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;;; quick-buffer-cycle.el --- Quickly cycle between buffers

;; Copyright (C) 2020  Mathias Dahl

;; Author: Mathias Dahl <mathias.dahl@gmail.com>
;; Maintainer: Mathias Dahl <mathias.dahl@gmail.com>
;; Version: 1.0.0
;; Keywords: convenience
;; URL: http://www.emacswiki.org/cgi-bin/wiki.pl?QuickBufferCycle

;; This file is not part of GNU Emacs.

;; This 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 2, or (at your option)
;; any later version.

;; This is distributed in the hope that it will be useful,
;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;; GNU General Public License for more details.

;; You should have received a copy of the GNU General Public License
;; along with GNU Emacs; see the file COPYING.  If not, write to the
;; Free Software Foundation, Inc., 59 Temple Place - Suite 330,
;; Boston, MA 02111-1307, USA.

;;; Commentary:
;; For a long time (*) I have had the F1 key bound to a command that
;; takes me to the previous buffer. It has been a very simple command
;; and it has served me well. I use this in every editing session more
;; or less.
;; * I think I first got the idea here: https://www.emacswiki.org/emacs/DedicatedKeys
;; Certain days I am repeating the same buffer switching pattern over
;; and over again. I switch to buffer A, then B, then C, and then A
;; again (or whatever). Those days, my convenient F1 binding does not
;; work for me and I have to use some sort of classic "buffer
;; selector" (sometimes I use the standard one in Emacs, but often I
;; use Anything (precursor to Helm)), which of course works but is
;; slower/boring.
;; This hack is my attempt to solve it. The idea is that, in a given
;; scenario, I will quickly learn that I first press F1 once, to
;; switch to buffer A and do some work there. Then I press the key
;; twice to switch to buffer C and work there, then press twice again
;; to switch to buffer B, and so on. In my use cases, probably I will
;; seldom press the key more than twice, but we'll see about that.
;; Before I wrote this hack I made an attempt to find if there were a
;; package like this already, but could not find one. Turns out I was
;; wrong (perhaps I did not want to find it, it is fun to reinvent the
;; wheel afterall...), but I learned this after developing this
;; one... At any rate, now it's done and I will try to use it to see
;; if I like it. Preliminary results shows that I will...
;; About the approach using a timer:
;; At first I tried an approach where I measured the time between each
;; call to the "switch" command. Long story short I could not get it
;; work the way I wanted to (it now seems others have succeeded and at
;; least one of those solution seems to be quite complex, which is
;; perhaps why I did not succeed) and got the idea that a timer would
;; be the solution - simply wait with switching buffers until the user
;; has stopped "cycling".
;; The solution is very simple, but it has one major drawback,
;; especially for the impatient: you can never switch to a buffer more
;; quickly than what the timer delay is set to (0.4 seconds by
;; default). You can tweak the timer value to whatever suits you. If
;; you have a fast computer and if you are a fast typist, you should
;; certainly try with 0.3 or even 0.2. If you can live with this
;; delay, this simple hack will probably work well for you.
;; Usage:
;; Bind `quick-buffer-cycle' to a convenient key. I prefer the F1 key:
;; (global-set-key [f1] 'quick-buffer-cycle)
;; Now, the most common scenario (for me) is that I want to switch to
;; the previous buffer. To do that, press the key once, and wait (the
;; default time to wait is 0.4 seconds). The previous buffer will be
;; selected.
;; If you know "where" in the buffer stack/list the buffer is that you
;; want to go to, quickly press the key as many times is needed to
;; select that buffer (let's say two times, if you know the buffer you
;; want is the one previous to the previous one). Here, "quickly"
;; means to press the key with a maximum delay of
;; `quick-buffer-cycle-timeout' (default 0.4 seconds).
;; To make it easier to select the correct buffer, when you pressed
;; the key the wrong number of times, there is a hint in the echo area
;; that shows how many key presses are needed to get to a certain
;; buffer after a buffer have been switched to. By default, the number
;; of buffers to show in the hint is 4 but this can be customized by
;; setting `quick-buffer-cycle-hint-limit' to the number of buffers to
;; be shown.

;;; Code:

(defun quick-buffer-cycle-get-buffers ()
  "Return the list of buffers to chose from."
  (cl-remove-if (lambda (buf)
                  (string-match "^ " (buffer-name buf)))

(defvar quick-buffer-cycle-timer nil
  "Timer used to activate the actual switch.")

(defcustom quick-buffer-cycle-hint-limit 4
  "The number of buffers to display in the hint.
If set to a number N, shows N buffer names.  If set to nil, do
not show any hint."
  :type 'integer)

(defun quick-buffer-cycle-show-hint ()
  "Show a hint with the first few buffers."
  (when quick-buffer-cycle-hint-limit
    (message "%s" (mapconcat
                   (lambda (n)
                     (format "%d: %s" n (quick-buffer-cycle-buffer-name n)))
                   (number-sequence 1 quick-buffer-cycle-hint-limit)
                   " "))))

(defun quick-buffer-cycle-switch ()
  "Switch to the last buffer that was cycled to."
  (switch-to-buffer (nth quick-buffer-cycle-count (quick-buffer-cycle-get-buffers)))
  (setq quick-buffer-cycle-count 0))

(defcustom quick-buffer-cycle-timeout 0.4
  "After this timeout a buffer will be switched to.
Depending on your typing speed and how fast computer you have,
you can experiment with values from 0.2 to 1.0 seconds.  A sweet
spot seems to be somewhere around 0.5 seconds."
  :type 'float)

(defvar quick-buffer-cycle-count 0
  "Track repeated invocations of the cycle command.")

(defun quick-buffer-cycle-start-timer ()
  "Start timer."
  (setq quick-buffer-cycle-timer
        (run-with-timer quick-buffer-cycle-timeout nil

(defun quick-buffer-cycle-stop-timer ()
  "Stop timer."
  (if (timerp quick-buffer-cycle-timer)
      (cancel-timer quick-buffer-cycle-timer)))

(defun quick-buffer-cycle-restart-timer ()
  "Restart timer."

(defun quick-buffer-cycle-buffer-name (N)
  "Get the name of buffer number N from the buffer list."
  (buffer-name (nth N (quick-buffer-cycle-get-buffers))))

(defun quick-buffer-cycle ()
  "Cycle between buffers quickly.
Successive rapid invocations cycle buffers further down on the
buffer list.  When pausing for the amount of time specified by
`quick-buffer-cycle-timeout', a buffer will be switched to.
After a buffer have been switched to, a hint is shown to help
switch to other buffers more easily."
  (setq quick-buffer-cycle-count (1+ quick-buffer-cycle-count))

(provide 'quick-buffer-cycle)

;;; quick-buffer-cycle.el ends here