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+;;; hscroll.el: Minor mode to automatically scroll truncated lines horizontally
+;;; Copyright (C) 1992, 1993, 1995, 1996 Free Software Foundation, Inc.
+
+;; Author: Wayne Mesard <wmesard@esd.sgi.com>
+;; Keywords: display
+
+;; This file is part of GNU Emacs.
+
+;; GNU Emacs is free software; you can redistribute it and/or modify
+;; it under the terms of the GNU General Public License as published by
+;; the Free Software Foundation; either version 2, or (at your option)
+;; any later version.
+
+;; GNU Emacs is distributed in the hope that it will be useful,
+;; but WITHOUT ANY WARRANTY; without even the implied warranty of
+;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;; GNU General Public License for more details.
+
+;; You should have received a copy of the GNU General Public License
+;; along with GNU Emacs; see the file COPYING.  If not, write to the
+;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
+;; Boston, MA 02111-1307, USA.
+
+;;; Commentary:a
+;;
+;;    Automatically scroll horizontally when the point moves off the
+;;    left or right edge of the window.  
+;;
+;;    - Type "M-x hscroll-mode" to enable it in the current buffer.
+;;    - Type "M-x hscroll-global-mode" to enable it in every buffer.
+;;    - "turn-on-hscroll" is useful in mode hooks as in:
+;;          (add-hook 'text-mode-hook 'turn-on-hscroll)
+;;
+;;    - hscroll-margin controls how close the cursor can get to the edge 
+;;      of the window.
+;;    - hscroll-step-percent controls how far to jump once we decide to do so.
+;;
+;;    Most users won't want to mess with the other variables defined
+;;    here.  But they're all documented, and they all start with
+;;    "hscroll-" if you're curious.
+;;
+;;    Oh, you should also know that if you set the hscroll-margin and
+;;    hscroll-step-percent large enough, you can get an interesting, but
+;;    undesired ping-pong effect as the point bounces from one edge to
+;;    the other.
+;;
+;;    wmesard@sgi.com
+
+;;; Code:
+
+;;; 
+;;; PUBLIC VARIABLES
+;;; 
+
+(defvar hscroll-version "2.2")
+
+(defvar hscroll-margin 5 
+  "*How many columns away from the edge of the window point is allowed to get
+before HScroll will horizontally scroll the window.")
+
+(defvar hscroll-snap-threshold 30
+  "*When point is this many columns (or less) from the left edge of the document, 
+don't do any horizontal scrolling.  In other words, be biased towards the left
+edge of the document.
+  Set this variable to zero to disable this bias.")
+
+(defvar hscroll-step-percent 25
+  "*How far away to place the point from the window's edge when scrolling.
+Expressed as a percentage of the window's width.")
+
+(defvar hscroll-mode-name " Hscr"
+  "*Horizontal scrolling mode line indicator.
+Set this to nil to conserve valuable mode line space.")
+
+(or (assq 'hscroll-mode minor-mode-alist)
+    (setq minor-mode-alist
+	  (cons '(hscroll-mode hscroll-mode-name) minor-mode-alist)))
+
+
+;;; 
+;;; PRIVATE VARIABLES
+;;; 
+
+(defvar hscroll-mode nil 
+  "Non-nil if HScroll mode is enabled.")
+(make-variable-buffer-local 'hscroll-mode)
+
+
+(defvar hscroll-old-truncate-local nil)
+(defvar hscroll-old-truncate-was-global nil)
+(make-variable-buffer-local 'hscroll-old-truncate)
+(make-variable-buffer-local 'hscroll-old-truncate-was-global)
+
+(defvar hscroll-old-truncate-default nil)
+
+;;; 
+;;; PUBLIC COMMANDS
+;;; 
+
+;;;###autoload
+(defun turn-on-hscroll ()
+  "Unconditionally turn on Hscroll mode in the current buffer."
+  (hscroll-mode 1))
+
+;;;###autoload
+(defun hscroll-mode (&optional arg)
+  "Toggle HScroll mode in the current buffer.
+With ARG, turn HScroll mode on if ARG is positive, off otherwise.
+In HScroll mode, truncated lines will automatically scroll left or
+right when point gets near either edge of the window.
+  See also \\[hscroll-global-mode]."
