On Thu, Jan 26, 2012 at 11:29 AM, Daniel James <dnljms@gmail.com> wrote:
On 26 January 2012 07:11, Andrey Semashev <andrey.semashev@gmail.com> wrote:
Is it possible to always use power-of-2 number of buckets and bitwise operations instead of modulus division?
Since the container can use arbitrary hash functions, and the standard's quality requirements for hash functions aren't very high, it's problematic. It might be possible to use a mix function on the hash value and then use a power of 2, although that adds to the platform specific issues, which I've been trying to avoid - I've had enough problems dealing with varying compilers. I'll have to look into how much faster it is (if at all). I originally developed this on 32 bit machines and it didn't seem worth it there.
I think, the problem of insufficient quality of a hash function should not be solved by the container itself. Those users who provide good hash functions will just waste CPU cycles if additional hash values mixing is done within the container. IMHO, the best way to address this problem is to provide a set of "good" hash functions for common types (I believe functional/hash already does this) and possibly a wrapper function that just does bit mixing in a user-provided hash function. Boost.Unordered docs should mention these tools and advise to use them to get better performance.