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/* rand.c: a simple pseudo-random number generator.
Copyright (C) 1992, 2011 Free Software Foundation, Inc.
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3, or (at your option)
any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */
#include "config.h"
#include "rand.h"
/* Current state of the random number generator. ANSI C says that the
default should be as if `srand' was called with the value 1. At all
times this will be a random number between 1 and RAND_MAX+1. */
static long rand_state = 2;
/* Set the state. It's ok if `seed' doesn't fit in a long, since we
check for negative values before we return anything. On the other
hand, we cannot allow our state to become zero, since then we will
produce zero forever. */
void
seed_rand (unsigned seed)
{
rand_state = seed + 1;
}
/* Return a pseudo-random number based on `x' in the range [0, RAND_MAX].
We use the Ghostscript computation (from the function `zrand' in the
file `zmath.c'). */
int
k_rand ()
{
/* We use an algorithm from CACM 31(10), pp. 1192-1201, October 1988.
According to a posting by Ed Taft on comp.lang.postscript, Level 2
(Adobe) PostScript interpreters use this algorithm too:
x[n+1] = (16807 * x[n]) mod (2^31 - 1)
The complication ensues from the fact that the multiplication may
lead to a 46-bit number. */
#define A 16807
#define M 0x7fffffff
#define Q 127773 /* M / A */
#define R 2836 /* M % A */
rand_state = A * (rand_state % Q) - R * (rand_state / Q);
while ( rand_state <= 0 ) rand_state += M;
/* OK, `rand_state' is our new random number between 1 and 2^31 - 1.
Now we have to subtract one, so that we return a number in the
range zero to RAND_MAX (inclusive). We obviously must define our
RAND_MAX to be 2^31 - 2, which we do (in `rand.h'). */
return rand_state - 1;
}
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