[boost] Re: [review results] hash functions accepted

From: Daniel James (daniel_at_[hidden])
Date: 2005-03-21 15:20:20

Thorsten Ottosen wrote:
> I would like to thank all who partcipated:

Thank you from me as well, and thanks to Thorsten for managing the review.

> Issues and solutions:
> ---------------------
> 
> 1. for those who don't want to include the whole std-lib, individual  headers
>     can be put in hash/std/XX.hpp (but see 3)

This should probably be a sub-directory of functional, as hash lives at 
boost/functional/hash.hpp.

> 4. avoid too many zero hash values by defaulting to something other than 0

I think the preferred solution was to add a constant to hash_combine 
(which seems to work well). Although, it might be good to have an 
'initial_seed' constant.

> 5. change implementation of hash values for pointers so undefined behavior is 
> gone (and consider adding  x + (x >> 3) )

I'm going to do that.

> 6. the design aim was predictibility and to minimize collision; for the latter 
> part, I
>     would like to know if similar values hash to similar buckets or not. If 
> very different
>     objects end up in the same buskets, wouldn't equality be much faster 
> rejected?
>     Should this be a concern?

Generally, values that collide should not be similar, (which is why 
collisions of sequences of zeros was a problem), although I'm sure that 
there are still exceptions to that rule. Especially because it's quite 
hard to define what 'similar' means in general.

Daniel