Re: [hash2] Review

From: Peter Dimov (pdimov_at_[hidden])
Date: 2024-12-13 17:24:35

Julien Blanc wrote:
...
> * remove default construction and too short key construction for algorithms
>   where this is actually harmful (such as siphash). I don't have an issue
>   with providing constructors with keys for algorithms that do not mandate it.
>   But boost code should encourage correct usage of the algorithms it propose,
>   not allow bad usage in the name of genericity.
...
> * makes copy construction requirement on hash algorithm a requirement only
>   if hash_append_unordered_range is used. Hashing unordeder ranges is an
>   open problem, some creative other solutions may be found, and i see no
>   reasons why they would involve duplicating the algorithm state. Actually,
>   if the hash is default constructible, then it is not needed. You need
>   either one of default or copy constructible, not both.

I already answered this (twice) - using default constructed hash algorithm
instances in hash_append_unordered_range makes collisions seed-independent.

But this aside, these two points also contradict each other. The first one wants
to remove default construction from SipHash; the second one wants to use
default construction in hash_append_unordered_range. If both are applied,
SipHash will not work in hash_append_unordered_range.

As for improving hash_append_unordered_range, my current ideas are here:

https://lists.boost.org/Archives/boost/2024/12/258744.php