Best way to add per-line information visually in emacs? - emacs

Question

I'm writing a minor mode for emacs which, at the very least, will calculate a numeric value for each line in a buffer. I want to display this visually, preferable neatly before each line.
I know some minor modes draw to the fringe, and I know overlays are an option too (are these related?), but I can't find a good example of what I want anywhere.
Basically, I want to have something like the line numbers from linum-mode, but they will need to change every time the buffer is modified (actually, only whenever the line they're on changes). Something like a character counter for each line would be a good example. And I'd like it to not break linum-mode, but not depend on it, etc, if possible.

Answer 1

Here is a quick example of one way to put an overlay after linum-mode numbers and before the line of text. I will need to give some thought about right-alignment of the character count.
NOTE:  This method contemplates that the linum-mode numbers are generated before the code that follows in this example. If the post-command-hook or the widow-scroll-functions hook is used to implement this proposed method, then those additions to the hooks would need to follow in time subsequently to the linum-mode functions attached to those same hooks.
The following example could be implemented with the post-command-hook and the window-scroll-functions hook. See the following link for an example of how to determine window-start and window-end before a redisplay occurs: https://stackoverflow.com/a/24216247/2112489
EDIT:  Added right-alignment of character count -- contemplates a maximum of three digits (i.e., up to 999 characters per line). The text after the character count overlays are now left-aligned.
(save-excursion
(let* (
(window-start (window-start))
(window-end (window-end)))
(goto-char window-end)
(while (re-search-backward "\n" window-start t)
(let* (
(pbol (point-at-bol))
(peol (point-at-eol))
(raw-char-count (abs (- peol pbol)))
(starting-column
(propertize (char-to-string ?\uE001)
'display
`((space :align-to 1)
(space :width 0))))
(colored-char-count
(propertize (number-to-string raw-char-count)
'face '(:background "gray50" :foreground "black")
'cursor t))
(one-spacer
(propertize (char-to-string ?\uE001)
'display
`((space :width 1))))
(two-spacers
(propertize (char-to-string ?\uE001)
'display
`((space :width 2))))
(final-char-count
(cond
((and
(< raw-char-count 100)
(> raw-char-count 9))
(concat one-spacer colored-char-count))
((< raw-char-count 10)
(concat two-spacers colored-char-count))
(t colored-char-count))) )
(overlay-put (make-overlay pbol pbol)
'before-string
(concat starting-column final-char-count two-spacers) )))))

Answer 2

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
;; M-x char-count-mode
(defvar char-count-p nil
"When `char-count-p` is non-`nil`, the overlays are present.")
(make-variable-buffer-local 'char-count-p)
(defvar char-count-this-command nil
"This local variable is set within the `post-command-hook`; and,
is also used by the `window-scroll-functions` hook.")
(make-variable-buffer-local 'char-count-this-command)
(defvar char-count-overlay-list nil
"List used to store overlays until they are removed.")
(make-variable-buffer-local 'char-count-overlay-list)
(defun char-count-post-command-hook ()
"Doc-string."
(setq char-count-this-command this-command)
(character-count-function))
(defun character-count-window-scroll-functions (win _start)
"Doc-string."
(character-count-function))
(defun equal-including-properties--remove-overlays (beg end name val)
"Remove the overlays using `equal`, instead of `eq`."
(when (and beg end name val)
(overlay-recenter end)
(dolist (o (overlays-in beg end))
(when (equal-including-properties (overlay-get o name) val)
(delete-overlay o)))))
(defun character-count-function ()
"Doc-string for the character-count-function."
(when
(and
char-count-mode
char-count-this-command
(window-live-p (get-buffer-window (current-buffer)))
(not (minibufferp))
(pos-visible-in-window-p (point)
(get-buffer-window (current-buffer) (selected-frame)) t) )
(remove-char-count-overlays)
(save-excursion
(let* (
counter
(selected-window (selected-window))
(window-start (window-start selected-window))
(window-end (window-end selected-window t)) )
(goto-char window-end)
(catch 'done
(while t
(when counter
(re-search-backward "\n" window-start t))
(when (not counter)
(setq counter t))
(let* (
(pbol (point-at-bol))
(peol (point-at-eol))
(raw-char-count (abs (- peol pbol)))
(starting-column
(propertize (char-to-string ?\uE001)
'display
`((space :align-to 1) (space :width 0))))
(colored-char-count
(propertize (number-to-string raw-char-count)
'face '(:background "gray50" :foreground "black")))
(one-spacer
(propertize (char-to-string ?\uE001)
'display
`((space :width 1))))
(two-spacers
(propertize (char-to-string ?\uE001)
'display
`((space :width 2))))
(final-char-count
(cond
((and
(< raw-char-count 100)
(> raw-char-count 9))
(concat one-spacer colored-char-count))
((< raw-char-count 10)
(concat two-spacers colored-char-count))
(t colored-char-count)))
(ov-string (concat starting-column final-char-count two-spacers)) )
(push ov-string char-count-overlay-list)
(overlay-put (make-overlay pbol pbol) 'before-string ov-string)
(when (<= pbol window-start)
(throw 'done nil)) )))
(setq char-count-p t)))
(setq char-count-this-command nil) ))
(defun remove-char-count-overlays ()
(when char-count-p
(require 'cl)
(setq char-count-overlay-list
(remove-duplicates char-count-overlay-list
:test (lambda (x y) (or (null y) (equal-including-properties x y)))
:from-end t))
(dolist (description char-count-overlay-list)
(equal-including-properties--remove-overlays (point-min) (point-max) 'before-string description))
(setq char-count-p nil) ))
(defun turn-off-char-count-mode ()
(char-count-mode -1))
(define-minor-mode char-count-mode
"A minor-mode that places the character count at the beginning of the line."
:init-value nil
:lighter " Char-Count"
:keymap nil
:global nil
:group nil
(cond
(char-count-mode
(setq scroll-conservatively 101)
(add-hook 'post-command-hook 'char-count-post-command-hook t t)
(add-hook 'window-scroll-functions
'character-count-window-scroll-functions t t)
(add-hook 'change-major-mode-hook 'turn-off-char-count-mode nil t)
(message "Turned ON `char-count-mode`."))
(t
(remove-char-count-overlays)
(remove-hook 'post-command-hook 'char-count-post-command-hook t)
(remove-hook 'window-scroll-functions
'character-count-window-scroll-functions t)
(remove-hook 'change-major-mode-hook 'turn-off-char-count-mode t)
(kill-local-variable 'scroll-conservatively)
(message "Turned OFF `char-count-mode`.") )))
(provide 'char-count)
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