On Mon, Mar 10, 2025 at 6:03 PM Jeremy Rifkin wrote:
>
> I'm thinking of a super simple implementation that could be done today
> that looks like:
>
> constexpr uint64_t compile_time_random(std::source_location location =
> std::source_location::current) {
> return random_from_digest(hash(std::format("{}:{}",
> location.file_name, location.line)));
> }
>
> Alternatively with evaluating differently (but deterministically) at
> different points in the file:
>
> constexpr uint64_t compile_time_random(int nonce, std::source_location
> location = std::source_location::current) {
> return random_from_digest(hash(std::format("{}:{} {}",
> location.file_name, location.line, nonce)));
> }
>
> #define compile_time_random() compile_time_random(__COUNTER__)


I was doing something similar with a constexpr implementation of AES128:

    // Here is a 128-Bit number which we use sort
    // of like a 'seed' for generating UUID's.
    constexpr __uint128_t my_secret_key = (0x1122334455667788u |
((__uint128_t)0x99aabbccddeeff00u << 64));

    constexpr void encrypt_one_block(__uint128_t &block, __uint128_t const &key)
    {
        // Let's pretend that this is a constexpr
        // implementation of the AES128 algorithm
        block ^= key;
    }

    constexpr __uint128_t uuid_implementation(long long unsigned const arg)
    {
        __uint128_t retval = arg;
        encrypt_one_block(retval, my_secret_key);
        return retval;
    }

    #define _UUID ( uuid_implementation( __COUNTER__ ) )

    int main(void)
    {
        constexpr auto x = _UUID;
    }

The only question though is what is the desired behaviour if a
constexpr UUID is generated in a header file? Should each translation
unit get a unique UUID, or should the same UUID be shared among all
the translation units (even if the constexpr variable has internal
linkage)?