lisp/reposition.el

   1 ;;; reposition.el --- center a Lisp function or comment on the screen
   2
   3 ;; Copyright (C) 1991, 1994, 2001-2017 Free Software Foundation, Inc.
   4
   5 ;; Author: Michael D. Ernst <mernst@theory.lcs.mit.edu>
   6 ;; Created: Jan 1991
   7 ;; Maintainer: emacs-devel@gnu.org
   8
   9 ;; This file is part of GNU Emacs.
  10
  11 ;; GNU Emacs is free software: you can redistribute it and/or modify
  12 ;; it under the terms of the GNU General Public License as published by
  13 ;; the Free Software Foundation, either version 3 of the License, or
  14 ;; (at your option) any later version.
  15
  16 ;; GNU Emacs is distributed in the hope that it will be useful,
  17 ;; but WITHOUT ANY WARRANTY; without even the implied warranty of
  18 ;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  19 ;; GNU General Public License for more details.
  20
  21 ;; You should have received a copy of the GNU General Public License
  22 ;; along with GNU Emacs.  If not, see <http://www.gnu.org/licenses/>.
  23
  24 ;;; Commentary:
  25
  26 ;; Reposition-window makes an entire function definition or comment visible,
  27 ;; or, if it is already visible, places it at the top of the window;
  28 ;; additional invocations toggle the visibility of comments preceding the
  29 ;; code.  For the gory details, see the documentation for reposition-window;
  30 ;; rather than reading that, you may just want to play with it.
  31
  32 ;; This tries pretty hard to do the recentering correctly; the precise
  33 ;; action depends on what the buffer looks like.  If you find a situation
  34 ;; where it doesn't behave well, let me know.  This function is modeled
  35 ;; after one of the same name in ZMACS, but the code is all-new and the
  36 ;; behavior in some situations differs.
  37
  38 ;;; Code:
  39
  40 ;;;###autoload
  41 (defun reposition-window (&optional arg)
  42   "Make the current definition and/or comment visible.
  43 Further invocations move it to the top of the window or toggle the
  44 visibility of comments that precede it.
  45   Point is left unchanged unless prefix ARG is supplied.
  46   If the definition is fully onscreen, it is moved to the top of the
  47 window.  If it is partly offscreen, the window is scrolled to get the
  48 definition (or as much as will fit) onscreen, unless point is in a comment
  49 which is also partly offscreen, in which case the scrolling attempts to get
  50 as much of the comment onscreen as possible.
  51   Initially `reposition-window' attempts to make both the definition and
  52 preceding comments visible.  Further invocations toggle the visibility of
  53 the comment lines.
  54   If ARG is non-nil, point may move in order to make the whole defun
  55 visible (if only part could otherwise be made so), to make the defun line
  56 visible (if point is in code and it could not be made so, or if only
  57 comments, including the first comment line, are visible), or to make the
  58 first comment line visible (if point is in a comment)."
  59   (interactive "P")
  60   (let* (;; (here (line-beginning-position))
  61          (here (point))
  62          ;; change this name once I've gotten rid of references to ht.
  63          ;; this is actually the number of the last screen line
  64          (ht (- (window-height) 2))
  65          (line (repos-count-screen-lines (window-start) (point)))
  66          (comment-height
  67           ;; The call to max deals with the case of cursor between defuns.
  68           (max 0
  69                (repos-count-screen-lines-signed
  70                 ;; the beginning of the preceding comment
  71                 (save-excursion
  72                   (if (not (eobp)) (forward-char 1))
  73                   (end-of-defun -1)
  74                   ;; Skip whitespace, newlines, and form feeds.
  75                   (if (re-search-forward "[^ \t\n\f]" nil t)
  76                       (backward-char 1))
  77                   (point))
  78                 here)))
  79          (defun-height
  80            (repos-count-screen-lines-signed
  81             (save-excursion
  82               (end-of-defun 1) ; so comments associate with following defuns
  83               (beginning-of-defun 1)
  84               (point))
  85             here))
  86          ;; This must be positive, so don't use the signed version.
