2013-01-24

pack

Sagittarius currently doesn't have pack procedure (or macro). That's simply because I wrote specific procedures to handle binary packing each time. However it's better to have generic one.

Then I need to consider its interface. I was thinking something similar with Industrial's pack however I have re-read this tweet (it's in Japanese):


Does it handle indefinte size?

I actually have 2 problems to implement: Firstly, I'm not good with pack stuff. Even when I was still using Perl, pack is only for hex to ascii or other way around (you can easily guess what is for :-) ). Secondly, if we support indefinate length, what would be the better solution?

The first thing, I just need to learn so it just takes fine time. The second one, I don't have much use cases so all what I can is guessing. There are, I think, 2 ways to implement indefinate length. One is like Perl way using some keyword inside of the format string (* or +?). The other one is providing a procedure to pre-compute the given data and generate format string. So it must be like this;
;; #\C is u8
(let ((fmt (generate-format-string #\C indefinite-bv)))
  (pack fmt indefinte-bv))
#|
Let's say indefinite-bv has 8 bytes then format string would be "CCCCCCCC".
Or if we use #\L as a bace character then format string would be "LL"
|#
The problem of this is that we can't optimise it in macro. So it always needs to be computed in runtime. I don't think this will be a big problem, though.

Ah, wait, format string can have indefinite marker if I check it macro expansion time. Hmm, which way is better?

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