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;;; follow.el --- synchronize windows showing the same buffer
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;; Copyright (C) 1995, 1996, 1997, 1999, 2001, 2002, 2003, 2004,
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;;   2005, 2006, 2007, 2008 Free Software Foundation, Inc.
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;; Author: Anders Lindgren <andersl@andersl.com>
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;; Maintainer: FSF (Anders' email bounces, Sep 2005)
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;; Created: 1995-05-25
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;; Keywords: display, window, minor-mode, convenience
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;; This file is part of GNU Emacs.

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;; 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
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;; the Free Software Foundation, either version 3 of the License, 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.  If not, see <http://www.gnu.org/licenses/>.
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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 display adjacent sections of the buffer.
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;;   This means that whenever one window is moved, all the
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;;   others will follow.  (Hence the name Follow mode.)
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;;
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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
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;; mode, use two full-height windows as though they are 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
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;; autoloaded when activated (see below).  Then do the following:
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;;
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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
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;;   immediately below the end of the left-hand window.  As long as
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;;   `follow-mode' is active, the two windows will follow each other!
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;;
;; * Play around and enjoy! Scroll one window and watch the other.
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;;   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
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;;   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 exist two system variables that control the appearence of
;; lines wider than the window containing them.  The default 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 functions that do
;; whatever you would like to do.  These functions are then added to
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;; 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"
;; and press Return.  To deactivate, do it again.
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;;
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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
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;;		Scroll text in a Follow mode window chain up.
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;;
;;	follow-scroll-down			 C-c . v
;;		Like `follow-scroll-up', but in the other direction.
;;
;;	follow-delete-other-windows-and-split	 C-c . 1
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;;		Maximize the visible area of the current buffer,
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;;		and enter Follow mode. 	This is a very convenient
;;		way to start Follow mode, hence we recomend that
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;;		this command be added to the global keymap.
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;;
;;	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
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;;		Switch buffer in all windows in the selected frame.
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;;
;;	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
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;;    you want to view two locations.  Of course, `follow' mode can
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;;    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
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;; 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
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;; 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.

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(require 'easymenu)

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(eval-when-compile
  (if (or (featurep 'bytecomp)
	  (featurep 'byte-compile))
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      (cond ((featurep 'xemacs)
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	     ;; 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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(defcustom follow-mode-hook nil
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  "Normal hook run by `follow-mode'."
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  :type 'hook
  :group 'follow)
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(defcustom follow-mode-off-hook nil
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  "Hooks to run when Follow mode is turned off."
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  :type 'hook
  :group 'follow)
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(make-obsolete-variable 'follow-mode-off-hook 'follow-mode-hook "22.2")
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;;{{{ Keymap/Menu
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;; 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
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;; loaded.  Also note that the only valid way to manipulate the
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;; keymap is to use `define-key'.  Don't change it using `setq' or
;; similar!
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(defcustom follow-mode-prefix "\C-c."
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  "Prefix key to use for follow commands in Follow mode.
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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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(defvar follow-mode-map
  (let ((mainmap (make-sparse-keymap))
        (map (make-sparse-keymap)))
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    (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)

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    (define-key mainmap follow-mode-prefix map)
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    ;; 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.)
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    (define-key mainmap [remap end-of-buffer] 'follow-end-of-buffer)
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    mainmap)
  "Minor mode keymap for Follow mode.")
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;; When the mode is not activated, only one item is visible to activate
;; the mode.
(defun follow-menu-filter (menu)
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  (if (bound-and-true-p follow-mode)
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      menu
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    '(["Follow mode"	follow-mode
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       :style toggle :selected follow-mode])))

;; If there is a `tools' menu, 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.
(easy-menu-add-item nil '("Tools")
  '("Follow"
    ;; The Emacs code used to just grey out operations when follow-mode was
    ;; not enabled, whereas the XEmacs code used to remove it altogether.
    ;; Not sure which is preferable, but clearly the preference should not
    ;; depend on the flavor.
    :filter follow-menu-filter
    ["Scroll Up"	follow-scroll-up	follow-mode]
    ["Scroll Down"	follow-scroll-down	follow-mode]
    "--"
    ["Delete Other Windows and Split" follow-delete-other-windows-and-split follow-mode]
    "--"
    ["Switch To Buffer"	follow-switch-to-buffer	follow-mode]
    ["Switch To Buffer (all windows)" follow-switch-to-buffer-all follow-mode]
    "--"
    ["First Window"	follow-first-window	follow-mode]
    ["Last Window"	follow-last-window	follow-mode]
    ["Next Window"	follow-next-window	follow-mode]
    ["Previous Window"	follow-previous-window	follow-mode]
    "--"
    ["Recenter"		follow-recenter		follow-mode]
    "--"
    ["Follow mode"	follow-mode :style toggle :selected follow-mode]))

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;;}}}
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(defcustom follow-mode-line-text " Follow"
  "Text shown in the mode line when Follow mode is active.
Defaults to \" Follow\".  Examples of other values
are \" Fw\", or simply \"\"."
  :type 'string
  :group 'follow)

(defcustom follow-auto nil
  "Non-nil activates Follow mode whenever a file is loaded."
  :type 'boolean
  :group 'follow)

(defcustom follow-intercept-processes (fboundp 'start-process)
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  "When non-nil, Follow mode will monitor process output."
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  :type 'boolean
  :group 'follow)

(defvar follow-avoid-tail-recenter-p (not (featurep 'xemacs))
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  "*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
the buffer.  Normally it is practical for the user that empty
windows are recentered automatically.  However, when using
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Follow mode it breaks the display when the end is displayed
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in a window \"above\" the last window.  This is for
example the case when displaying a short page in info.

