Performance of hashes and associative arrays

[Raphaël Jakse]

Le 11/11/2012 12:38, Manfred Nowak a écrit :
> Jakse wrote:
>
>> It would also be interesting to have ideas for the general
>> case
>
> Yes, ideas might be interesting. :-)
>
> A root of "good" hashing is incorporated in algorithm2 of your
> posting:
>
> spread the values produced by the hash function uniformly over
> the interval  size_t.min .. size_t.max.
>
> Therefore:
>
> Compute the hash value for a given key by multiplying each byte
> of the key with a factor, which increases from the byte with
> lowest significance to the byte with highest significance; of
> course add the results.
>
> Remarks:
> a)
> Because of the general case: assume that the original key
> contains much redundance. Therefore do not use the bytes of the
> original key but compute a ( at least close to) lossless
> compression of the original key.
>
> b)
> The factors in the factor list should be primes for spreading
> the hash value uniformly over the intended range
>
> c)
> The quotint of two consecutives primes in the factor list should
> be greater than 256, because the used key might not contain any
> reduundancy.
>
> -manfred
>

Thanks for this interesting answer.
Is compressing for performance reasons ? is it more interesting to 
compress and then hash than just hash ?