Formatting an Integer using ISO-prefixes for kB,MB,GB,... and kiB,MiB,GiB,

Question

I looking for the function that prints file size on the mode-line in size-indication-mode. I have searched for size-indication-mode in the source but cannot find a code-references to it. So where is the function that prints for example 22k

when file is approximately 22 kilobytes big?

What about difference between - kilobytes (kB), 1000 bytes, and - kibibytes (KiB), 1024 bytes, as defined at

Shouldn't Emacs support both?

This is of course not that hard to write but why reinvent the wheel?

http://en.wikipedia.org/wiki/Kibibyte

Answer 1

Here's the function I use.

(defconst number-to-string-approx-suffixes
  '("k" "M" "G" "T" "P" "E" "Z" "Y"))
(defun number-to-string-approx-suffix (n &optional binary)
  "Return an approximate decimal representation of NUMBER as a string,
followed by a multiplier suffix (k, M, G, T, P, E, Z, Y). The representation
is at most 5 characters long for numbers between 0 and 10^19-5*10^16.
Uses a minus sign if negative.
NUMBER may be an integer or a floating point number.
If the optional argument BINARY is non-nil, use 1024 instead of 1000 as
the base multiplier."
  (if (zerop n)
      "0"
    (let ((sign "")
          (b (if binary 1024 1000))
          (suffix "")
          (bigger-suffixes number-to-string-approx-suffixes))
      (if (< n 0)
          (setq n (- n)
                sign "-"))
      (while (and (>= n 9999.5) (consp bigger-suffixes))
        (setq n (/ n b) ; TODO: this is rounding down; nearest would be better
              suffix (car bigger-suffixes)
              bigger-suffixes (cdr bigger-suffixes)))
      (concat sign
                  (if (integerp n)
                  (int-to-string n)
                (number-to-string (floor n)))
              suffix))))

I use it in the size column of the buffer menu.

(defvar Buffer-menu-buffer+size-shorten 'binary)
(defadvice Buffer-menu-buffer+size (before Buffer-menu-shorten-size
                                    compile activate)
  "Shorten the size column in a buffer menu by using multiplier suffixes
\(k, M, G, T\).
This is done only if `Buffer-menu-buffer+size-shorten' is non-nil.
If `Buffer-menu-buffer+size-shorten' is the symbol `binary', use binary
multipliers (powers of 1024). Otherwise use decimal (powers of 1000)
multipliers."
  (if Buffer-menu-buffer+size-shorten
      (let ((binary (eq Buffer-menu-buffer+size-shorten 'binary)))
        (save-match-data
          (if (string-match "^[0-9]+$" size)
              (setq size (number-to-string-approx-suffix (string-to-number size)
                                                         binary)))))))