Re: Why are tables constructed randomly?

[Philipp Janda <siffiejoe@...>]

Am 24.10.2013 09:48 schröbte Coda Highland:
> On Thu, Oct 24, 2013 at 12:16 AM, Luther <lutheroto@gmail.com> wrote:
> > The loop shows two identical sequences of keys. However, in different
> > runs of the program, the order of keys changes, as if the order is
> > completely random. I know Lua doesn't give any guarantees about the
> > order of keys, but I can't think of any reason why the hash table would
> > be constructed differently on each run. Can anyone tell me what's going
> > on here?
> >
> > (I know C, so you can point me to the relevant source code if you think
> > it will help me understand.)
> >
> > Luther
>
> Because Lua's tables are hash tables. They're an unordered collection
> -- they're organized according to an internal hash code for
> performance, and that hashing function has some randomization involved
> to help avoid pathological worst cases or maliciously-constructed
> attacks.

See here[1] and here[2].

  [1]: http://lua-users.org/lists/lua-l/2011-12/msg00813.html
  [2]: http://lua-users.org/lists/lua-l/2012-01/msg00497.html

> I don't know the Lua internals well enough to point you at the code,
> but hash tables are fairly common so if you want to know more about
> the general concept you should have no trouble looking it up.

Look for `makeseed` in `lstate.c`.

> /s/ Adam

Philipp