Re: [hash2] Comments

From: Peter Dimov (pdimov_at_[hidden])
Date: 2024-12-07 00:16:28

Andrey Semashev wrote:
> 2. Somewhat related, on the use case of extending the size of the hash value
> by repeatedly calling result(). Is this something that should be encouraged? I'm
> not a specialist, but my understanding is that extending the hash value this
> way does not improve its quality, as you get a series of bits in the value that
> are predetermined based on the lower bits. Isn't that effectively equivalent to
> just e.g. zero-extending the hash value?

No, it isn't. Hashes often have more internal state than they output as the
final result, so extension can actually produce non-redundant bits. This is
true, for example, for xxhash and siphash, and there's an example in the docs
of extending xxhash.

Even if the effective number of bits doesn't change, it's still not the same
as zero-extending, because a zero-extended hash is of very poor quality.
(The constant zero bits, being constant and zero, are completely unaffected
by the input.)

Somewhat related, if you ask get_integral_result for more bits than there
are in the input value, it takes care to not just give you something zero-
extended, for the same reason.

Ideally, each bit in the hash value should have about 50% probability of
being 1. This is called "avalanche property".