Re: Byte swapping and bindat

[Stefan Monnier]

Stefan Monnier via Users list for the GNU Emacs text editor [2022-06-10 
09:32:31] wrote:
>> Is there a way to specify a bindat type with fields whose endianess
>> depends on an external parameter (or another field of the type?)
>
> I'm so glad you asked:
>
>     (let* ((threshold 32)
>            (type (bindat-type (kind   uint 8)
>                               (length sint 32 (> kind threshold)))))
>       (list (bindat-unpack type "\x10\x00\x00\x01\x01")
>             (bindat-unpack type "\x80\x00\x00\x01\x01")))
>     =>
>     (((kind . 16)  (length . 257))
>      ((kind . 128) (length . 16842752)))
>
> That's one of the benefits of the new Bindat :-)

Oh, and if you want to do that for `uint` (which currently doesn't take
a "reverse" argument (patch welcome)), you can do:

    (let* ((threshold 32)
           (type (bindat-type (kind   uint 8)
                              (length if (> kind threshold)
                                         (uintr 32) (uint 32)))))
      (list (bindat-unpack type "\x10\x00\x00\x01\x01")
            (bindat-unpack type "\x80\x00\x00\x01\x01")))
    =>
    (((kind . 16)  (length . 257))
     ((kind . 128) (length . 16842752)))


-- Stefan