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### Re: Re: Predict Random Numbers

by no_slogan (Deacon)
 on Mar 14, 2002 at 18:06 UTC ( #151765=note: print w/replies, xml ) Need Help??

in reply to Re: Predict Random Numbers

For drand48(), the manpage tells you the constants a, b, and m. To get n (the number of outputs from int(rand*256) that are used), I used trial and error. There's a mathematical derivation of what values of n are likely to work in another paper, but that doesn't seem to be available online.

Determining a, b, and m using srand() can be automated in some cases, but it takes a bit of guesswork. If m is a power of 2 and b is small enough, you'll see an obvious pattern in the output of:

```for (0..30) {
srand(1<<\$_);
printf "%030b\n", rand()*(2**30);
}
You can work out a and take a guess at m and b from that -- a is the part that moves, b is the part that stays still, and m is whatever seems big enough to hold it all.

There are several papers (including the one I mentioned before) on determining those constants using only the output from rand(). Some of them use the L3 algorithm as well.

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