Re: Random Number Generation

From:

Joshua Cranmer <Pidgeot18@verizon.invalid>

Newsgroups:

comp.lang.java.programmer

Date:

Fri, 12 Sep 2008 20:26:10 -0400

Message-ID:

<gaf1b2$qd0$1@news-int.gatech.edu>

adrian.bartholomew@gmail.com wrote:

    public void shuffle() {
        for (int i=0; i<10; i++) {
            random.setSeed(System.currentTimeMillis());
            Collections.shuffle(cards, random);
        }
    }

From experience, I can tell you that you've shuffled at most twice (if
you're lucky), and probably once, assuming you have no more than 52
cards. A millisecond is an awful long time for a computer, and the
method does not in general have millisecond-scale resolution (10 and 16
are common, IIRC).

Resetting the seed so rapidly kills your entropy big time. Even if each
shuffle takes 1 ms, you'd essentially only see changes in the last 4-6
bits, throwing away several bits of good information. Don't reset the
seed, you don't need to.

Also, note that, although using the current time in milliseconds might
be fair entropy across invocations, using it multiple times in the same
invocation hurts entropy. The random number generator, without a seed,
will generate its own without any problems, and it does so in a far
smarter way.

--
Beware of bugs in the above code; I have only proved it correct, not
tried it. -- Donald E. Knuth