Author: Don Dailey
Date: 18:59:10 10/22/98
Go up one level in this thread
On October 18, 1998 at 05:46:50, Ed Schröder wrote:
>Since ages I use the following formula for creating a set of random
>integers for hashing at program start:
>
>#define LENGTH 64*12
>
>int random [LENGTH];
>
>int a,b,c,d,x;
>
>srand(9); // initialize prime number
>
>for (x=0; x<LENGTGH; x++)
> { a=rand(); b=rand(); if (b & 1) a=a | 0x8000;
> c=rand(); d=rand(); if (d & 1) c=c | 0x8000;
> random[x]=(a<<16)+c;
> }
>
>I wonder how good such a system is and how others do it.
>
>- Ed -
Hi Ed,
I noticed this question generated a lot of responses. The way
I actually generated my random number table (I use 64 bit entries)
was using the random number generator that comes with gcc and
then I xor each entry with the "true random number generator"
supplied by most linux distributions. I guess I am paranoid
and liked the combination of doing both! But I precomputed
the table and declare it somewhere in the code.
The "true random generator" in linux is based on lots of chaotic
state changes inside your computer. It looks like a device
in unix and you just read them from a file. They look at things
like disk latency, interupts keyboard presses and anything they
can find to act as a source of random state to seed into their
generator. I don't know how random it actually is but I'm
happy.
The hamming stuff though is probably the right way to do it if
you are willing to have a precomputed table. You could probably
get away with any lousy generator if you are doing the right
hamming measurements. Since the hamming technique throws out
numbers that don't pass, you will probably get a better distribution
anyway.
- Don
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