drivers/staging/lustre/lnet/libcfs/prng.c

/*
 * GPL HEADER START
 *
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License version 2 only,
 * as published by the Free Software Foundation.
 *
 * 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 version 2 for more details (a copy is included
 * in the LICENSE file that accompanied this code).
 *
 * You should have received a copy of the GNU General Public License
 * version 2 along with this program; If not, see
 * http://www.gnu.org/licenses/gpl-2.0.html
 *
 * GPL HEADER END
 */
/*
 * Copyright (c) 2007, 2010, Oracle and/or its affiliates. All rights reserved.
 * Use is subject to license terms.
 */
/*
 * This file is part of Lustre, http://www.lustre.org/
 * Lustre is a trademark of Sun Microsystems, Inc.
 *
 * libcfs/libcfs/prng.c
 *
 * concatenation of following two 16-bit multiply with carry generators
 * x(n)=a*x(n-1)+carry mod 2^16 and y(n)=b*y(n-1)+carry mod 2^16,
 * number and carry packed within the same 32 bit integer.
 * algorithm recommended by Marsaglia
 */

#include <linux/libcfs/libcfs.h>

/*
 * From: George Marsaglia <geo@stat.fsu.edu>
 * Newsgroups: sci.math
 * Subject: Re: A RANDOM NUMBER GENERATOR FOR C
 * Date: Tue, 30 Sep 1997 05:29:35 -0700
 *
 * You may replace the two constants 36969 and 18000 by any
 * pair of distinct constants from this list:
 * 18000 18030 18273 18513 18879 19074 19098 19164 19215 19584
 * 19599 19950 20088 20508 20544 20664 20814 20970 21153 21243
 * 21423 21723 21954 22125 22188 22293 22860 22938 22965 22974
 * 23109 23124 23163 23208 23508 23520 23553 23658 23865 24114
 * 24219 24660 24699 24864 24948 25023 25308 25443 26004 26088
 * 26154 26550 26679 26838 27183 27258 27753 27795 27810 27834
 * 27960 28320 28380 28689 28710 28794 28854 28959 28980 29013
 * 29379 29889 30135 30345 30459 30714 30903 30963 31059 31083
 * (or any other 16-bit constants k for which both k*2^16-1
 * and k*2^15-1 are prime)
 */

#define RANDOM_CONST_A 18030
#define RANDOM_CONST_B 29013

static unsigned int seed_x = 521288629;
static unsigned int seed_y = 362436069;

/**
 * cfs_rand - creates new seeds
 *
 * First it creates new seeds from the previous seeds. Then it generates a
 * new pseudo random number for use.
 *
 * Returns a pseudo-random 32-bit integer
 */
unsigned int cfs_rand(void)
{
	seed_x = RANDOM_CONST_A * (seed_x & 65535) + (seed_x >> 16);
	seed_y = RANDOM_CONST_B * (seed_y & 65535) + (seed_y >> 16);

	return ((seed_x << 16) + (seed_y & 65535));
}
EXPORT_SYMBOL(cfs_rand);

/**
 * cfs_srand - sets the initial seed
 * @seed1 : (seed_x) should have the most entropy in the low bits of the word
 * @seed2 : (seed_y) should have the most entropy in the high bits of the word
 *
 * Replaces the original seeds with new values. Used to generate a new pseudo
 * random numbers.
 */
void cfs_srand(unsigned int seed1, unsigned int seed2)
{
	if (seed1)
		seed_x = seed1; /* use default seeds if parameter is 0 */
	if (seed2)
		seed_y = seed2;
}
EXPORT_SYMBOL(cfs_srand);

/**
 * cfs_get_random_bytes - generate a bunch of random numbers
 * @buf : buffer to fill with random numbers
 * @size: size of passed in buffer
 *
 * Fills a buffer with random bytes
 */
void cfs_get_random_bytes(void *buf, int size)
{
	int *p = buf;
	int rem, tmp;

	LASSERT(size >= 0);

	rem = min((int)((unsigned long)buf & (sizeof(int) - 1)), size);
	if (rem) {
		get_random_bytes(&tmp, sizeof(tmp));
		tmp ^= cfs_rand();
		memcpy(buf, &tmp, rem);
		p = buf + rem;
		size -= rem;
	}

	while (size >= sizeof(int)) {
		get_random_bytes(&tmp, sizeof(tmp));
		*p = cfs_rand() ^ tmp;
		size -= sizeof(int);
		p++;
	}
	buf = p;
	if (size) {
		get_random_bytes(&tmp, sizeof(tmp));
		tmp ^= cfs_rand();
		memcpy(buf, &tmp, size);
	}
}
EXPORT_SYMBOL(cfs_get_random_bytes);