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; Footnote behavior customization
; + left-align footnote text to first character of first paragraph,
;   not to left margin.
; + Instructions:
;   + toggle the feature using 'mode-map-prefix M-q' (usually 'C-c ! M-q')
;   + whenever you run `auto-fill' (M-q), it will act
;   + if you set visual-line-mode, it acts automatically
(require 'footnote)

(defcustom Footnote-align-to-fn-text t
"Nil if footnote text is to be aligned flush left with left side
of the footnote number. Non-nil if footnote text is to be aligned
left with the first character of footnote text."
  :type  'boolean
  :group 'footnote)
(make-variable-buffer-local 'Footnote-align-to-fn-text)

(defvar Footnote--body-auto-fill-prefix nil
"When Footnmote mode modifies variable `fill-prefix', store the
prior value here, so that it can be restored when leaving the
footnote area.")
(make-variable-buffer-local 'Footnote--body-auto-fill-prefix)

(defun Footnote--calc-fn-alignment-column()
"Calculate the left alignment for footnote text."
  (+ footnote-body-tag-spacing
    (length
       (concat footnote-start-tag  footnote-end-tag
         (Footnote-index-to-string
            (caar (last footnote-text-marker-alist)))))))

(defun Footnote--align-to-fn()
  "For the purpose of this feature proposal (only), this function is an advice :after
function `Footnote-add-footnote'. For implementation within emacs, the code would
be included at the end of that function."
  ; TODO: If the length has changed, eg. when adding footnote [10] after having
  ;       footnote [9], refresh all footnotes with new alignment.
  (when Footnote-align-to-fn-text
    (setq Footnote--body-auto-fill-prefix (or fill-prefix ""))
    (setq fill-prefix (make-string (Footnote--calc-fn-alignment-column) 32))))

(defun Footnote--align-to-body()
  (when (not Footnote-align-to-fn-text)
    (setq fill-prefix Footnote--body-auto-fill-prefix)
    (setq Footnote--body-auto-fill-prefix nil)))

(defun Footnote--return-to-body(arg)
  "For the purpose of this feature proposal (only), this function is an advice :after
function `Footnote-back-to-message'. For implementation within emacs, the code would
be included at the end of that function."
  (setq fill-prefix Footnote--body-auto-fill-prefix)
  (setq Footnote--body-auto-fill-prefix nil))

(defun Footnote--point-in-body-p()
  "Return `t' if point is in the buffer text area, ie. before the beginning of
the footnote area."
  ; NOTE: This code is shared with a patch I've proposed for emacs-bug#29756
  (let
    ((p (point))
     (q (if (not footnote-text-marker-alist) (point-max)
         (if (string-equal footnote-section-tag "")
           (cdr (first footnote-text-marker-alist))
          (goto-char (cdr (first footnote-text-marker-alist)))
          (if (re-search-backward
                (concat "^" footnote-section-tag-regexp) nil t)
            (match-beginning 0)
           ; This `else' should never happen, and indicates an error, ie. footnotes
           ; already exist and a footnote-section-tag is defined, but the section tag
           ; hasn't been found. We choose to assume that the user deleted it
           ; intentionally and wants us to behave in this buffer as if the section tag
           ; was set "", so we do that, now.
           (setq footnote-section-tag "")
           ; HOWEVER: The rest of footnote mode does not currently honor or account
           ; for this.
           ;
           ; To illustrate the difference in behavior, create a few footnotes, delete
           ; the section tag, and create another footnote. Then undo, comment the
           ; above line (that sets the tag to ""), re-evaluate this function, and repeat.
           (cdr (first footnote-text-marker-alist))
         )))))
   (goto-char p) ; undo `re-search-backward' side-effect
   (if (< p q) t nil)))

(defun Footnote--fill-paragraph-in-body(orig-function &optional justify region)
  "For the purpose of this feature proposal (only), this function is an advice :before
function `fill-paragraph' to ensure that variable `fill-prefix' is correctly restored
and maintained when dealing with footnotes potentially possessing a different
`fill-prefix' value. This is meant to cover cases of the user manually returning to
the buffer text from the footnote area instead of using the function
`Footnote-back-to-message'.
  For the purpose of implementation within emacs, this function would be a remap of
`fill-paragraph', specific to the `Footnote-mode' minor mode."
  (when (and footnote-mode (Footnote--point-in-body-p) Footnote--body-auto-fill-prefix)
    (setq fill-prefix Footnote--body-auto-fill-prefix)
    (setq Footnote--body-auto-fill-prefix nil)))

(defun Footnote-toggle-alignment()
  "Change whether footnote text is aligned flush left with the
left of a footnote's number, or flush left with the first
character of footnote text on the footnote's first line."
  (interactive)
  (setq Footnote-align-to-fn-text (not Footnote-align-to-fn-text))
  (when footnote-text-marker-alist
    (if (>= (point) (cdr (first footnote-text-marker-alist)))
      (if Footnote-align-to-fn-text
        (Footnote--align-to-fn)
       (Footnote--align-to-body))))
  (if Footnote-align-to-fn-text
    (message "Footnotes will left-align to footnote text")
   (message "Footnotes will left-align to body text")))


(define-key footnote-mode-map
  (kbd "M-q") 'Footnote-toggle-alignment)

(advice-add 'Footnote-add-footnote :after #'Footnote--align-to-fn)
(advice-add 'Footnote-back-to-message :after #'Footnote--return-to-body)
(advice-add 'fill-paragraph :before #'Footnote--fill-paragraph-in-body)
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