pack-length-bytes

[Sammo]

In case anyone's interested, here is function 'pack-length-bytes' to compute and return the number of bytes that a proper 'pack' format string would produce. It works by cleverly replacing the elements of the format string with the number of bytes for each element, then summing those numbers.

Code Select Expand

;; pack-length-bytes
;; return the number of bytes defined by a well-formed
;; 'pack' format string
;;
;; e.g.,
;; (pack-length-bytes ">ucc c s27 s4 n1 s3 n22 n2") --> 64
;; (pack-length-bytes "cccccs27s4n1s3n22n2") --> 64
;;
;; Knowing that 'replace' is destructive is the key to
;; understanding this function.
;;
;; The (cons 0 (map ...)) hack returns 0 for the
;; legal "" str-format.
;;
(define (pack-length-bytes str-format)
  (let
    ( TABLE '((">"  ""  )   ;order of table entries matters
              ("<"  ""  )
              ("ld" "4 ")   ;"ld" must precede "d"
              ("lf" "8 ")   ;"lf" must precede "f"
              ("lu" "4 ")   ;"lu" must precede "u"
              ("b"  "1 ")
              ("c"  "1 ")
              ("d"  "2 ")
              ("f"  "4 ")
              ("u"  "2 ")
              ("s"  " " )
              ("n"  " " ))
    )
  ;body of let
    (map (fn (x) (replace (x 0) str-format (x 1))) TABLE)
    (apply + (cons 0 (map integer (parse str-format)))) ))