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+;;; hl-sentence-length.el --- Highlights sentences by word count
+
+;; Copyright (C) 2022 Ferdinand Pieper
+
+;; Author: Ferdinand Pieper <mail@pie.tf>
+;; URL: https://github.com/fpiper/hl-sentence-length
+
+;; This file is not part of GNU Emacs.
+
+;; This file 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 file 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; see the file COPYING. If not, see
+;; <https://www.gnu.org/licenses/>.
+
+;;; Commentary:
+;;
+;; Minor mode to highlight sentences based on the number of words.
+;; This is intended to help writers focus on varying their sentence
+;; lengths. By default it cycles trough eight different colors. To
+;; increase this set `hl-sentence-length-cycle' and customize the
+;; faces for higher word counts or define new ones.
+;;
+;;; Code:
+
+(defface hl-sentence-length-face-1
+  '((t (:background "#7d9fb2")))
+  "Face for sentences of length 1.")
+(defface hl-sentence-length-face-2
+  '((t (:background "#d5b195")))
+  "Face for sentences of length 2.")
+(defface hl-sentence-length-face-3
+  '((t (:background "#6f9e91")))
+  "Face for sentences of length 3.")
+(defface hl-sentence-length-face-4
+  '((t (:background "#cc8fb1")))
+  "Face for sentences of length 4.")
+(defface hl-sentence-length-face-5
+  '((t (:background "#a3d0e9")))
+  "Face for sentences of length 5.")
+(defface hl-sentence-length-face-6
+  '((t (:background "#e6d1a1")))
+  "Face for sentences of length 6.")
+(defface hl-sentence-length-face-7
+  '((t (:background "#f0eba8")))
+  "Face for sentences of length 7.")
+(defface hl-sentence-length-face-8
+  '((t (:background "#707070")))
+  "Face for sentences of length 8.")
+;; Faces 9-12 declared just for convenience. Easily customize using
+;; `customize-face' or use-package's `:custom-face' keyword.
+(defface hl-sentence-length-face-9
+  '((t (:background "#ffffff")))
+  "Face for sentences of length 9.")
+(defface hl-sentence-length-face-10
+  '((t (:background "#ffffff")))
+  "Face for sentences of length 10.")
+(defface hl-sentence-length-face-11
+  '((t (:background "#ffffff")))
+  "Face for sentences of length 11.")
+(defface hl-sentence-length-face-12
+  '((t (:background "#ffffff")))
+  "Face for sentences of length 12.")
+
+(defvar hl-sentence-length-cycle 8
+  "Number of faces to cycle trough.")
+
+(defun hl-sentence-length-sentence-beginning ()
+  "Return point at beginning of the current sentence."
+  (save-excursion
+    (backward-sentence)
+    (point)))
+
+(defun hl-sentence-length-sentence-end ()
+  "Return point at end of the current sentence."
+  (save-excursion
+    (backward-sentence)
+    (forward-sentence)
+    (point)))
+
+(defun hl-sentence-length-sentence-length ()
+  "Return length of current sentence."
+  (save-mark-and-excursion
+    (backward-sentence)
+    (set-mark (point))
+    (forward-sentence)
+    (count-words-region (region-beginning) (region-end))))
+
+(defun hl-sentence-length-sentence-face ()
+  "Return the appropriate face for the current sentence."
+  (let* ((length (hl-sentence-length-sentence-length))
+         (modulo (mod length hl-sentence-length-cycle))
+         (facenumber (if (eq modulo 0)
+                         hl-sentence-length-cycle
+                       modulo)))
+    (intern (format "hl-sentence-length-face-%i" facenumber))))
+
+(defun hl-sentence-length-highlight-sentence ()
+  "Highlight the current sentence based on its length."
+  (let ((face (hl-sentence-length-sentence-face))
+        (overlay (make-overlay (hl-sentence-length-sentence-beginning) (hl-sentence-length-sentence-end))))
+    (overlay-put overlay 'origin 'hl-sentence-length)
+    (overlay-put overlay 'face face)))
+
+(define-minor-mode hl-sentence-length-mode
+  "Highlight the word count of sentences."
+  nil nil nil
+  (if hl-sentence-length-mode
+      (add-hook 'post-command-hook 'hl-sentence-length-update nil t)
+    (progn
+      (remove-hook 'post-command-hook 'hl-sentence-length-update t)
+      (hl-sentence-length-remove-highlights))))
+
+(defun hl-sentence-length-create-highlights ()
+  (save-mark-and-excursion
+    (let ((pmax (point-max)))
+      (beginning-of-buffer)
+      (while (< (point) pmax)
+        (forward-sentence)
+        (hl-sentence-length-highlight-sentence)))))
+
+(defun hl-sentence-length-remove-highlights ()
+  (remove-overlays nil nil 'origin 'hl-sentence-length))
+
+(defun hl-sentence-length-update ()
+  "Update the highlighted sentence lengths."
+  ;; FIXME There is probably a better way to do this.
+  (hl-sentence-length-remove-highlights)
+  (hl-sentence-length-create-highlights))
+
+;;; hl-sentence-length.el ends here