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Re: how to calculate the size of string in bytes?

From: tomas
Subject: Re: how to calculate the size of string in bytes?
Date: Tue, 18 Aug 2015 12:13:52 +0200
User-agent: Mutt/1.5.21 (2010-09-15)

Hash: SHA1

On Tue, Aug 18, 2015 at 02:11:54AM -0700, Sam Halliday wrote:
> Hi all,
> We've had to change the ENSIME protocol to be more friendly to other editors 
> and this has meant changing how we frame TCP messages.
> We used to have a 6 character hex number at the start of each message that 
> counted the number of multibyte characters, but we'd like to change it to be 
> the number of bytes in the message.
> We're sending the string to `process-send-string' and `read'ing from the 
> associated network buffer. But when calculating the outgoing length of the 
> string that we want to send, we use `length' --- but we need this to be 
> `length-in-bytes' not the number of multibyte chars. Is there a built in 
> function to do this or am I going to have to iterate the string and count the 
> byte size of each character?
> A quick test shows that
>   (length (encode-coding-string "EURO" 'raw-text))
> seems to give the correct result (1 for ASCII, 2 for Pound Sterling, 3 for 
> Euro), but I am not 100% sure if this is correct.

Raw is, afaik, Emacs's internal coding system. You don't want traces of it
in the network :-)

I'd expect you to use whichever coding system the network protocol prescribes
(these days it'd be UTF-8 by default). Things will (mostly) work for raw-text
since it's nearly UTF-8.

The really correct way to do this (AFAICS) would be to find out which encoding
process-send-string is going to use (via process-coding-system) and use *that*
in the length calculation -- this way you won't lie :-)

So I'd try this (slightly reordering the let*)

  (let* ((msg (concat (ensime-prin1-to-string sexp) "\n"))
         (coding-system (cdr (process-coding-system proc)))
         (string (concat (ensime-net-encode-length (length encode-coding-string 
msg coding-system)) msg))

It seems somewhat wasteful to encode msg (to find its length) just
to let process-send-string encode again -- perhaps there's a better
idiom around for that. The use case seems common enough. Anyone?

- -- tomás
Version: GnuPG v1.4.12 (GNU/Linux)


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