sentence-highlight-word-count-minor-mode.el

;; A minor mode inspired by Gary Provost's notes on making sentence length music
;; via https://www.aerogrammestudio.com/2014/08/05/this-sentence-has-five-words/
;;
;; The sentence at point gains its colour from how many words it has:
;;   yellow - one or two words
;;   pink   - three or four words
;;   red    - five words
;;   green  - medium sentences
;;   blue   - long sentences
;;
;; An overview mode colorizing the whole buffer by sentence length would be nice
;; too, but I will leave that as an exercise for people with a less rusty elisp.
;;
;; And it'd probably be nicer if empty lines didn't colorize the sentence on the
;; most recent previous line.
;;
;; (And I bet I am not doing ideal cleanup, or the most idiomatic code, either.)

(defun sentence-highlight-minor-mode ()
    (interactive)
    (make-local-variable 'sentence-highlight-mode)
    (if sentence-highlight-mode
        (progn
          (setq sentence-highlight-mode nil)
          (remove-hook 'post-command-hook 'sentence-highlight-current)
          (move-overlay sentence-extent (point) (point) (current-buffer)))
      (setq sentence-highlight-mode t)
      (add-hook 'post-command-hook 'sentence-highlight-current)))

(setq sentence-end "[^.].[.?!]+\\([]\"')}]*\\|<[^>]+>\\)\\($\\| $\\|\t\\| \\)[ \t\n]*")

(setq sentence-color-lt3  "#f7f5ca") ; yellow - staccato sentences
(setq sentence-color-lt5  "#f5dcf6") ; pink   - short sentences
(setq sentence-color-lt6  "#efbbb7") ; red    - five-word sentences
(setq sentence-color-lt12 "#d3f3d0") ; green  - medium sentences
(setq sentence-color-long "#b2f4f5") ; blue   - long sentences

;; These probably ought to be done with defface and/or macros instead
(setq sentence-face-bg-lt3 (make-face 'sentence-face-bg-lt3))
(set-face-background sentence-face-bg-lt3 sentence-color-lt3)
(setq sentence-face-bg-lt5 (make-face 'sentence-face-bg-lt5))
(set-face-background sentence-face-bg-lt5 sentence-color-lt5)
(setq sentence-face-bg-lt6 (make-face 'sentence-face-bg-lt6))
(set-face-background sentence-face-bg-lt6 sentence-color-lt6)
(setq sentence-face-bg-lt12 (make-face 'sentence-face-bg-lt12))
(set-face-background sentence-face-bg-lt12 sentence-color-lt12)
(setq sentence-face-bg-long (make-face 'sentence-face-bg-long))
(set-face-background sentence-face-bg-long sentence-color-long)

(defun sentence-begin-pos () (save-excursion (unless (= (point) (point-max)) (forward-char)) (backward-sentence) (point)))
(defun sentence-end-pos () (save-excursion (unless (= (point) (point-max)) (forward-char)) (backward-sentence) (forward-sentence) (point)))

(setq sentence-highlight-mode nil)
(setq sentence-extent (make-overlay 0 0))

(defun sentence-highlight-current (&rest ignore)
  "Highlight current sentence."    
    (and sentence-highlight-mode (> (buffer-size) 0)
    (progn
      (and (boundp 'sentence-extent)
        sentence-extent
        (let*
            ((head (sentence-begin-pos))
             (tail (sentence-end-pos))
             (words (count-words head tail))
             (face
              (cond
               ((< words 3) sentence-face-bg-lt3)
               ((< words 5) sentence-face-bg-lt5)
               ((< words 6) sentence-face-bg-lt6)
               ((< words 12) sentence-face-bg-lt12)
               (t sentence-face-bg-long))))
          (progn
            (move-overlay sentence-extent head tail (current-buffer))
            (overlay-put sentence-extent 'face face)))
        ))))

;;;;; Now, enable this for Outline mode only:

;;; (add-hook 'outline-mode-hook (function (lambda ()
;;;
;;;  ; .... any other outline mode tweaks or key definitions you may need
;;;
;;;  (make-local-variable 'sentence-highlight-mode)
;;;  (setq sentence-highlight-mode t)
;;;  (add-hook 'post-command-hook 'sentence-highlight-current)
;;; )))