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Re: [COSE] [Cbor] [Rats] RAM requirements for COSE/CWT

Carsten Bormann <cabo@tzi.org> Tue, 01 March 2022 10:16 UTC

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From: Carsten Bormann <cabo@tzi.org>
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Date: Tue, 1 Mar 2022 11:16:13 +0100
Cc: Laurence Lundblade <lgl@island-resort.com>, "rats@ietf.org" <rats@ietf.org>, "cbor@ietf.org" <cbor@ietf.org>, "cose@ietf.org" <cose@ietf.org>
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To: Jeremy O'Donoghue <jodonogh@qti.qualcomm.com>
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Subject: Re: [COSE] [Cbor] [Rats] RAM requirements for COSE/CWT
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On 1. Mar 2022, at 10:41, Jeremy O'Donoghue <jodonogh@qti.qualcomm.com> wrote:
> 
> One thing I would really like to see, and haven’t found so far, is a reliable mechanism to determine the preferred encoding for floats. Currently my implementations require the caller to decide on the float encoding (float16, float32, float64) as I haven’t been able to find a reference that explains how to straightforwardly determine the most efficient *encoding* for a float without loss of precision.

Ready for copying:

https://github.com/cabo/cbor-ruby/blob/master/ext/cbor/packer.h#L138

(Don’t be confused by the “msgpack” prefixes; this code was adapted from the msgpack implementation, and to enable updates from upstream, the renaming is done with a bunch of #defines in renamer.h.)

Note the magic lines

 float fv = v;
  if (fv == v) {     … encode as binary16 or binary32 instead of binary64

Which do all the work here.

Unfortunately C doesn’t have a portable way to say the same for binary16, so there is some more extensive code following.

Grüße, Carsten

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