Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!munnari.oz.au!uhccux!waikato!ldo
From: ldo@waikato.ac.nz (Lawrence D'Oliveiro, Waikato University)
Newsgroups: comp.sys.mac.programmer
Subject: Re: Is Random() random?
Message-ID: <2430.275c1f0b@waikato.ac.nz>
Date: 4 Dec 90 09:11:23 GMT
References: <HAIRSTON.90Dec2210114@henry.ece.cmu.edu>
Organization: University of Waikato, Hamilton, New Zealand
Lines: 40

The QuickDraw Random function uses a reasonably good algorithm.
It's basically the same as the "minimal standard" one described
in the article "Random Number Generators: Good Ones are Hard to Find",
by Stephen K Park and Keith W Miller, in Communications of the ACM,
volume 31, number 10, October 1988.

However, to obey this algorithm *exactly*, you have to ignore the
function result from Random, and look at the value of the RandSeed
QuickDraw global instead. This takes on uniformly-distributed values
from 1 up to 2147483647 (2 ** 31 - 1) inclusive.

Here's an example routine which returns a uniformly distributed
real number from 0 up to (but not including) 1:

	Procedure GetRandomReal
	  (
	    var Result : Real
	  );

	    Var
		Ignore : Integer;

	  Begin
	    Ignore := Random; { cause a change to RandSeed }
	    Result := (RandSeed - 1) / 2147483647.0
	  End {GetRandomReal};

As for the result of the Random function itself, I don't know what
its properties (e g autocorrelation and other signs of non-randomness)
are. But if you read the article I mentioned above, you'd be less
likely to trust any old random number algorithm (or variation thereof)
until you've seen the results of some very careful testing.

Lawrence D'Oliveiro                       fone: +64-71-562-889
Computer Services Dept                     fax: +64-71-384-066
University of Waikato            electric mail: ldo@waikato.ac.nz
Hamilton, New Zealand    37^ 47' 26" S, 175^ 19' 7" E, GMT+13:00
"...so she tried to break into the father bear's computer, but it was
too hard. Then she tried to break into the mother bear's computer, but
that was too easy..."