Re: [Boost-users] functional/hash: hash_value with context

From: Daniel James (daniel_james_at_[hidden])
Date: 2006-12-04 15:00:44

Boris Mansencal wrote:
>
> indeed,
> return (size_t)(p.x()*P_height + p.y())
> seems to give slightly better results...
> (but actually I still do not really grasp why...)
> 

If P_height happens to be a multiple of the number of buckets in the 
container and the container uses a plain modulus of the hash value then 
the x value will have no effect on the chosen bucket. If you want your 
program to be portable to different container implementations this could 
be a problem.

A possible way to work around this is to apply something like Thomas 
Wang's integer hash function to the result.

http://www.cris.com/~Ttwang/tech/inthash.htm

Although the function is specific to the number of bits.

But I'm surprised that boost::hash is slow. With full optimization, I'd 
expect it to work okay. Maybe a simpler hash_value might be better:

     std::size_t hash_value(point const& p)
     {
         std::size_t seed = p.x();
         seed ^= (std::size_t) p.y() + (seed<<6) + (seed>>2);
         return seed;
     }

If possible, I'd be interested in looking at your tests (you can email 
me off list about them).

Daniel