ((not (and (boundp 'follow-mode) follow-mode))
	 (scroll-up arg))
	(arg
	 (save-excursion (scroll-up arg))
	 (setq follow-internal-force-redisplay t))
	(t
	 (let* ((windows (follow-all-followers))
		(end (window-end (car (reverse windows)))))
	   (if (eq end (point-max))
	       (signal 'end-of-buffer nil)
	     (select-window (car windows))
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	     ;; `window-end' might return nil.
	     (if end
		 (goto-char end))
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	     (vertical-motion (- next-screen-context-lines))
	     (set-window-start (car windows) (point)))))))


(defun follow-scroll-down (&optional arg)
  "Scroll text in a Follow Mode window chain down.

If called with no ARG, the `next-screen-context-lines' top lines of
the top window in the chain will be visible in the bottom window.

If called with an argument, scroll ARG lines down.
Negative ARG means scroll upward.

Works like `scroll-up' when not in Follow Mode."
  (interactive "P")
  (cond ((not (and (boundp 'follow-mode) follow-mode))
	 (scroll-up arg))
	(arg
	 (save-excursion (scroll-down arg)))
	(t
	 (let* ((windows (follow-all-followers))
		(win (car (reverse windows)))
		(start (window-start (car windows))))
	   (if (eq start (point-min))
	       (signal 'beginning-of-buffer nil)
	     (select-window win)
	     (goto-char start)
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	     (vertical-motion (- (- (window-height win)
				    1
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				    next-screen-context-lines)))
	     (set-window-start win (point))
	     (goto-char start)
	     (vertical-motion (- next-screen-context-lines 1))
	     (setq follow-internal-force-redisplay t))))))

;;}}}
;;{{{ Buffer

;;;###autoload
(defun follow-delete-other-windows-and-split (&optional arg)
  "Create two side by side windows and enter Follow Mode.

Execute this command to display as much as possible of the text
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in the selected window.  All other windows, in the current
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frame, are deleted and the selected window is split in two
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side-by-side windows. Follow Mode is activated, hence the
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two windows always will display two successive pages.
\(If one window is moved, the other one will follow.)

If ARG is positive, the leftmost window is selected.  If it negative,
the rightmost is selected.  If ARG is nil, the leftmost window is
selected if the original window is the first one in the frame.

To bind this command to a hotkey, place the following line
in your `~/.emacs' file, replacing [f7] by your favourite key:
    (global-set-key [f7] 'follow-delete-other-windows-and-split)"
  (interactive "P")
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  (let ((other (or (and (null arg)
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			(not (eq (selected-window)
				 (frame-first-window (selected-frame)))))
		   (and arg
			(< (prefix-numeric-value arg) 0))))
	(start (window-start)))
    (delete-other-windows)
    (split-window-horizontally)
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    (if other
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	(progn
	  (other-window 1)
	  (set-window-start (selected-window) start)
	  (setq follow-internal-force-redisplay t)))
    (follow-mode 1)))

(defun follow-switch-to-buffer (buffer)
  "Show BUFFER in all windows in the current Follow Mode window chain."
  (interactive "BSwitch to Buffer: ")
  (let ((orig-window (selected-window))
	(windows (follow-all-followers)))
    (while windows
      (select-window (car windows))
      (switch-to-buffer buffer)
      (setq windows (cdr windows)))
    (select-window orig-window)))


(defun follow-switch-to-buffer-all (&optional buffer)
  "Show BUFFER in all windows on this frame.
Defaults to current buffer."
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  (interactive (list (read-buffer "Switch to Buffer: "
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				  (current-buffer))))
  (or buffer (setq buffer (current-buffer)))
  (let ((orig-window (selected-window)))
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    (walk-windows
     (function
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      (lambda (win)
	(select-window win)
	(switch-to-buffer buffer))))
    (select-window orig-window)
    (follow-redisplay)))


(defun follow-switch-to-current-buffer-all ()
  "Show current buffer in all windows on this frame, and enter Follow Mode.

