It is 2^61, not 2^64: The most random UUID version has 122 random bits. They also contain version data or sometimes timestamps. Sometimes the random bits are partly used to define classes of UUIDs.   As the UUIDs have a defined format,  - it is neither enough to just provide a 128 bit random number, nor  - to put together two 64 bit random numbers   -----Ursprüngliche Nachricht----- Von:Jeremy Rifkin via Std-Proposals <std-proposals_at_[hidden]> Gesendet:Fr 14.03.2025 05:54 Betreff:Re: [std-proposals] Random numbers in identical builds An:std-proposals_at_[hidden]; CC:Jeremy Rifkin <rifkin.jer_at_[hidden]>; > And then we'd provide a constexpr function for the platforms that have a 128-Bit integer type  You still haven’t addressed how you think this will be used on platforms that don’t have such a type.  > For us mere mortals here on Earth, it's accepted that 128 bits is enough. There might be a species somewhere else in the universe that have spread to multiple galaxies; I mean the universe is 13.8 billion  You’ve gone from making claims about cryptography, mathematicians, and being unique at the scale of the universe to talking about “mere mortals on earth.” Again, I recommend being specific about the criteria and guarantees you want instead of hand-waving.  > But for the rudimentary stuff we do here on Earth, we can populate a database with 128-Bit randomly-generated identifiers and be sure that we won't get a collision even if we combine all the world's databases together into one.  According to Rivery, all the world’s data as of 2024 was about 149ZB. 2^64 bytes is 0.001 of that. And that number is only going up and accelerating. As of 2023 amazon AWS *alone* held 280 trillion items. This is only a few orders of magnitude off of 2^64. Just so you understand the scale of the claims you’re making. Again, 128 random bits fundamentally can only provide at most 64 bits of collision resistance and 2^64 is only the point at which there’s a 50% chance of collision. You can still have substantial chances of collision at much lower amounts.  I hope you see why your claim below is not correct to say during this day and age:  > But yeah 128-Bit is enough bits for humans to say "We won't get a collision ever".  Once again, I recommend being really specific about your goals, what functionality you’re trying to provide, what guarantees you want to make about random number quality, and what use cases you want to serve.  Cheers, Jeremy On Thu, Mar 13, 2025 at 18:32 Frederick Virchanza Gotham via Std-Proposals <std-proposals_at_[hidden] <mailto:std-proposals_at_[hidden]> > wrote: On Thu, Mar 13, 2025 at 7:06 PM Jeremy Rifkin wrote: > > > Maybe we could make it: > >    std::array< char unsigned, 128u / CHAR_BIT > > > or maybe even: > >    std::array< char unsigned, (128u / CHAR_BIT) + !!(128u % CHAR_BIT) > > > but also provide an extra constexpr function to convert the array to > > __uint128_t (where a 128-Bit integer type exists). > > But what if it doesn’t exist? Then you haven’t solved the fundamental problem with this interface. The main type would be:     typedef std::array< char unsigned, (128u / CHAR_BIT) + !!(128u % CHAR_BIT) > uuid_t; And then we'd provide a constexpr function for the platforms that have a 128-Bit integer type:     #ifdef __GNUC__         constexpr __uint128_t to_uint128(uuid_t const &);     #endif although maybe std::bit_cast would be an alternative here. > 128 random bits can provide *at most* 64 bits of collision resistance > thanks to the birthday paradox. 2^64 is large, but not that large. For us mere mortals here on Earth, it's accepted that 128 bits is enough. There might be a species somewhere else in the universe that have spread to multiple galaxies; I mean the universe is 13.8 billion years old, and life started on Earth 3.5 billion years ago . . . so maybe if a species started out 9 billion years ago and had billions of years to spread outside their solar system, around their galaxy and into neighbouring galaxies. Maybe that multigalactic species needs 256 bits. But for the rudimentary stuff we do here on Earth, we can populate a database with 128-Bit randomly-generated identifiers and be sure that we won't get a collision even if we combine all the world's databases together into one. Sorry I'm watching 'Signs' with Mel Gibson tonight. What a film. Up there with the Sixth Sense. But yeah 128-Bit is enough bits for humans to say "We won't get a collision ever". That's why a UUID is 128-Bit. -- Std-Proposals mailing list Std-Proposals_at_[hidden] <mailto:Std-Proposals_at_[hidden]> https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals -- Std-Proposals mailing list Std-Proposals_at_[hidden] https://lists.isocpp.org/mailman/listinfo.cgi/std-proposals Is is 2^