[GiNaC-list] [PATCH] add configure option to enable deterministic sorting

[Valery Yundin]

Hi,

On 29 May 2014 07:31, Alexei Sheplyakov <alexei.sheplyakov at gmail.com> wrote:
> I'm terribly sorry, but everything which makes the hash function worse
> (slower or produces more collisions) is not acceptable.
If you insist on that regardless of how it affects overall
performance, it is not a technical problem and cannot be fixed with a
patch.

In any case, if my previous patches are not acceptable for whatever
reasons GiNaC developers might have, I suggest to close this
discussion by adding a configure option. So users can decide for
themselves if (non-)determinism is worth few per-cent speed
difference. Below is the simplest patch one can imagine.

By default it keeps things as they are now. If user specifies
--enable-determinism then make_hash_seed simply returns 0. Objects
with the same calchash() implementation *and* the same contents *but*
different types will have a hash collision, which be resolved in
base::compare by using typeid_1.before(typeid_2) (on Linux it is a
simple strcmp). Since it happens quite rarely (depends on a particular
code of course), it is better than always using crc32 as in windows
version of your code. In fact it can sometimes be faster than current
"fast" hash.

Best,
Valery
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