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in your `~/.emacs' file:
	(global-set-key [f7] 'follow-switch-to-current-buffer-all)"
  (interactive)
  (or (and (boundp 'follow-mode) follow-mode)
      (follow-mode 1))
  (follow-switch-to-buffer-all))

;;}}}
;;{{{ Movement

;; Note, these functions are not very useful, atleast not unless you
;; rebind the rather cumbersome key sequence `C-c . p'.

(defun follow-next-window ()
  "Select the next window showing the same buffer."
  (interactive)
  (let ((succ (cdr (follow-split-followers (follow-all-followers)))))
    (if succ
	(select-window (car succ))
      (error "%s" "No more windows"))))


(defun follow-previous-window ()
  "Select the previous window showing the same buffer."
  (interactive)
  (let ((pred (car (follow-split-followers (follow-all-followers)))))
    (if pred
	(select-window (car pred))
      (error "%s" "No more windows"))))


(defun follow-first-window ()
  "Select the first window in the frame showing the same buffer."
  (interactive)
  (select-window (car (follow-all-followers))))


(defun follow-last-window ()
  "Select the last window in the frame showing the same buffer."
  (interactive)
  (select-window (car (reverse (follow-all-followers)))))

;;}}}
;;{{{ Redraw

(defun follow-recenter (&optional arg)
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  "Recenter the middle window around point.
Rearrange all other windows around the middle window.
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With a positive argument, place the current line ARG lines
from the top.  With a negative, place it -ARG lines from the
bottom."
  (interactive "P")
  (if arg
      (let ((p (point))
	    (arg (prefix-numeric-value arg)))
	(if (>= arg 0)
	    ;; Recenter relative to the top.
	    (progn
	      (follow-first-window)
	      (goto-char p)
	      (recenter arg))
	  ;; Recenter relative to the bottom.
	  (follow-last-window)
	  (goto-char p)
	  (recenter arg)
	  ;; Otherwise, our post-command-hook will move the window
	  ;; right back.
	  (setq follow-internal-force-redisplay t)))
    ;; Recenter in the middle.
    (let* ((dest (point))
	   (windows (follow-all-followers))
	   (win (nth (/ (- (length windows) 1) 2) windows)))
      (select-window win)
      (goto-char dest)
      (recenter)
      ;;(setq follow-internal-force-redisplay t)
      )))


(defun follow-redraw ()
  "Arrange windows displaying the same buffer in successor order.
This function can be called even if the buffer is not in Follow mode.

Hopefully, there should be no reason to call this function when in
Follow mode since the windows should always be aligned."
  (interactive)
  (sit-for 0)
  (follow-redisplay))

;;}}}
;;{{{ End of buffer

(defun follow-end-of-buffer (&optional arg)
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  "Move point to the end of the buffer, Follow Mode style.
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If the end is not visible, it will be displayed in the last possible
window in the Follow Mode window chain.

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The mark is left at the previous position.  With arg N, put point N/10
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of the way from the true end."
  (interactive "P")
  (let ((followers (follow-all-followers))
	(pos (point)))
    (cond (arg
	   (select-window (car (reverse followers))))
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	  ((follow-select-if-end-visible
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	    (follow-windows-start-end followers)))
	  (t
	   (select-window (car (reverse followers)))))
    (goto-char pos)
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    (with-no-warnings
      (end-of-buffer arg))))
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;;}}}

;;}}}

;;{{{ Display

;;;; The display routines

;;{{{ Information gathering functions

(defun follow-all-followers (&optional testwin)
  "Return all windows displaying the same buffer as the TESTWIN.
The list contains only windows displayed in the same frame as TESTWIN.
If TESTWIN is nil the selected window is used."
  (or (and testwin (window-live-p testwin))
      (setq testwin (selected-window)))
  (let* ((top (frame-first-window (window-frame testwin)))
	 (win top)
	 (done nil)
	 (windows '())
	 (buffer (window-buffer testwin)))
    (while (and (not done) win)
      (if (eq (window-buffer win) buffer)
	  (setq windows (cons win windows)))
      (setq win (next-window win 'not))
      (if (eq win top)
	  (setq done t)))
    (nreverse windows)))


(defun follow-split-followers (windows &optional win)
  "Split the WINDOWS into the sets: predecessors and successors.
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Return `(PRED . SUCC)' where `PRED' and `SUCC' are ordered starting
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from the selected window."
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  (or win
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      (setq win (selected-window)))
  (let ((pred '()))
    (while (not (eq (car windows) win))
      (setq pred (cons (car windows) pred))
      (setq windows (cdr windows)))
    (cons pred (cdr windows))))


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;; This function is optimized function for speed!
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(defun follow-calc-win-end (&optional win)
  "Calculate the presumed window end for WIN.

