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From: Tobias Schwinger (tschwinger_at_[hidden])
Date: 2005-03-11 10:03:02

Daniel James wrote:
> Tobias Schwinger wrote:
>> I am missing a way to parametrize how hash values are combined.
>> This is especially desirable because the skeleton could then be
>> exploited to implement double hashing for compound data (a comon
>> technique to avoid collisions by varying the hash function) and can be
>> useful in other situations (e.g. a small hash table in memory vs. a
>> large one on disk).
> I think if you want to combine hash values in a different manner, it's
> best just write your own version of hash_combine. It could save a little
> code if we supplied a version of hash_range that works for different
> combining functions, but I don't think it'll be that useful (and would
> require that your hash_combine function had the same signature as ours,
> you might want to do it differently).

Well, this was not exactly the point I was trying to make:

Example: let's say we have a disk-stored hash table and an in-memory
hash table for caching. The same objects are stored but it is desirable
to have two different hashing functions. Unfortunately boost::hash can
only be used for at most one of them.

If I wanted to implement double hashing I'ld have to duplicate the whole
facility (which may include writing a lot of "hash_value2" functions for
a grown project) because there can be only one 'hash_value' per type
which can only use one algorithm for combining the hash values, as the
current design does not provide a way to get parametrization in there.

Looking at the source underlines this impression: hash_combine seems
like a forgotten hard wired strategy in something that actually wants to
be a facade.

>> The for loop in hash_float_value is a nice one to be unrolled via
>> metaprogramming.
> Yes, for anyone who hasn't looked at the code it loops a fixed number of
> times based on the accuracy of the float type. Compilers could unroll
> that loop themselves, but that's probably too much to expect.

Especially since it is very questionable if these global 'unroll loops'
options do any good in the big picture.



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