[Monotone-devel] Re: pointer arithmetics in C++

[Lapo Luchini]

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Nathaniel Smith wrote:
> If that's really what you want, you need something like
>
> x1 ^ x2 ^ x3 ^ reinterpret_cast<int>(my_pointer);
>
> (Or perhaps multiplication would be better, or something, hash
> functions are not that easy to design.)
In fact if x1 and x2 are likely to change by 1 it would be quite easy
to have the case (x1+1 ^ x2+1 = x1 ^ x2) when the hash is a collision
(if x1 and x2 had a 0 lower bit) or similar cases.
If speed is not a real concern, it really would be better something like:
  x1*p1 + x2*p2 + x3*p3
where p1,p2,p3 are some "big" different primes.

(and me not being a cryptographer I guess it would have "other
problems" anyway, but at least it wouldn't be "weak" to such
frequently-happening use cases ^_^)

- --
Lapo Luchini
address@hidden (OpenPGP & X.509)
www.lapo.it (Jabber, ICQ, MSN)
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