[fpc-pascal] 32-bit random numbers

Mark Morgan Lloyd markMLl.fpc-pascal at telemetry.co.uk
Fri Apr 21 19:22:20 CEST 2017

On 21/04/17 17:00, Peter wrote:
> On 21/04/17 10:11, Mark Morgan Lloyd wrote:> Am I correct in interpreting the documentation as implying that if I> want to get a full-range 32-bit unsigned random number I have to use> something like Random($100000000) ?> > Internally, that gets a 64-bit random by calling genrand_MT19937 twice> and throwing away one of the results. Not only does that waste time, but> it also means that if I use a "well known" seed I only see every other> value from the expected sequence e.g. http://oeis.org/A221557>
> For a 32bit unsigned, I would be inclined to use random to a int64, andmask out the upper bits.
> You could also write an XORShift generator if you are prepared to forgothe presence of zero in the sequence.

Trying to read successive 32-bit randoms only gives you the odd members 
of the Mersenne Twister sequence. Easily fixable, I'll put a patch on 
Mantis later.

Mark Morgan Lloyd
markMLl .AT. telemetry.co .DOT. uk

[Opinions above are the author's, not those of his employers or colleagues]

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