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Must be set before Follow mode is loaded.
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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!")

(defvar follow-cache-command-list
  '(next-line previous-line forward-char backward-char)
  "List of commands that don't require recalculation.

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.")

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


;; Internal variables:

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

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

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

;; This inline function must be as small as possible!
;; Maybe we should define a macro that expands to nil if
;; the variable is not set.

(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)))
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;;}}}
;;{{{ Cache

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(dolist (cmd follow-cache-command-list)
  (put cmd 'follow-mode-use-cache t))
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;;}}}

;;{{{ The mode

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


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

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(put 'follow-mode 'permanent-local t)
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;;;###autoload
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(define-minor-mode follow-mode
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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
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  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
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side-by-side windows are used.  The user can, with the help of Follow
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mode, use two full-height windows as though they would have been
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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.

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

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This command runs the normal hook `follow-mode-hook'.
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Keys specific to Follow mode:
\\{follow-mode-map}"
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  :keymap follow-mode-map
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  (when (and follow-mode follow-intercept-processes)
    (follow-intercept-process-output))
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  (cond (follow-mode ; On
         ;; XEmacs: If this is non-nil, the window will scroll before
         ;; the point will have a chance to get into the next window.
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         (when (boundp 'scroll-on-clipped-lines)
	   (setq scroll-on-clipped-lines nil))
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         (force-mode-line-update)
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         (add-hook 'post-command-hook 'follow-post-command-hook t))
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        ((not follow-mode) ; Off
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         (force-mode-line-update))))
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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 ()
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  "Find-file hook for Follow mode.  See the variable `follow-auto'."
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  (if follow-auto (follow-mode t)))

;;}}}

;;{{{ User functions

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

;;{{{ Scroll

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;; `scroll-up' and `-down', but for windows in Follow mode.
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;;
;; 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)
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  "Scroll text in a Follow mode window chain up.
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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.

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Works like `scroll-up' when not in Follow mode."
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  (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)
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  "Scroll text in a Follow mode window chain down.
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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.

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Works like `scroll-up' when not in Follow mode."
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  (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)
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				    (if header-line-format 2 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)
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  "Create two side by side windows and enter Follow mode.
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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.)

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If ARG is positive, the leftmost window is selected.  If negative,
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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)
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  "Show BUFFER in all windows in the current Follow mode window chain."
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  (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 ()
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  "Show current buffer in all windows on this frame, and enter Follow mode.
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To bind this command to a hotkey place the following line
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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

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;; Note, these functions are not very useful, at least not unless you
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;; 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
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from the top.  With a negative argument, place it -ARG lines
from the bottom."
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  (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
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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."
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  (or (window-live-p testwin)
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      (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)"
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  (if (featurep 'xemacs)
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      ;; 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))
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	    (setq height
		  (- (window-height)
		     (if header-line-format 2 1)))
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	    (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)
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      (setq pos (car (follow-calc-win-end (car windows))))
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      (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."
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  (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)
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  "Non-nil when the `window-point' is visible in all windows."
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  (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))
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    (unless (bolp)
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      (vertical-motion 0 win)
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      (unless (eq (point) (window-start win))
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	(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."
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  (let (win)
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    (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.
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      (when (follow-pos-visible dest (caar win-start-end) win-start-end)
	(setq win (caar win-start-end))
	(select-window win))
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      (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
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;; is nil.  Start every window directly after the end of the previous
;; window, to make sure long lines are displayed correctly.
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(defun follow-redisplay (&optional windows win)
  "Reposition the WINDOWS around WIN.
Should the point be too close to the roof we redisplay everything
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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)))
  (or windows (setq windows (follow-all-followers win)))
  ;; Calculate the start of the first window.
  (let* ((old-win-start (window-start win))
	 (try-first-start (follow-estimate-first-window-start
			   windows win old-win-start))
	 (try-win-start (follow-calc-win-start
			 windows try-first-start win))
	 (start (cond ((= try-win-start old-win-start)
		       (follow-debug-message "exact")
		       try-first-start)
		      ((< try-win-start old-win-start)
		       (follow-debug-message "above")
		       (follow-calculate-first-window-start-from-above
			windows try-first-start win old-win-start))
		      (t
		       (follow-debug-message "below")
		       (follow-calculate-first-window-start-from-below
			windows try-first-start win old-win-start)))))
    (dolist (w windows)
      (set-window-start w start)
      (setq start (car (follow-calc-win-end w))))))
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(defun follow-estimate-first-window-start (windows win start)
  "Estimate the position of the first window.
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The estimate is computed by assuming that the window WIN, which
should be a member of WINDOWS, starts at position START."
  (let ((windows-before (car (follow-split-followers windows win))))
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    (save-excursion
      (goto-char start)
      (vertical-motion 0 win)
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      (dolist (w windows-before)
	(vertical-motion (- 1 (window-text-height w)) w))
      (point))))
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;; 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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	      (setq done t res (point)))))
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      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)
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      ;; Always calculate what happens when no line is displayed in the first
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      ;; 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!
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	    (setq done t res (point-min))
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	  (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