Re: [Cbor] 65-bit negatives, big nums conflict between CDE and dCBOR drafts?

[Wolf McNally <[e-mail]>]

LL,

> On Apr 10, 2024, at 6:56 PM, lgl island-resort.com <[e-mail]> wrote:
>> On Apr 10, 2024, at 12:29 AM, Wolf McNally <[e-mail]> wrote:
>> 
>> Further, it was never our expectation that -2^64 would be encoded as a float, and in fact we deliberately set no such expectations.
> 
> It seems so by implication. The choices are float, type 1 or tag 3. It couldn’t be type 1 because of (former) prohibition of 65-bit negatives. It couldn’t be tag 3 because that’s not part of dCBOR.

Indeed. And now as of I-D 09, the prohibition is indeed “former."

>> So if dCBOR’s current proscription of type 1 negative 65-bit integers indeed conflicts with this, then it must go. Obviously, this leaves a dangling range of basic integers expressible in dCBOR that are not expressible in either signed or unsigned 64-bit machine integers and which dCBOR (indeed all CBOR) implementations that do not support wider integers will have to reject.
> 
> A lot of basic decoders like tinyCBOR return the type and the value, so they’ll return “type 1” and a “uint64”. It’s up to the app to figure out what to do next for this range of values. Apps can error out, invoke a big number library, manually handle it or maybe they don’t have to do anything put print it out or translate it to JSON.

Generally, our implementations keep an internal CBOR representation, and our API allows direct interrogation of that representation if you want to know it, or they just let you specify an extraction type, and if the stored type can be converted to the extraction type with no loss, then it’s simply returned as that type. If loss would be incurred in extraction, then the API refuses.

> Protocols that want to clearly and specifically spare the native code implementer trouble can do this in CDDL:
> 
>    xxx = int .gt -9223372036854775809
> 
> Might even be considered good CDDL hygiene to constrain the ranges of integers in a protocols.

I agree that is a best practice.

~ Wolf