+  (interactive "P")
+  (make-local-hook 'post-command-hook)
+  (let ((newmode (if (null arg)
+		      (not hscroll-mode)
+		    (> (prefix-numeric-value arg) 0))))
+
+    (if newmode
+	;; turn it on
+	(if (not hscroll-mode)
+	    ;; it was off
+	    (let ((localp (local-variable-p 'truncate-lines)))
+	      (if localp
+		  (setq hscroll-old-truncate-local truncate-lines))
+	      (setq hscroll-old-truncate-was-global (not localp))
+	      (setq truncate-lines t)
+	      (add-hook 'post-command-hook 
+			(function hscroll-window-maybe) nil t)
+	      ))
+      ;; turn it off
+      (if hscroll-mode
+	  ;; it was on
+	  (progn
+	    (if hscroll-old-truncate-was-global
+		(kill-local-variable 'truncate-lines)
+	      (setq truncate-lines hscroll-old-truncate-local))
+	    (if (not truncate-lines)
+		(set-window-hscroll (selected-window) 0))
+	    (remove-hook 'post-command-hook
+			 (function hscroll-window-maybe) t)
+	    ))
+      )
+
+    (setq hscroll-mode newmode)
+    (force-mode-line-update nil)
+    ))
+
+
+;;;###autoload
+(defun hscroll-global-mode  (&optional arg)
+  "Toggle HScroll mode in all buffers.
+With ARG, turn HScroll mode on if ARG is positive, off otherwise.
+If a buffer ever has HScroll mode set locally (via \\[hscroll-mode]), 
+it will forever use the local value (i.e., \\[hscroll-global-mode] 
+will have no effect on it).
+  See also \\[hscroll-mode]."
+  (interactive "P")
+  (let* ((oldmode (default-value 'hscroll-mode))
+	 (newmode (if (null arg)
+		      (not oldmode)
+		    (> (prefix-numeric-value arg) 0))))
+
+    (if newmode
+	;; turn it on
+	(if (not hscroll-mode)
+	    ;; it was off
+	    (progn
+	      (setq hscroll-old-truncate-default (default-value truncate-lines))
+	      (setq hscroll-old-truncate-was-global t)
+	      (setq-default truncate-lines t)
+	      (add-hook 'post-command-hook (function hscroll-window-maybe))
+	      ))
+      ;; turn it off
+      (if hscroll-mode
+	  ;; it was on
+	  (progn
+	    (setq-default truncate-lines hscroll-old-truncate-default)
+	    (remove-hook 'post-command-hook (function hscroll-window-maybe))
+	    ))
+      )
+
+    (setq-default hscroll-mode newmode)
+    (force-mode-line-update t)
+    ))
+
+(defun hscroll-window-maybe ()
+  "Scroll horizontally if point is off or nearly off the edge of the window.
+This is called automatically when in HScroll mode, but it can be explicitly
+invoked as well (i.e., it can be bound to a key)."
+  (interactive)
+  ;; Only consider scrolling if truncate-lines is true, 
+  ;; the window is already scrolled or partial-widths is true and this is
+  ;; a partial width window.  See display_text_line() in xdisp.c.
+  (if (and hscroll-mode
+	   (or truncate-lines
+	       (not (zerop (window-hscroll)))
+	       (and truncate-partial-width-windows
+		    (< (window-width) (frame-width)))))
+      (let ((linelen (save-excursion (end-of-line) (current-column)))
+	    (rightmost-char (+ (window-width) (window-hscroll)))
+	    )
+ 	(if (< (current-column) hscroll-snap-threshold)
+ 	    (set-window-hscroll 
+ 	     (selected-window) 
+ 	     (- (window-hscroll)))
+ 	  (if (>= (current-column)
+		(- rightmost-char hscroll-margin
+		   ;; Off-by-one if the left edge is scrolled
+		   (if (not (zerop (window-hscroll))) 1 0)
+		   ;; Off by one if the right edge is scrolled
+		   (if (> linelen rightmost-char) 1 0)
+		   ))
+	    ;; Scroll to the left a proportion of the window's width.
+	    (set-window-hscroll 
+	     (selected-window) 
+	     (- (+ (current-column) 
+		   (/ (* (window-width) hscroll-step-percent) 100))
+		(window-width)))
+	  (if (< (current-column) (+ (window-hscroll) hscroll-margin))
+	      ;; Scroll to the right a proportion of the window's width.
+	      (set-window-hscroll
+	       (selected-window)
+	       (- (current-column) (/ (* (window-width) hscroll-step-percent) 100)))
+	    )))
+	)))
+
+;;; 
+;;; It's not a bug, it's a *feature*
+;;; 
+
+(provide 'hscroll)
+
+;;; hscroll.el ends here