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+-*- mode: org -*-
+
+* About
+
+  =Emacs-Redshift= (aka =redshift.el=) provides an Emacs interface to
+  [[https://github.com/jonls/redshift][Redshift]] for changing display
+  color temperature.
+
+* Installation
+
+  Emacs-Redshift can be used from a Git checkout and can be installed
+  using Guix.
+
+** Git checkout
+
+   Clone a Git repository:
+
+   #+BEGIN_SRC sh
+     git clone https://github.com/wigust/emacs-redshift
+   #+END_SRC
+
+   Add it to Emacs =load-path= in your init file and load:
+
+   #+BEGIN_SRC emacs-lisp
+     (add-to-list 'load-path (expand-file-name "~/emacs-redshift"))
+     (require 'redshift)
+   #+END_SRC
+
+** Guix
+
+   Clone a Git repository:
+
+   #+BEGIN_SRC sh
+     git clone https://github.com/wigust/emacs-redshift
+     cd emacs-redshift
+   #+END_SRC
+
+   Install a Guix package with:
+
+   #+BEGIN_SRC sh
+     guix package -f guix.scm
+   #+END_SRC
+
+* Usage
+
+  Call =M-x redshift-set-temp= to set a display temperature.
+
+  Call =M-x redshift-increase-temp= to increase a display temperature.
+
+  Call =M-x redshift-decrease-temp= to decrease a display temperature.
+
+  To change increment for =redshift-increase-temp= and
+  =redshift-decrease-temp= set =redshift-temp-increment= to a number.
+
+  #+BEGIN_SRC emacs-lisp
+    (setq redshift-temp-increment 100)
+  #+END_SRC
+
+* Keys
+
+  You could use [[https://github.com/abo-abo/hydra][Hydra]] to get an
+  interactive interface:
+
+  #+BEGIN_SRC emacs-lisp
+    (defhydra hydra-redshift () "redshift"
+        ("g" redshift-set-temp "set")
+        ("n" redshift-decrease-temp "decrease")
+        ("p" redshift-increase-temp "increase"))
+  #+END_SRC
diff --git a/guix.scm b/guix.scm
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+;;; guix.scm --- Emacs interface to Redshift
+
+;; Copyright © 2018 Oleg Pykhalov <go.wigust@gmail.com>
+
+;; This file is part of Emacs-Redshift.
+
+;; Emacs-Redshift 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.
+;;
+;; Emacs-Redshift 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 Emacs-Redshift.
+;; If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This file contains Guix package for development version of
+;; Emacs-redshift.  To build or install, run:
+;;
+;;   guix build --file=guix.scm
+;;   guix package --install-from-file=guix.scm
+
+;;; Code:
+
+(use-modules ((guix licenses) #:prefix license:)
+             (guix build utils)
+             (guix build-system emacs)
+             (guix gexp)
+             (guix git-download)
+             (guix packages)
+             (ice-9 popen)
+             (ice-9 rdelim))
+
+(define %source-dir (dirname (current-filename)))
+
+(define (git-output . args)
+  "Execute 'git ARGS ...' command and return its output without trailing
+newspace."
+  (with-directory-excursion %source-dir
+    (let* ((port   (apply open-pipe* OPEN_READ "git" args))
+           (output (read-string port)))
+      (close-port port)
+      (string-trim-right output #\newline))))
+
+(define (current-commit)
+  (git-output "log" "-n" "1" "--pretty=format:%H"))
+
+(define emacs-redshift
+  (let ((commit (current-commit)))
+    (package
+      (name "emacs-redshift")
+      (version (string-append "0.0.1" "-" (string-take commit 7)))
+      (source (local-file %source-dir
+                          #:recursive? #t
+                          #:select? (git-predicate %source-dir)))
+      (build-system emacs-build-system)
+      (home-page "https://github.com/wigust/emacs-redshift")
+      (synopsis "Emacs interface to Redshift")
+      (description
+       "This package provides an Emacs interface to Redshift")
+      (license license:gpl3+))))
+
+emacs-redshift
+
+;;; guix.scm ends here
diff --git a/redshift.el b/redshift.el
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+;;; redshift.el --- Emacs interface to Redshift
+
+;; Copyright © 2018 Oleg Pykhalov <go.wigust@gmail.com>
+
+;; Author: Oleg Pykhalov <go.wigust@gmail.com>
+;; Version: 0.0.1
+;; URL: https://wigust.github.io/emacs-redshift/
+;; Keywords: tools
+;; Package-Requires: ((emacs "24.3")
+
+;; This file is part of Emacs-Redshift.
+
+;; Emacs-Redshift 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.
+;;
+;; Emacs-Redshift 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 Emacs-Redshift.
+;; If not, see <http://www.gnu.org/licenses/>.
+
+;;; Commentary:
+
+;; This file provides functions to control Redshift.  It allows you:
+;;
+;; - to decrease a display temperature with `redshift-decrease-temp';
+;;
+;; - to increase a display temperature with `redshift-increase-temp';
+;;
+;; - to set a display temperature with `redshift-set-temp'.
+
+;;; Code:
+
+(defvar redshift-temp 5500 "Display color temperature")
+
+(defcustom redshift-temp-increment 500
+  "Display color temperature increment."
+  :type 'number
+  :group 'redshift)
+
+(defcustom redshift-program "redshift"
+  "The name by which to invoke Redshift."
+  :type 'string
+  :group 'redshift)
+
+;;;###autoload
+(defun redshift-set-temp (temp)
+  "Set display color temperature of display to TEMP and return TEMP."
+  (interactive "nColor temperature: ")
+  (and (start-process "redshift" nil redshift-program
+                      "-O" (number-to-string temp))
+       temp))
+
+;;;###autoload
+(defun redshift-increase-temp ()
+  "Increase display color temperature of display to TEMP."
+  (interactive)
+  (setq redshift-temp
+        (redshift-set-temp (+ redshift-temp
+                              redshift-temp-increment))))
+
+;;;###autoload
+(defun redshift-decrease-temp ()
+  "Decrease display color temperature of display to TEMP."
+  (interactive)
+  (setq redshift-temp
+        (redshift-set-temp (- redshift-temp
+                              redshift-temp-increment))))
+
+(provide 'redshift)
+
+;;; redshift.el ends here