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;;; chop.el -- Interactive binary search for a line within a window.
;;; Written by Luke Gorrie <luke@bluetail.com>. Version 1.0, May 2002.
;;; Probably only works in GNU Emacs 21.
;; Interactively-driven binary search to move the point to a
;; particular line on the screen, by successive choppings in half.
;; Setup:
;;
;; Bind move-chop-{up,down} to keys. Otherwise they probably won't
;; work right, since M-x mucks with `last-command'.
;; Instructions:
;;
;; Use the following algorithm to move to any visible line in O(log N)
;; steps for a window N lines high:
;;
;; 1. Choose a line, L.
;; 2. Press either UP or DOWN. This starts by moving the point to the center.
;; 3. Repeat until L is the current line (or close enough):
;; If L is above the point, press UP. Otherwise press DOWN.
;;
;; In practice, a few chops usually gets you pretty close. Then you
;; can zero in with line/paragraph/defun-based motion.
(defvar chop-size nil
"Number of lines that the next \"chop\" will contain, as floating-point.
Only meaningful for consecutive chops.")
(defvar chop-current-line nil
"Current line number, as floating-point.
Only meaningful for consecutive chops.")
(defun chop-move-up ()
"Move by one 'chop' into the upper half of the remaining space."
(interactive)
(chop-move -1))
(defun chop-move-down ()
"Move by one 'chop' into the lower half of the remaining space."
(interactive)
(chop-move 1))
(defun chop-move (dir)
"Move by one 'chop'. DIR is the direction: -1 for upwards, 1 for downwards."
(setq this-command 'chop-move)
(if (chop-new-p)
(chop-first)
(chop-next dir)))
(defun chop-new-p ()
(or current-prefix-arg
(not (eq last-command 'chop-move))))
(defun chop-first ()
"Make a first chop, leaving the point in the middle of the window."
(let ((half (/ (chop-last-line-number) 2.0)))
(setq chop-size half)
(setq chop-current-line half)
(move-to-window-line (round half))))
(defun chop-next (dir)
"Make the next chop."
(setq chop-size (/ chop-size 2))
(incf chop-current-line (* dir chop-size))
(move-to-window-line (min (chop-last-line-number)
(round chop-current-line))))
(defun chop-last-line-number ()
"Window height, minus 1 to index from 0, minus 1 to account for modeline."
(- (window-height) 2))
(provide 'chop)
;;; chop.el ends here
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