Path: utzoo!mnetor!uunet!portal!cup.portal.com!Uly
From: Uly@cup.portal.com
Newsgroups: comp.lang.c
Subject: Re: Random Numbers
Message-ID: <3128@cup.portal.com>
Date: 11 Feb 88 10:11:07 GMT
References: <22926@ucbvax.BERKELEY.EDU>
Organization: The Portal System (TM)
Lines: 44
XPortal-User-Id: 1.1001.3486



Here's my favorite random number generator.  It's Knuth's algorithm A
from Seminumerical Algorithms.  The primary advantage is speed over
the linear congruential methods.  The primary disadvantage is that 
as of 1981 (correct me if I'm out of date) the theoretical underpinnings
for this technique were not well-understood.  In any case, I use this
routine constantly and have never had a problem with it.  The initializing
routine can use any 55 int array, I normally preset it with known values
and change only the first word based on the current clock ticks.  Note
that the array must possess at least one odd integer.  The period of this
generator is guaranteed to be at least 2^55-1.

Uly
----------------------------------------------------------------------------
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----------------------------------------------------------------------------
(Apologies for weak c code, I invariably code this one in assembler)

static int Y[55],K,J;


initrandom(seed)
int *seed;
{       int i=55;
        while(i--) Y[i]= *seed++;
        Y[0]=13;                        /* array must have one odd val */
        J=23;
        K=54;
        }


random(maxval)                  /* Return psuedorandom val mod maxval */
int maxval;
{       maxval= (Y[K--]+=Y[J--])%maxval;
        if(J<0) J=54;
        if(K<0) K=54;
        if(maxval<0) return -maxval;
        return maxval;
        }

...............