Re: [PATCH 13/10] tests for various pack index features

From: Nicolas Pitre <nico@cam.org>
Date: Apr 11, 2007, 17:29 UTC
Message-ID: <alpine.LFD.0.98.0704111316290.28181@xanadu.home>
In-Reply-To: <20070411145103.GP5436@spearce.org>

On Wed, 11 Apr 2007, Shawn O. Pearce wrote:

Show 14 quoted lines

> Olivier Galibert <galibert@pobox.com> wrote:
> > On Wed, Apr 11, 2007 at 08:57:09AM -0400, Nicolas Pitre wrote:
> > > Hmmm what we need is a random data generator that always produces the 
> > > same thing.  I'll hack something to replace urandom.
> > 
> > Don't hack something, ues the standard reference, the Mersenne Twister.
> > 
> >   http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/emt.html
> > 
> > PRNGs are the same as cryptosystems, it's very easy to hack up
> > something and get it very, very wrong.  And it's unnecessary, since
> > there are very good ones available.
> 
> Indeed.

Please don't get too excited.

We don't want a full fledged random number generator with a period of 2^30000 that is faster than light and impossible to predict, etc, etc.

The _only_ thing we want is a convenient way to produce large files with garbage that is neither compressible nor deltifiable, but still reproducible. And for that matter the Mersenne Twister algo is _way_ too heavy for our needs.

The one that I just implemented basically boils down to:

	while (count--) {
		next = next * 1103515245 + 12345;
		putchar((next >> 16) & 0xff);
	}

and that does the job perfectly well.

Nicolas