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

From: Dave Harris (brangdon_at_[hidden])
Date: 2005-03-16 12:07:14

In-Reply-To: <01c301c52989$d50a2f40$6801a8c0_at_pdimov>
pdimov_at_[hidden] (Peter Dimov) wrote (abridged):
> I think that it is better to fix the zero trap in hash_combine, because 
> this does not depend on users remembering to initialize the seed to 
> default_seed, instead of zero. 

Did you see my suggestion of changing the primitive hash_value functions 
instead? That also does not depend on users remembering to initialise the 
seed. 

    size_t hash_value( int v ) {
        return size_t(v) + default_seed;
    }

And similarly for long, etc. Is there a reason to reject this?

> 2. Add a hash_range overload that takes a seed.
>
> 2a. void hash_range( size_t & seed, It first, It last );
> 2b. size_t hash_range( size_t seed, It first, It last );
> 2c. size_t hash_range( It first, It last, size_t seed );
> 
> which are listed in order of my preference.

So we're talking about a family of 3 functions:

2a:
    void hash_combine( size_t &seed, T value );
    void hash_range( size_t &seed, It first, It last );
    size_t hash_range( It first, It last );

2b:
    void hash_combine( size_t &seed, T value );
    size_t hash_range( size_t seed, It first, It last );
    size_t hash_range( It first, It last );

2c:
    void hash_combine( size_t &seed, T value );
    size_t hash_range( It first, It last, size_t seed=0 );
    
I think 2b is very error-prone, because of the lack of consistency between 
how hash_combine and the seeded version of hash_range return their 
results. 2a is less error-prone, but it seems needlessly confusing to have 
the two versions of hash_range return their results in different ways. In 
general there's a lot of similarity between hash_combine and hash_range 
which those versions fail to bring out.

I'd rather be more consistent, eg:

(a)
    size_t hash_combine( size_t seed, T value );
    size_t hash_range( size_t seed, It first, It last );

or:

(b)
    void hash_combine( size_t &seed, T value );
    void hash_range( size_t &seed, It first, It last );

(Perhaps with overloads to catch passing a non-size_t first argument.) I 
don't see much value in adding a hash_range which doesn't take a seed. 
Especially with (a), because we can easily pass 0 explicitly.

-- Dave Harris, Nottingham, UK