Re: trying to predict next rand value
From: Skybuck Flying (nospam_at_hotmail.com)
Date: 10/29/04
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Date: Fri, 29 Oct 2004 01:24:07 +0200
Well I figured it out myself now lol...
This C line is nicely obfuscated... ;)
I was wondering what was returned... holdrand after or before calculating...
return(((holdrand = holdrand * 214013L + 2531011L) >> 16) & 0x7fff);
I should read this as:
(holdrand = holdrand * 214013L + 2531011L);
return ((holdrand >> 16) & 0x7fff);
Bingo ! ;)
( So it does actually two things in one line
1. Update the holdrand variable
2. Calculate some random value and return it ;)
Bye,
Skybuck.
"Mok-Kong Shen" <mok-kong.shen@t-online.de> wrote in message
news:clru70$hqe$05$1@news.t-online.com...
>
>
> Skybuck Flying wrote:
>
> > "Mok-Kong Shen" <mok-kong.shen@t-online.de> wrote in message
> > news:clrsp3$egh$01$1@news.t-online.com...
> >
> >>
> >>Skybuck Flying wrote:
> >>
> >>
> >>>Well the question is what does this code do ?
> >>>
> >>>int __cdecl rand (void)
> >>>{
> >>>#ifdef _MT
> >>> _ptiddata ptd = _getptd();
> >>> return( ((ptd->_holdrand = ptd->_holdrand * 214013L + 2531011L) >>
> >
> > 16) &
> >
> >>>0x7fff );
> >>>#else /* _MT */
> >>> return(((holdrand = holdrand * 214013L + 2531011L) >> 16) & 0x7fff);
> >>>#endif /* _MT */
> >>>}
> >>>
> >>>I am not a C programmer but to me it looks like the next time rand() is
> >>>called... the new value will be based on the old value of rand() ?
> >>>
> >>>But maybe I am reading it wrong :)
> >>
> >>Perhaps it's best that you look at a textbook like Knuth,
> >>'The Art of Computer Programming', vol. 2, to see how the PRNGs
> >>commonly function.
> >
> >
> > Neh perhaps it s better if I ask this question in the C lang newsgroup
;)
>
> I believe that you are quite surely right.
>
> M. K. Shen
>
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