[boost] Re: [review] hash functions

From: Daniel James (daniel_at_[hidden])
Date: 2005-03-09 08:31:08

Peter Dimov wrote:

> It is a deliberate design decision that the algorithms are fully 
> specified. The goal is precisely to not allow you (as an implementor) to 
> change them.. ;-)
> 
> As usual, this is a tradeoff between giving more freedom to implementors 
> to improve the behavior and giving more portability guarantees to users. 
> Varying the hash function can lead to dramatic changes in performance 
> and not specifying the exact algorithm will mean that an application 
> using the default hash functions would need to be benchmarked again 
> after every change of the standard library.

Yes, I agree for the final standard, but we're not there yet.

I'm also a little worried about how the hash functions will vary with 
different platforms. For example, if sizeof(long) > sizeof(std::size_t), 
then hash_value(long) might be weak for values which only vary in the 
higher bits.

But I do want to stay close to your design for now. It's far better than 
what I would have come up with and it'll be good to get more experience 
of how well it works before considering any changes.

> Of course the fact that hash_range gives the same result as hash_value 
> on the corresponding container is no accident, either.

Yes, the reason that this came up, is because this wasn't true for 
Jeremy's original implementation. But I think his hash function for 
strings was faster, which might be desirable.

Daniel