  87          (defun-depth (repos-count-screen-lines here
  88                                                 (save-excursion
  89                                                   (end-of-defun 1)
  90                                                   (point))))
  91          (defun-line-onscreen-p
  92            (and (<= defun-height line)
  93                 (<= (- line defun-height) ht))))
  94     (cond ((or (= comment-height line)
  95                (and (= line ht)
  96                     (> comment-height line)
  97                     ;; if defun line offscreen, we should be in case 4
  98                     defun-line-onscreen-p))
  99            ;; Either first comment line is at top of screen or (point at
 100            ;; bottom of screen, defun line onscreen, and first comment line
 101            ;; off top of screen).  That is, it looks like we just did
 102            ;; recenter-definition, trying to fit as much of the comment
 103            ;; onscreen as possible.  Put defun line at top of screen; that
 104            ;; is, show as much code, and as few comments, as possible.
 105
 106            (if (and arg (> defun-depth (1+ ht)))
 107                ;; Can't fit whole defun onscreen without moving point.
 108                (progn (end-of-defun) (beginning-of-defun) (recenter 0))
 109              (recenter (max defun-height 0)))
 110            ;;(repos-debug-macro "1")
 111            )
 112
 113           ((or (= defun-height line)
 114                (= line 0)
 115                (and (< line comment-height)
 116                     (< defun-height 0)))
 117            ;; Defun line or cursor at top of screen, OR cursor in comment
 118            ;; whose first line is offscreen.
 119            ;; Avoid moving definition up even if defun runs offscreen;
 120            ;; we care more about getting the comment onscreen.
 121
 122            (cond ((= line ht)
 123                   ;; cursor on last screen line (and so in a comment)
 124                   (if arg (progn (end-of-defun) (beginning-of-defun)))
 125                   (recenter 0)
 126                   ;;(repos-debug-macro "2a")
 127                   )
 128
 129                  ;; This condition, copied from case 4, may not be quite right
 130
 131                  ((and arg (< ht comment-height))
 132                   ;; Can't get first comment line onscreen.
 133                   ;; Go there and try again.
 134                   (forward-line (- comment-height))
 135                   (beginning-of-line)
 136                   ;; was (reposition-window)
 137                   (recenter 0)
 138                   ;;(repos-debug-macro "2b")
 139                   )
 140                  (t
 141                   (recenter (min ht comment-height))
 142                   ;;(repos-debug-macro "2c")
 143                   ))
 144            ;; (recenter (min ht comment-height))
 145            )
 146
 147           ((and (> (+ line defun-depth -1) ht)
 148                 defun-line-onscreen-p)
 149            ;; Defun runs off the bottom of the screen and the defun line
 150            ;; is onscreen.
 151            ;; Move the defun up.
 152            (recenter (max 0 (1+ (- ht defun-depth)) defun-height))
 153            ;;(repos-debug-macro "3")
 154            )
 155
 156           (t
 157            ;; If on the bottom line and comment start is offscreen
 158            ;; then just move all comments offscreen, or at least as
 159            ;; far as they'll go.
 160
 161            ;; Try to get as much of the comments onscreen as possible.
 162            (if (and arg (< ht comment-height))
 163                ;; Can't get defun line onscreen; go there and try again.
 164                (progn (forward-line (- defun-height))
 165                       (beginning-of-line)
 166                       (reposition-window))
 167              (recenter (min ht comment-height)))
 168            ;;(repos-debug-macro "4")
 169            ))))
 170
 171 ;;; Auxiliary functions
 172
 173 ;; Return number of screen lines between START and END.
 174 (defun repos-count-screen-lines (start end)
 175   (save-excursion
 176     (save-restriction
 177       (narrow-to-region start end)
 178       (goto-char (point-min))
 179       (vertical-motion (- (point-max) (point-min))))))
 180
 181 ;; Return number of screen lines between START and END; returns a negative
 182 ;; number if END precedes START.
 183 (defun repos-count-screen-lines-signed (start end)
 184   (let ((lines (repos-count-screen-lines start end)))
 185     (if (< start end)
 186         lines
 187       (- lines))))
 188
 189 ;; (defmacro repos-debug-macro (case-no)
 190 ;;   `(message (concat "Case " ,case-no ": %s %s %s %s %s")
 191 ;;             ht line comment-height defun-height defun-depth))
 192
 193 (provide 'reposition)
 194
 195 ;;; reposition.el ends here