lisp/auto-show.el

   1 ;;; auto-show.el --- perform automatic horizontal scrolling as point moves
   2 ;;; This file is in the public domain.
   3
   4 ;;; Keywords: scroll display minor-mode
   5 ;;; Author: Pete Ware <ware@cis.ohio-state.edu>
   6 ;;; Maintainer: FSF
   7
   8 ;;; Commentary:
   9
  10 ;;; This file provides functions that
  11 ;;; automatically scroll the window horizontally when the point moves
  12 ;;; off the left or right side of the window.
  13
  14 ;;; Once this library is loaded, automatic horizontal scrolling
  15 ;;; occurs whenever long lines are being truncated.
  16 ;;; To request truncation of long lines, set the variable
  17 ;;; Setting the variable `truncate-lines' to non-nil.
  18 ;;; You can do this for all buffers as follows:
  19 ;;;
  20 ;;; (set-default 'truncate-lines t)
  21
  22 ;;; Here is how to do it for C mode only:
  23 ;;;
  24 ;;; (set-default 'truncate-lines nil)   ; this is the original value
  25 ;;; (defun my-c-mode-hook ()
  26 ;;;   "Run when C-mode starts up.  Changes ..."
  27 ;;;   ... set various personal preferences ...
  28 ;;;   (setq truncate-lines t))
  29 ;;; (add-hook 'c-mode-hook 'my-c-mode-hook)
  30 ;;;
  31 ;;;
  32 ;;; As a finer level of control, you can still have truncated lines but
  33 ;;; without the automatic horizontal scrolling by setting the buffer
  34 ;;; local variable `auto-show-mode' to nil.  The default value is t.
  35 ;;; The command `auto-show-mode' toggles the value of the variable
  36 ;;; `auto-show-mode'.
  37
  38 ;;; Code:
  39
  40 (defvar auto-show-mode t
  41   "*Non-nil enables automatic horizontal scrolling, when lines are truncated.
  42 The default value is t.  To change the default, do this:
  43         (set-default 'auto-show-mode nil)
  44 See also command `auto-show-mode'.
  45 This variable has no effect when lines are not being truncated.")
  46
  47 (make-variable-buffer-local 'auto-show-mode)
  48
  49 (defvar auto-show-shift-amount 8
  50   "*Extra columns to scroll. for automatic horizontal scrolling.")
  51
  52 (defvar auto-show-show-left-margin-threshold 50
  53   "*Threshold column for automatic horizontal scrolling to the right.
  54 If point is before this column, we try to scroll to make the left margin
  55 visible.  Setting this to 0 disables this feature.")
  56
  57 (defun auto-show-truncationp ()
  58   "True if line truncation is enabled for the selected window."
  59   (or truncate-lines
  60       (and truncate-partial-width-windows
  61            (< (window-width) (frame-width)))))
  62
  63 ;;;###autoload
  64 (defun auto-show-mode (arg)
  65   "Turn automatic horizontal scroll mode on or off.
  66 With arg, turn auto scrolling on if arg is positive, off otherwise."
  67   (interactive "P")
  68   (setq auto-show-mode
  69         (if (null arg)
  70             (not auto-show-mode)
  71           (> (prefix-numeric-value arg) 0))))
  72
  73 (defun auto-show-make-point-visible (&optional ignore-arg)
  74   "Scroll horizontally to make point visible, if that is enabled.
  75 This function only does something if `auto-show-mode' is non-nil
  76 and longlines are being truncated in the selected window.
  77 See also the command `auto-show-toggle'."
  78   (interactive)
  79   (if (and auto-show-mode (auto-show-truncationp)
  80            (equal (window-buffer) (current-buffer)))
  81       (let* ((col (current-column))     ;column on line point is at
  82              (scroll (window-hscroll))  ;how far window is scrolled
  83              (w-width (- (window-width)
  84                          (if (> scroll 0)
  85                              2 1)))     ;how wide window is on the screen
  86              (right-col (+ scroll w-width)))
  87         (if (and (< col auto-show-show-left-margin-threshold)
  88                  (< col (window-width))
  89                  (> scroll 0))
  90             (scroll-right scroll)
  91           (if (< col scroll)            ;to the left of the screen
  92               (scroll-right (+ (- scroll col) auto-show-shift-amount))
  93             (if (or (> col right-col)   ;to the right of the screen
  94                     (and (= col right-col)
  95                          (not (eolp))))
  96                 (scroll-left (+ auto-show-shift-amount
  97                                 (- col (+ scroll w-width))))
  98               )
  99             )
 100           )
 101         )
 102     )
 103   )
 104
 105 ;; Do auto-scrolling after commands.
 106 (add-hook 'post-command-hook 'auto-show-make-point-visible)
 107
 108 ;; Do auto-scrolling in comint buffers after process output also.
 109 (add-hook 'comint-output-filter-functions 'auto-show-make-point-visible t)
 110
 111 (provide 'auto-show)
 112
 113 ;; auto-show.el ends here
 114