On Fri, Mar 14, 2025 at 2:31 PM Thiago Macieira wrote:
>
> On Friday, 14 March 2025 Frederick Virchanza Gotham wrote:
>>
> > But nowadays, whenever I need a UUID for anything, I just generate a
> > 128-Bit random number. I don't manually set the 6 bits to indicate that
> > it's version 4. And I think that's what everyone else is doing now too.
>
> No, most people who call it "UUID" do set the proper bits. Even some that
> don't call it that, like systemd's ID128, do it too.
>
> $ uuidgen; uuidgen; uuidgen; uuidgen
<snip>
> $ cat /proc/sys/kernel/random/{uuid,uuid,uuid,uuid}
<snip>
> $ systemd-id128 new -u; systemd-id128 new -u; systemd-id128 new -u; systemd-



Okay then I'll accommodate this as follows:

    constexpr __uint128_t SetUUIDv4(__uint128_t arg) noexcept
    {
        // Set to version 4 and IETF variant
        arg &= 0xffff'ffff'ffff'003f'ff0f'ffff'ffff'ffff;
        arg |= 0x0000'0000'0000'0080'0040'0000'0000'0000;
        return arg;
    }

    consteval __uint128_t uuid(char const *const name) noexcept
    {
        // The following line shouldn't be needed
        // but I'm a fan of belt and braces
        static_assert( std::is_constant_evaluated() );
        md5::details::Context c;
        c.append( name, md5::details::const_strlen(name) );
        c.append( "This is my salt!", sizeof "This is my salt!" - 1u );
        __uint128_t const digest = c.final();
    #ifdef UUID_ONLY_122_BITS
        return setUUIDv4(digest);
    #endif
        return digest;
    }

I'm going to turn this into a header-only library that will work
properly on as many compilers as possible. Once I'm satisfied it's
good enough, I'll have a think about whether it should be in the
standard library.

Oh and of course the Salt (i.e. the seed) would be set in the
Makefile, so then the C++ code would be:

#ifdef UUID_SEED
        c.append( UUID_SEED, sizeof UUID_SEED - 1u );
#else
        c.append( "This is my salt!", sizeof "This is my salt!" - 1u );
#endif

If you want identical reproducible builds, just leave the salt alone.
If you want to shake things up, change the salt.