;;; pixel-scroll.el --- Scroll a line smoothly

;; Package-Requires: ((emacs "24.5"))
;; Version: 1.0.0
;; Package-Version: 20170419.2104
;; Keywords: convenience, usability

;;; This file is part of GNU Emacs

;;; License

;; This program 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, or (at your option)
;; any later version.

;; This program 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 this program; see the file COPYING.  If not, write to
;; the Free Software Foundation, Inc., 51 Franklin Street, Fifth
;; Floor, Boston, MA 02110-1301, USA.

;; 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 Emacs scroll a
;; line smoothly.

;; The smooth scroll-up is carried out using `set-window-vscroll',
;; `window-vscroll', and `scroll-up'.  Scrolling text upward a line by
;; pixels using `set-window-vscroll' and by a line using `scroll-up'
;; gives similar visual feedback when vscroll location is @0.  Note
;; that vscroll location is vertical shift obtained by
;; `window-vscroll'.  Line height by pixel is obtained by
;; `frame-char-height'.  Following two lines scroll text in similar
;; fashion, visually.
;;
;;   (scroll-up 1)
;;   (set-window-vscroll nil (frame-char-height) t)
;;
;; Scrolling text upward by a pixel and a line yields similar result
;; when vscroll location is at the last pixel.  Following two lines
;; scroll text in similar fashion, visually.
;;
;;   (scroll-up 1)
;;   (set-window-vscroll nil (1- (frame-char-height) t)) (scroll-up 1)
;;
;; When vscroll gets larger and as soon as point is beyond beginning
;; of a window, vscroll is set to zero.  To user, scope is changed
;; suddenly without point moved.  This package tries to scroll text
;; upward by a line with pixel-by-pixel transition by a following
;; sequence.
;;
;;   (progn
;;     (vertical-motion 1)
;;     (dolist (vs (number-sequence 1 (1- (frame-char-height))))
;;       (set-window-vscroll nil vs t) (sit-for 0.1))
;;     (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-p 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 undos the effect of the pixel-level
scroll.")

(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

  (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
alternative of `scroll-up'.  Scope moves downward."
  (interactive)
  (or arg (setq arg 1))
  (dotimes (ii arg) ; move scope downward
    (if (<= (count-lines (window-start) (window-end)) 2)
        (scroll-up 1) ; when end of scroll is close, relay on robust guy
      (when (or (pixel-point-at-top-p) ; prevent too late
                (and scroll-preserve-screen-position
                     (not (pixel-point-at-bottom-p)))) ; prevent too fast
        (vertical-motion 1)) ; move point downward
      (pixel-scroll-pixel-up (if pixel-resolution-fine-p
                                 1
                               (pixel-line-height)))))) ; move scope downward

(defun pixel-scroll-down (&optional arg)
  "Scroll text of selected window down ARG lines.  This is
alternative of `scroll-down'.  Scope moves upward."
  (interactive)
  (or arg (setq arg 1))
  (dotimes (ii arg)
    (if (equal (window-start) (point-min))
        (scroll-down 1) ; when beginning-of-buffer is seen, relay on robust guy
      (while (or (pixel-point-at-bottom-p) ; prevent too late (try several lines)
                 (and scroll-preserve-screen-position
                      (not (pixel-point-at-top-p)))) ; prevent too fast
        (vertical-motion -1)))
    (pixel-scroll-pixel-down (if pixel-resolution-fine-p
                                 1
                               (pixel-line-height -1)))))

(defun pixel-point-at-top-p ()
  "Return if point is at top of a window."
  (let ((y (cdr (posn-x-y (posn-at-point)))))
    (if y
        (< y ; top margin
           (pixel-line-height))
      ;; when point is out of scope by hscroll
      (< (count-lines (window-start) (point)) 1))))

(defun pixel-point-at-bottom-p ()
  "Return if point is at bottom of a window."
  (let* ((y (cdr (posn-x-y (posn-at-point))))
         (edges (window-inside-pixel-edges))
         (height (- (nth 3 edges) (nth 1 edges)))) ; (- bottom top)
    (if y
        (< (- height (+ y (line-pixel-height))) ; bottom margin
           (pixel-line-height -1)) ; coming line
      ;; when point is out of scope by hscroll
      (< (count-lines (point) (window-end)) 4))))

(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 of POS in the selected
window.  When POS is nil or negative, height of the first line or
the coming line above the first line is provided."
  (or pos (setq pos (window-start)))
  (if (< pos 0)
      (setq pos (pixel-point-at-coming-line)))
  (save-excursion
    (goto-char pos)
    (line-pixel-height))) ; frame-char-height

(defun pixel-point-at-coming-line ()
  "Return the character position of the first character on the
coming line just above the scope of current window.  This code is
presented at
https://lists.gnu.org/archive/html/emacs-devel/2017-04/msg00475.html."
  (let* ((win0 (window-start))
         (vscroll0 (window-vscroll nil t))
         (pos
          (save-excursion
            (goto-char win0)
            (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))))))
    (set-window-start nil win0 t) ; restore the window
    (set-window-vscroll nil vscroll0 t) ; restore the window
    pos))

(defun pixel-scroll-down-and-set-window-vscroll (vscroll)
  "Scroll down a line and set VSCROLL in pixels.  This code is
presented at
https://lists.gnu.org/archive/html/emacs-devel/2017-04/msg00366.html.
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-coming-line) t)
  (set-window-vscroll nil vscroll t))

(provide 'pixel-scroll)
;;; pixel-scroll.el ends here