Actually, the position returned is the start of the next
window, normally is the end plus one.

If WIN is nil, the selected window is used.

Returns (end-pos end-of-buffer-p)"
  (if follow-emacs-version-xemacs-p
      ;; XEmacs can calculate the end of the window by using
      ;; the 'guarantee options. GOOD!
      (let ((end (window-end win t)))
	(if (= end (funcall (symbol-function 'point-max)
			    (window-buffer win)))
	    (list end t)
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	  (list (+ end 1) nil)))
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    ;; Emacs: We have to calculate the end by ourselves.
    ;; This code works on both XEmacs and Emacs, but now
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    ;; that XEmacs has got custom-written code, this could
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    ;; be optimized for Emacs.
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    (let ((orig-win (and win (selected-window)))
	  height
	  buffer-end-p)
      (if win (select-window win))
      (prog1
	  (save-excursion
	    (goto-char (window-start))
	    (setq height (- (window-height) 1))
	    (setq buffer-end-p
		  (if (bolp)
		      (not (= height (vertical-motion height)))
		    (save-restriction
		      ;; Fix a mis-feature in `vertical-motion':
		      ;; The start of the window is assumed to
		      ;; coinside with the start of a line.
		      (narrow-to-region (point) (point-max))
		      (not (= height (vertical-motion height))))))
	    (list (point) buffer-end-p))
	(if orig-win
	    (select-window orig-win))))))


;; Can't use `save-window-excursion' since it triggers a redraw.
(defun follow-calc-win-start (windows pos win)
  "Calculate where WIN will start if the first in WINDOWS start at POS.

If WIN is nil the point below all windows is returned."
  (let (start)
    (while (and windows (not (eq (car windows) win)))
      (setq start (window-start (car windows)))
      (set-window-start (car windows) pos 'noforce)
      (setq pos (car (inline (follow-calc-win-end (car windows)))))
      (set-window-start (car windows) start 'noforce)
      (setq windows (cdr windows)))
    pos))


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;; The result from `follow-windows-start-end' is cached when using
;; a handful simple commands, like cursor movement commands.

(defsubst follow-cache-valid-p (windows)
  "Test if the cached value of `follow-windows-start-end' can be used.
Note that this handles the case when the cache has been set to nil."
  (let ((res t)
	(cache follow-windows-start-end-cache))
    (while (and res windows cache)
      (setq res (and (eq (car windows)
			 (car (car cache)))
		     (eq (window-start (car windows))
			 (car (cdr (car cache))))))
      (setq windows (cdr windows))
      (setq cache (cdr cache)))
    (and res (null windows) (null cache))))


(defsubst follow-invalidate-cache ()
  "Force `follow-windows-start-end' to recalculate the end of the window."
  (setq follow-windows-start-end-cache nil))


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;; Build a list of windows and their start and end positions.
;; Useful to avoid calculating start/end position whenever they are needed.
;; The list has the format:
;; ((Win Start End End-of-buffer-visible-p) ...)

;; Used to have a `save-window-excursion', but it obviously triggered
;; redraws of the display. Check if I used it for anything.


