site_archiver@lists.apple.com Delivered-To: darwin-dev@lists.apple.com User-agent: Mutt/1.5.18 (2008-05-17) On Wed, Nov 25, 2009 at 03:18:17PM -0500, Clark Cox scratched on the wall:

On Wed, Nov 25, 2009 at 2:47 PM, Derek Gaston <friedmud@gmail.com> wrote:

...

Indeed... note that this wasn't about passing the _same_ value... it was about passing similar values to srand... like singularly increasing integers...

I was responding specifically to:

"Are you (and everyone else that replied) trying to tell me that the first number out of rand() (after reseeding) should always be the predictable?"

You're misinterpreting the term "predictable." The OP states, in the same post you are quoting from, that they know the first number out is always the same for a given seed. In fact, their application design depends on it. This quote doesn't say, "for a specific seed value." Their point was that if you know the first number out for a given seed, you can predict-- with a fairly high degree of accuracy-- what the first number out will be for a very similar (numerically close) seeds. In other words, there is a highly structured relation between the seed value and the first value out, making it easy to accurately estimate-- i.e. predict-- what the first value out will be for seed values we haven't tested yet. To put it yet another way, if you know the first value from srand(N) and srand(N+2), you can make a good prediction of the first value from srand(N+1) for any value of N. This makes the first value out predictable, not in terms of a specific N, but as a general property of the PRNG. That's clearly a poor property for a PRNG, just as it would be for a hash. -j -- Jay A. Kreibich < J A Y @ K R E I B I.C H > "Our opponent is an alien starship packed with atomic bombs. We have a protractor." "I'll go home and see if I can scrounge up a ruler and a piece of string." --from Anathem by Neal Stephenson _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-dev mailing list (Darwin-dev@lists.apple.com) Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/darwin-dev/site_archiver%40lists.appl... This email sent to site_archiver@lists.apple.com