c-radix-helper.el

(defun read-c-radix (s)
  (let ((c (substring s 0 (min (length s) 1))))
    (cond ((string= s "2") 2)
          ((string= c "b") 2)
          ((string= s "8") 8)
          ((string= c "o") 8)
          ((string= s "16") 16)
          ((string= c "h") 16)
          ((string= s "10") 10)
          ((string= c "d") 10))))

(defun parse-c-radix-number (s)
  (let* ((m (string-match "^\\(0b\\([01]+\\)\\|0o?\\([0-7]+\\)\\|0x\\([0-9a-fA-F]+\\)\\|\\([1-9][0-9]*\\|0\\)\\)$" s))
         (b (match-string 2 s))
         (o (match-string 3 s))
         (x (match-string 4 s))
         (d (match-string 5 s)))
    (cond ((stringp b) (string-to-number b 2))
          ((stringp o) (string-to-number o 8))
          ((stringp x) (string-to-number x 16))
          ((stringp d) (string-to-number d 10)))))

(defun format-c-radix-binary (n &optional x)
  (concat
   (if x "0b" "")
   (let ((n1 (ash n -1)))
     (if (> n1 0)
         (format-c-radix-binary n1)
       ""))
   (if (= (logand n 1) 1) "1" "0")))

(defun format-c-radix-number (n r)
  (if ((= r 2) (format-c-radix-binary n 't))
      ((format (cond ((= r 8) "0%o")
                     ((= r 16) "0x%x")
                     ('t "%d"))
               n))))

(defun convert-c-radix (radix)
  "Changes radix of number at cursor."
  (interactive
   (list (completing-read
          "Result radix: "
          '("bin" "oct" "hex" "dec" "2" "8" "16" "10"))))
  (let* ((r (read-c-radix radix))
         (b (bounds-of-thing-at-point 'word))
         (s (buffer-substring-no-properties (car b) (cdr b)))
         (n (parse-c-radix-number s)))
    (if (and (numberp n) (numberp r))
        (save-excursion (delete-region (car b) (cdr b))
                        (insert (format-c-radix-number n r))))
    ))