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+* ~hl-sentence-length~ mode
+
+[[file:example.png]]
+
+Varying sentence lengths helps engage the reader. However it is hard to keep track of sentence length while writing. To help this mode highlights all sentences in the current buffer based on their word count.
+
+~hl-sentence-length-cycle~ can be customized to change the number of different faces that are used.
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diff --git a/hl-sentence-length.el b/hl-sentence-length.el
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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