(defun follow-windows-start-end (windows)
  "Builds a list of (WIN START END BUFFER-END-P) for every window in WINDOWS."
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  (if (follow-cache-valid-p windows)
      follow-windows-start-end-cache
    (let ((win-start-end '())
	  (orig-win (selected-window)))
      (while windows
	(select-window (car windows))
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	(setq win-start-end
	      (cons (cons (car windows)
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			  (cons (window-start)
				(follow-calc-win-end)))
		    win-start-end))
	(setq windows (cdr windows)))
      (select-window orig-win)
      (setq follow-windows-start-end-cache (nreverse win-start-end))
      follow-windows-start-end-cache)))


(defsubst follow-pos-visible (pos win win-start-end)
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  "Non-nil when POS is visible in WIN."
  (let ((wstart-wend-bend (cdr (assq win win-start-end))))
    (and (>= pos (car wstart-wend-bend))
	 (or (< pos (car (cdr wstart-wend-bend)))
	     (nth 2 wstart-wend-bend)))))


;; By `aligned' we mean that for all adjecent windows, the end of the
;; first is equal with the start of the successor.  The first window
;; should start at a full screen line.

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(defsubst follow-windows-aligned-p (win-start-end)
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  "Non-nil if the follower WINDOWS are aligned."
  (let ((res t))
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    (save-excursion
       (goto-char (window-start (car (car win-start-end))))
       (if (bolp)
	   nil
	 (vertical-motion 0 (car (car win-start-end)))
	 (setq res (eq (point) (window-start (car (car win-start-end)))))))
    (while (and res (cdr win-start-end))
      ;; At least two followers left
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      (setq res (eq (car (cdr (cdr (car win-start-end))))
		    (car (cdr (car (cdr win-start-end))))))
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      (setq win-start-end (cdr win-start-end)))
    res))


;; Check if the point is visible in all windows. (So that
;; no one will be recentered.)

(defun follow-point-visible-all-windows-p (win-start-end)
  "Non-nil when the window-point is visible in all windows."
  (let ((res t))
    (while (and res win-start-end)
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      (setq res (follow-pos-visible (window-point (car (car win-start-end)))
				    (car (car win-start-end))
				    win-start-end))
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      (setq win-start-end (cdr win-start-end)))
    res))


;; Make sure WIN always starts at the beginning of an whole screen
;; line. If WIN is not aligned the start is updated which probably
;; will lead to a redisplay of the screen later on.
;;
;; This is used with the first window in a follow chain.  The reason
;; is that we want to detect that the point is outside the window.
;; (Without the update, the start of the window will move as the
;; user presses BackSpace, and the other window redisplay routines
;; will move the start of the window in the wrong direction.)

(defun follow-update-window-start (win)
  "Make sure that the start of WIN starts at a full screen line."
  (save-excursion
    (goto-char (window-start win))
    (if (bolp)
	nil
      (vertical-motion 0 win)
      (if (eq (point) (window-start win))
	  nil
	(vertical-motion 1 win)
	(set-window-start win (point) 'noforce)))))

;;}}}
;;{{{ Selection functions

;; Make a window in WINDOWS selected if it currently
;; is displaying the position DEST.
;;
;; We don't select a window if it just has been moved.

(defun follow-select-if-visible (dest win-start-end)
  "Select and return a window, if DEST is visible in it.
Return the selected window."
  (let ((win nil))
    (while (and (not win) win-start-end)
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      ;; Don't select a window that was just moved. This makes it
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      ;; possible to later select the last window after a `end-of-buffer'
      ;; command.
      (if (follow-pos-visible dest (car (car win-start-end)) win-start-end)
	  (progn
	    (setq win (car (car win-start-end)))
	    (select-window win)))
      (setq win-start-end (cdr win-start-end)))
    win))


;; Lets select a window showing the end. Make sure we only select it if it
;; it wasn't just moved here. (i.e. M-> shall not unconditionally place
;; the point in the selected window.)
;;
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;; (Compability cludge: in Emacs `window-end' is equal to `point-max';
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;; in XEmacs, it is equal to `point-max + 1'. Should I really bother
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;; checking `window-end' now when I check `end-of-buffer' explicitly?)
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(defun follow-select-if-end-visible (win-start-end)
  "Select and return a window, if end is visible in it."
  (let ((win nil))
    (while (and (not win) win-start-end)
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      ;; Don't select a window that was just moved. This makes it
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      ;; possible to later select the last window after a `end-of-buffer'
      ;; command.
      (if (and (eq (point-max) (nth 2 (car win-start-end)))
	       (nth 3 (car win-start-end))
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	       ;; `window-end' might return nil.
	       (let ((end (window-end (car (car win-start-end)))))
		 (and end
		      (eq (point-max) (min (point-max) end)))))
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	  (progn
	    (setq win (car (car win-start-end)))
	    (select-window win)))
      (setq win-start-end (cdr win-start-end)))
    win))


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;; Select a window that will display the point if the windows would
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;; be redisplayed with the first window fixed. This is useful for
;; example when the user has pressed return at the bottom of a window
;; as the point is not visible in any window.

