--- /dev/null
+;;; pixel-scroll.el --- Scroll a line smoothly
+
+;; Copyright (C) 2017 Free Software Foundation, Inc.
+;; Author: Tak Kunihiro <tkk@misasa.okayama-u.ac.jp>
+;; Keywords: mouse
+;; Package: emacs
+
+;; 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 3 of the License, 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. If not, see <http://www.gnu.org/licenses/>.
+
+;; Usage:
+;;
+;; To interactively toggle the mode:
+;;
+;; M-x pixel-scroll-mode RET
+;;
+;; To make the mode permanent, put these in your init file:
+;;
+;; (require 'pixel-scroll)
+;; (pixel-scroll-mode 1)
+
+;;; Commentary:
+
+;; This package offers a global minor mode which makes mouse-wheel
+;; scroll a line smoothly.
+;;
+;; Scrolling a line up by `set-window-vscroll' and that by `scroll-up'
+;; give similar display as shown below.
+;;
+;; A: (scroll-up 1)
+;; B: (set-window-vscroll nil (frame-char-height) t)
+;;
+;; Also scrolling a pixel up by `set-window-vscroll' and that by
+;; `scroll-up' give similar display, when vscroll is the last pixel of
+;; the line, as shown below.
+;;
+;; A: (scroll-up 1)
+;; B: (set-window-vscroll nil (1- (frame-char-height) t)) (scroll-up 1)
+;;
+;; When point reaches to the top of a window on scroll by
+;; `set-window-vscroll', vscroll is set to zero. To scroll a line
+;; smoothly and continuously, this package scrolls a line by following
+;; sequences.
+;;
+;; (vertical-motion 1)
+;; (dolist (vs (number-sequence 1 (1- (frame-char-height))))
+;; (set-window-vscroll nil vs t) (sit-for 0))
+;; (scroll-up 1)
+
+;;; Todo:
+;;
+;; Allowing pixel-level scrolling in Emacs requires a thorough review
+;; of the related functionalities, to make sure none of them zeroes
+;; out vscroll where users won't want that.
+
+;;; Code:
+
+(require 'mwheel)
+
+(defvar pixel-wait 0
+ "Idle time on each step of pixel scroll specified in second.
+More wait will result in slow and gentle scroll.")
+
+(defvar pixel-resolution-fine-flag nil
+ "Set scrolling resolution to a pixel instead of a line.
+After a pixel scroll, typing C-n or C-p scrolls the window to
+make it fully visible, and undoes the effect of the pixel-level
+scroll.")
+
+;;;###autoload
+(define-minor-mode pixel-scroll-mode
+ "A minor mode to scroll text pixel-by-pixel.
+With a prefix argument ARG, enable Pixel Scroll mode if ARG is positive,
+and disable it otherwise. If called from Lisp, enable Pixel Scroll mode
+if ARG is omitted or nil."
+ :init-value nil
+ :group 'scrolling
+ :global t
+ :version "26.1"
+
+ (if pixel-scroll-mode
+ (setq mwheel-scroll-up-function 'pixel-scroll-up
+ mwheel-scroll-down-function 'pixel-scroll-down)
+ (setq mwheel-scroll-up-function 'scroll-up
+ mwheel-scroll-down-function 'scroll-down)))
+
+(defun pixel-scroll-up (&optional arg)
+ "Scroll text of selected window up ARG lines.
+This is an alternative of `scroll-up'. Scope moves downward."
+ (interactive)
+ (or arg (setq arg 1))
+ (dotimes (ii arg) ; move scope downward
+ (if (pixel-eob-at-top-p) ; when end-of-the-buffer is close
+ (scroll-up 1) ; relay on robust method
+ (when (pixel-point-at-top-p) ; prevent too late
+ (vertical-motion 1)) ; move point downward
+ (pixel-scroll-pixel-up (if pixel-resolution-fine-flag
+ 1
+ (pixel-line-height)))))) ; move scope downward
+
+(defun pixel-scroll-down (&optional arg)
+ "Scroll text of selected window down ARG lines.
+This is and alternative of `scroll-down'. Scope moves upward."
+ (interactive)
+ (or arg (setq arg 1))
+ (dotimes (ii arg)
+ (if (or (pixel-bob-at-top-p) ; when beginning-of-the-buffer is seen
+ (pixel-eob-at-top-p)) ; for file with a long line
+ (scroll-down 1) ; relay on robust method
+ (while (pixel-point-at-bottom-p) ; prevent too late (multi tries)
+ (vertical-motion -1))
+ (pixel-scroll-pixel-down (if pixel-resolution-fine-flag
+ 1
+ (pixel-line-height -1))))))
+
+(defun pixel-bob-at-top-p ()
+ "Return non-nil if beginning of buffer is at top of window."
+ (equal (window-start) (point-min)))
+
+(defun pixel-eob-at-top-p ()
+ "Return non-nil if end of buffer is at top of window."
+ (<= (count-lines (window-start) (window-end)) 2)) ; count-screen-lines
+
+(defun pixel-posn-y-at-point ()
+ "Return y coordinates of point in pixels of current window."
