infer-indentation-mode.el

(define-minor-mode infer-indentation-mode
  "infer-indentation"
  ""
  :keymap nil
  
  ;; store the original settings so we can switch back
  (when (not (boundp 'infer-indentation-original-style))
    (set (make-local-variable 'infer-indentation-original-style) indent-tabs-mode))
  (when (not (boundp 'infer-indentation-original-width))
    (set (make-local-variable 'infer-indentation-original-width) tab-width))
  
  (defun detect-line-indentation-style (line)
    "Detect if a line is indented with tabs or spaces"
    (message (concat "testing:'" line "'"))
    (when (> (length line) 0)
      (let ((first-char (substring line 0 1)))
	(cond
	 ((string-equal first-char " ") (cons :spaces (get-indentation-width line)))
	 ((string-equal first-char "\t") (cons :tabs 1))))))
  
  (defun get-indentation-width (line &optional count)
    "Detect how many spaces are used for indenting"
    (let ((count (if count count 0)))
      (cond
       ((= (length line) 0) count)
       ((string-equal (substring line 0 1) " ")
	(get-indentation-width (substring line 1) (+ 1 count)))
       (t count))))
  
  (defun detect-current-line-indentation-style ()
    "Detect if the current line is indented with tabs or spaces"
    (detect-line-indentation-style
     (buffer-substring-no-properties (point-at-bol) (point-at-eol))))
  
  (defun detect-current-buffer-indentation-style ()
    "Detect it the current buffer is indenting with tabs or spaces"
    (let ((indentation-style (detect-current-line-indentation-style)))
      (if indentation-style
	  indentation-style
	;; couldn't use current line for indentation, try the next line
	(progn
	  (forward-line 1)
	  (detect-current-buffer-indentation-style)))))
  
  (defun infer-indentation-style ()
    ""
    (save-excursion
      (progn
	(goto-char (point-min))
	(detect-current-buffer-indentation-style))))

  (defun infer-indentation ()
    "Calculate how the current buffer is being indented and set the indentations settings to that"
    (let ((style (infer-indentation-style)))
      (cond
       ((equal (car style) :spaces)
	(progn
	  (message (concat "Indenting with " (number-to-string (cdr style)) " spaces"))
	  (setq indent-tabs-mode nil)
	  (setq tab-width (cdr style))))
       ((equal (car style) :tabs)
	(progn
	  (message "Indenting with tabs")
	  (setq indent-tabs-mode t))))))
  
  (defun use-original-indentation ()
    (message "Using original indentation style")
    (setq indent-tabs-mode infer-indentation-original-style)
    (setq tab-width infer-indentation-original-width)
    )
  
  ;; toggle infer-indentation
  (if infer-indentation-mode
      (use-original-indentation)
    (infer-indentation))
  )