Re: [boost] [functional/hash] integer hash function

Subject: Re: [boost] [functional/hash] integer hash function
From: Peter Dimov (pdimov_at_[hidden])
Date: 2011-10-05 06:32:53

Christopher Jefferson wrote:

> A default hash function certainly can shuffle bits, to create a better 
> distribution.

What does "better distribution" mean?

> It might still be worth documenting this weakness however.

This is not a weakness. The definition of a hash function is that equal 
inputs produce the same output and different inputs should produce different 
outputs. This is exactly what this function does. If an algorithm doesn't 
work well with this hash function, this is a sign that the algorithm itself 
has a weakness, because it doesn't work well with functions that satisfy the 
requirements for a hash function. In other words, the algorithm places 
additional requirements on its hash function, requirements that a hash 
function author has no way of knowing.

Now, to be honest, I know what the perceived defect is, that the default 
hash function is not cryptographic enough. In particular, that a single bit 
change in the input results in a predictable single bit change in the 
output, whereas one would like it to result in random bit changes all over 
the size_t. But, again, if your algorithm requires this property, it is not 
generic and will not necessarily work with a hash function written by 
someone who is not aware of this requirement. If you require the output bits 
to be properly shuffled, do that yourself, this is something that all hash 
functions may potentially need.

Which is basically what you said. :-)

>  I would say not, as it is simple to add once a shuffle/distribution 
> function, which can be applied to the hash, if the user wants one.