Re: [boost] [functional] boost::hash with 64-bit types

From: Peter Dimov (pdimov_at_[hidden])
Date: 2006-03-20 17:50:19

Daniel James wrote:
> Daniel James wrote:
>> Peter Dimov wrote:
>>> For larger integer types, we probably need something along the
>>> lines of
>>>
>>> std::size_t hash_value( val )
>>> {
>>>     size_t r = val;
>>>
>>>     while( val >>= size_t_bits )
>>>     {
>>>         hash_combine( r, (size_t)val );
>>>     }
>>>
>>>     return r;
>>> }
>>>
>>> It has the property of preserving the hash_value of all _values_
>>> that can fit into a size_t, even though the underlying type is
>>> capable of holding bigger integers.
>>
>> I don't think it does.
>
> Actually, it does for positive number. Sorry I misread it. What you
> were doing clicked a couple of seconds after I clicked on 'send'. But
> I'm not sure it works for negative numbers.

You're right, it doesn't. It'd need a check against (size_t)-1 instead of 0 
in the while loop.

std::size_t hash_value( signed T val )
{
    unsigned T v2( val );

    size_t r = v2;
    size_t s = val >= 0? 0: (size_t)-1;

    while( (size_t)(v2 >>= size_t_bits) != s )
    {
        hash_combine( r, (size_t)v2 );
    }

    return r;
}

or something like that.

Now that I think about it, I'm not sure whether under the existing 
implementation -1 and -1L are guaranteed to hash to the same value, so it 
might not matter.