(defun follow-select-if-visible-from-first (dest windows)
  "Select and return a window with DEST, if WINDOWS are redrawn from top."
  (let ((win nil)
	end-pos-end-p)
    (save-excursion
      (goto-char (window-start (car windows)))
      ;; Make sure the line start in the beginning of a real screen
      ;; line.
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      (vertical-motion 0 (car windows))
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      (if (< dest (point))
	  ;; Above the start, not visible.
	  nil
	;; At or below the start. Check the windows.
	(save-window-excursion
	  (while (and (not win) windows)
	    (set-window-start (car windows) (point) 'noforce)
	    (setq end-pos-end-p (follow-calc-win-end (car windows)))
	    (goto-char (car end-pos-end-p))
	    ;; Visible, if dest above end, or if eob is visible inside
	    ;; the window.
	    (if (or (car (cdr end-pos-end-p))
		    (< dest (point)))
		  (setq win (car windows))
		(setq windows (cdr windows)))))))
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    (if win
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	(select-window win))
    win))


;;}}}
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;;{{{ Redisplay
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;; Redraw all the windows on the screen, starting with the top window.
;; The window used as as marker is WIN, or the selcted window if WIN
;; is nil.

(defun follow-redisplay (&optional windows win)
  "Reposition the WINDOWS around WIN.
Should the point be too close to the roof we redisplay everything
from the top. WINDOWS should contain a list of windows to
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redisplay, it is assumed that WIN is a member of the list.
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Should WINDOWS be nil, the windows displaying the
same buffer as WIN, in the current frame, are used.
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Should WIN be nil, the selected window is used."
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  (or win
      (setq win (selected-window)))
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  (or windows
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      (setq windows (follow-all-followers win)))
  (follow-downward windows (follow-calculate-first-window-start windows win)))


;; Redisplay a chain of windows. Start every window directly after the
;; end of the previous window, to make sure long lines are displayed
;; correctly.

(defun follow-downward (windows pos)
  "Redisplay all WINDOWS starting at POS."
  (while windows
    (set-window-start (car windows) pos)
    (setq pos (car (follow-calc-win-end (car windows))))
    (setq windows (cdr windows))))


;;(defun follow-downward (windows pos)
;;  "Redisplay all WINDOWS starting at POS."
;;  (let (p)
;;    (while windows
;;      (setq p (window-point (car windows)))
;;      (set-window-start (car windows) pos)
;;      (set-window-point (car windows) (max p pos))
;;      (setq pos (car (follow-calc-win-end (car windows))))
;;      (setq windows (cdr windows)))))


;; Return the start of the first window.
;;
;; First, estimate the position. It the value is not perfect (i.e. we
;; have somewhere splited a line between windows) we try to enhance
;; the value.
;;
;; The guess is always perfect if no long lines is split between
;; windows.
;;
;; The worst case peformace of probably very bad, but it is very
;; unlikely that we ever will miss the correct start by more than one
;; or two lines.

(defun follow-calculate-first-window-start (windows &optional win start)
  "Calculate the start of the first window.

WINDOWS is a chain of windows to work with.  WIN is the window
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to recenter around.  It is assumed that WIN starts at position
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START."
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  (or win
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      (setq win (selected-window)))
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  (or start
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      (setq start (window-start win)))
  (let ((guess (follow-estimate-first-window-start windows win start)))
    (if (car guess)
	(cdr guess)
      ;; The guess wasn't exact, try to enhance it.
      (let ((win-start (follow-calc-win-start windows (cdr guess) win)))
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	(cond ((= win-start start)
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	       (follow-debug-message "exact")
	       (cdr guess))
	      ((< win-start start)
	       (follow-debug-message "above")
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	       (follow-calculate-first-window-start-from-above
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		windows (cdr guess) win start))
	      (t
	       (follow-debug-message "below")
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	       (follow-calculate-first-window-start-from-below
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		windows (cdr guess) win start)))))))


;; `exact' is disabled due to XEmacs and fonts of variable
;; height.
(defun follow-estimate-first-window-start (windows win start)
  "Estimate the position of the first window.

