From: Daniel James (daniel_james_at_[hidden])
Date: 2006-03-20 17:17:39
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. It needs to be done in the reverse order, and
needs to use a different combine function which hashes 0's to 0. For
example, for 10, that would do:
r = 10;
Which would not give 10.
Negative numbers would be a problem, so I was thinking of inverting the
higher parts (ie. a 64-bit -2, would convert to the 32-bit values FF,
FE, then I'd invert the first one, to get 00, FE and combine it to make
FE) but that'd be a little slow as it requires a comparison. So I was
going to try to come up with something better.
Sorry if that isn't very clear. I wanted to spend a bit of time writing
this up and I've been thinking about other possibilities as well.
Boost list run by bdawes at acm.org, gregod at cs.rpi.edu, cpdaniel at pacbell.net, john at johnmaddock.co.uk