My colleague asked me to write function written to convert selected binary constants (ex. 0b11101) to decimal view and back.

bit-converter

;; The fist version. Tricky and workarounded.

;; you can test it on this:
;; var BITS = ( 29, 0, 63, 85, 34 )


(defun bit-compact (start end)
  (interactive "r")
  (defun int-to-binary-string (i)
    "convert an integer into it's binary representation in string format"
    (let ((res ""))
      (while (not (= i 0))
	    (setq res (concat (if (= 1 (logand i 1)) "1" "0") res))
	    (setq i (lsh i -1)))
      (if (string= res "")
	    (setq res "0"))
      res))
  (defun format-bit (bit-string from-base)
    "return either decimal string or python-style binary string"
    (let ((number (string-to-number bit-string from-base)))
      (cond ((= from-base 2) (int-to-string number))
	        ((= from-base 10) (concat "0b" (int-to-binary-string number))))))
  (defun do-convert (regex base)
    "search for regexp and convert numbers: dec <-> binary"
    (while (search-forward-regexp regex nil t)
      (replace-match
       (format-bit
	    (match-string 1)
	    base))))
  (save-excursion
    (if (not (use-region-p))
	    (progn (setq start (line-beginning-position)) (setq end (line-end-position))))
    (goto-char start)
    (narrow-to-region start end)
    (cond ((> (how-many "0b[01]+" start end) 0) (do-convert "0b\\([01]+\\)" 2))
	      ((> (how-many "[0-9]+" start end) 0) (do-convert "\\([0-9]+\\)" 10)))
    (fill-region start end)
    (widen)))