Returns (EXACT . POS). If EXACT is non-nil, POS is the starting
position of the first window. Otherwise it is a good guess."
  (let ((pred (car (follow-split-followers windows win)))
	(exact nil))
    (save-excursion
      (goto-char start)
      ;(setq exact (bolp))
      (vertical-motion 0 win)
      (while pred
	(vertical-motion (- 1 (window-height (car pred))) (car pred))
	(if (not (bolp))
	  (setq exact nil))
	(setq pred (cdr pred)))
      (cons exact (point)))))


;; Find the starting point, start at GUESS and search downward.
;; The returned point is always a point below GUESS.

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(defun follow-calculate-first-window-start-from-above
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       (windows guess win start)
  (save-excursion
    (let ((done nil)
	  win-start
	  res)
      (goto-char guess)
      (while (not done)
	(if (not (= (vertical-motion 1 (car windows)) 1))
	    ;; Hit bottom! (Can we really do this?)
	    ;; We'll keep it, since it ensures termination.
	    (progn
	      (setq done t)
	      (setq res (point-max)))
	  (setq win-start (follow-calc-win-start windows (point) win))
	  (if (>= win-start start)
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	      (progn
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		(setq done t)
		(setq res (point))))))
      res)))


;; Find the starting point, start at GUESS and search upward.  Return
;; a point on the same line as GUESS, or above.
;;
;; (Is this ever used? I must make sure it works just in case it is
;; ever called.)

(defun follow-calculate-first-window-start-from-below
       (windows guess &optional win start)
  (setq win (or win (selected-window)))
  (setq start (or start (window-start win)))
  (save-excursion
    (let ((done nil)
	  win-start
	  res)
      ;; Always calculate what happend when no line is displayed in the first
      ;; window. (The `previous' res is needed below!)
      (goto-char guess)
      (vertical-motion 0 (car windows))
      (setq res (point))
      (while (not done)
	(if (not (= (vertical-motion -1 (car windows)) -1))
	    ;; Hit roof!
	    (progn
	      (setq done t)
	      (setq res (point-min)))
	  (setq win-start (follow-calc-win-start windows (point) win))
	  (cond ((= win-start start)	; Perfect match, use this value
		 (setq done t)
		 (setq res (point)))
		((< win-start start)	; Walked to far, use preious result
		 (setq done t))
		(t			; Store result for next iteration
		 (setq res (point))))))
      res)))

;;}}}
;;{{{ Avoid tail recenter

;; This sets the window internal flag `force_start'. The effect is that
;; windows only displaying the tail isn't recentered.
;; Has to be called before every redisplay... (Great isn't it?)
;;
;; XEmacs doesn't recenter the tail, GOOD!
;;
;; A window displaying only the tail, is a windows whose
;; window-start position is equal to (point-max) of the buffer it
;; displays.
;;
;; This function is also added to `post-command-idle-hook', introduced
;; in Emacs 19.30.  This is needed since the vaccine injected by the
;; call from `post-command-hook' only works until the next redisplay.
;; It is possible that the functions in the `post-command-idle-hook'
;; can cause a redisplay, and hence a new vaccine is needed.
;;
;; Sometimes, calling this function could actually cause a redisplay,
;; especially if it is placed in the debug filter section.  I must
;; investigate this further...

(defun follow-avoid-tail-recenter (&rest rest)
  "Make sure windows displaying the end of a buffer aren't recentered.

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This is done by reading and rewriting the start position of
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non-first windows in Follow Mode."
  (if follow-avoid-tail-recenter-p
      (let* ((orig-buffer (current-buffer))
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