Re: UUID v7

> On 20 Nov 2024, at 00:06, Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> 
> On Tue, Nov 19, 2024 at 9:45 AM Andrey M. Borodin <x4mmm@yandex-team.ru> wrote:
>> 
>> 
>> 
>>> On 19 Nov 2024, at 14:31, Andrey M. Borodin <x4mmm@yandex-team.ru> wrote:
>>> 
>>> Done.
>> 
>> Here's v33 intact + one more patch to add 2 bits of entropy on MacOS (to compensate lack of nanoseconds).
>> What do you think?
>> 
> 
> Thank you for updating the patch!
> 
> I've reviewed the v33 patch and made some changes mostly for cosmetic
> things. Please review it to see if we accept these changes.

Your changes look good to me. I particularly like sortability test.
I see that you removed implementation of clock_gettime() for Windows. Well, this makes sense.


> 
> I have one question about the additional patch:
> 
> +#if defined(__darwin__)
> + /*
> + * On MacOS real time is truncted to microseconds. Thus, 2 least
> + * significant bits of increased_clock_precision are neither random
> + * (CSPRNG), nor time-dependent (in a sense - truly random). These 2 bits
> + * are dependent on other time-specific bits, thus they do not contribute
> + * to uniqueness. To make these bit random we mix in two bits from CSPRNG.
> + */
> + uuid->data[7] = uuid->data[7] ^ (uuid->data[8] >> 6);
> +#endif
> 
> I thought that the whole 12 bits in "rand_a" is actually
> time-dependent since we store 1/4096 fraction of sub-milliseconds. Am
> I missing something?

We have 12 bits in increaesd_clock_precission but only 1000 possible values of these bits. 2 least significant bits are defined by other 10 bits.
These bits are not equal to 0, they are changing.
True, these bits are time-dependent in a sense that these bits are be computed from a full timestamp. I wanted to express the fact that timestamp cannot be altered in a way so only these 2 bits are changed.


Best regards, Andrey Borodin.