+ (let ((hscroll0 (window-hscroll))
+ (y (cdr (posn-x-y (posn-at-point)))))
+ ;; when point is out of scope by hscroll
+ (unless y
+ (save-excursion
+ (set-window-hscroll nil (current-column))
+ (setq y (cdr (posn-x-y (posn-at-point))))
+ (set-window-hscroll nil hscroll0)))
+ y))
+
+(defun pixel-point-at-top-p ()
+ "Return if point is located at top of a window."
+ (let* ((y (pixel-posn-y-at-point))
+ (top-margin y))
+ (< top-margin (pixel-line-height))))
+
+(defun pixel-point-at-bottom-p ()
+ "Return if point is located at bottom of a window."
+ (let* ((y (pixel-posn-y-at-point))
+ (edges (window-inside-pixel-edges))
+ (height (- (nth 3 edges) (nth 1 edges))) ; (- bottom top)
+ (bottom-margin (- height (+ y (line-pixel-height))))) ; bottom margin
+ (< bottom-margin (pixel-line-height -1)))) ; coming unseen line
+
+(defun pixel-scroll-pixel-up (amt)
+ "Scroll text of selected windows up AMT pixels.
+Scope moves downward."
+ (while (>= (+ (window-vscroll nil t) amt)
+ (pixel-line-height))
+ (setq amt (- amt (pixel--whistlestop-line-up)))) ; major scroll
+ (pixel--whistlestop-pixel-up amt)) ; minor scroll
+
+(defun pixel-scroll-pixel-down (amt)
+ "Scroll text of selected windows down AMT pixels.
+Scope moves upward."
+ (while (> amt 0)
+ (let ((vs (window-vscroll nil t)))
+ (if (equal vs 0)
+ (pixel-scroll-down-and-set-window-vscroll
+ (1- (pixel-line-height -1)))
+ (set-window-vscroll nil (1- vs) t))
+ (setq amt (1- amt))
+ (sit-for pixel-wait))))
+
+(defun pixel--whistlestop-line-up ()
+ "Scroll text upward a line with each pixel whistlestopped.
+When `vscroll' is non-zero, complete scrolling a line. When
+`vscroll' is larger than height of multiple lines, for example
+88, this flushes multiple lines. At the end, `vscroll' will be
+zero. This assumes that the lines are with the same height.
+Scope moves downward. This function returns number of pixels
+that was scrolled."
+ (let* ((src (window-vscroll nil t)) ; EXAMPLE (initial) @0 @8 @88
+ (height (pixel-line-height)) ; 25 25 23
+ (line (1+ (/ src height))) ; catch up + one line Ä1 Ä1 Ä4
+ (dst (* line height)) ; goal @25 @25 @92
+ (delta (- dst src))) ; pixels to be scrolled 25 17 4
+ (pixel--whistlestop-pixel-up (1- delta)) ; until one less @24 @24 @91
+ (scroll-up line) (sit-for pixel-wait) ; scroll 1 pixel @0 @0 @0
+ delta))
+
+(defun pixel--whistlestop-pixel-up (n)
+ "Scroll text upward by N pixels with each pixel whistlestopped.
+Scope moves downward."
+ (when (> n 0)
+ (let ((vs0 (window-vscroll nil t)))
+ (dolist (vs (number-sequence (1+ vs0) (+ vs0 n)))
+ (set-window-vscroll nil vs t) (sit-for pixel-wait)))))
+
+(defun pixel-line-height (&optional pos)
+ "Return height in pixels of text line at POS in the selected window.
+When POS is nil or negative, height of the first line or the coming
+unseen line above the first line, respectively, is provided."
+ (or pos (setq pos (window-start)))
+ (when (< pos 0)
+ (setq pos (pixel-point-at-unseen-line)))
+ (save-excursion
+ (goto-char pos)
+ (line-pixel-height))) ; frame-char-height
+
+(defun pixel-point-at-unseen-line ()
+ "Return the character position of line above the selected window.
+The returned value is the position of the first character on the
+unseen line just above the scope of current window."
+ (let* ((pos0 (window-start))
+ (vscroll0 (window-vscroll nil t))
+ (pos
+ (save-excursion
+ (goto-char pos0)
+ (if (bobp)
+ (point-min)
+ ;; When there's an overlay string at window-start,
+ ;; (beginning-of-visual-line 0) stays put.
+ (let ((ppos (point))
+ (tem (beginning-of-visual-line 0)))
+ (if (eq tem ppos)
+ (vertical-motion -1))
+ (point))))))
+ ;; restore initial position
+ (set-window-start nil pos0 t)
+ (set-window-vscroll nil vscroll0 t)
+ pos))
+
+(defun pixel-scroll-down-and-set-window-vscroll (vscroll)
+ "Scroll down a line and set VSCROLL in pixels.
+It is important to call `set-window-start' to force the display
+engine use that particular position as the window-start point.
+Otherwise, redisplay will reset the window's vscroll."
+ (set-window-start nil (pixel-point-at-unseen-line) t)
+ (set-window-vscroll nil vscroll t))
+
+(provide 'pixel-scroll)
+;;; pixel-scroll.el ends here