bsd/libkern/random.c

   1 /*
   2  * Copyright (c) 2000 Apple Computer, Inc. All rights reserved.
   3  *
   4  * @APPLE_LICENSE_HEADER_START@
   5  *
   6  * The contents of this file constitute Original Code as defined in and
   7  * are subject to the Apple Public Source License Version 1.1 (the
   8  * "License").  You may not use this file except in compliance with the
   9  * License.  Please obtain a copy of the License at
  10  * http://www.apple.com/publicsource and read it before using this file.
  11  *
  12  * This Original Code and all software distributed under the License are
  13  * distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, EITHER
  14  * EXPRESS OR IMPLIED, AND APPLE HEREBY DISCLAIMS ALL SUCH WARRANTIES,
  15  * INCLUDING WITHOUT LIMITATION, ANY WARRANTIES OF MERCHANTABILITY,
  16  * FITNESS FOR A PARTICULAR PURPOSE OR NON-INFRINGEMENT.  Please see the
  17  * License for the specific language governing rights and limitations
  18  * under the License.
  19  *
  20  * @APPLE_LICENSE_HEADER_END@
  21  */
  22 /*-
  23  * Copyright (c) 1992, 1993
  24  *      The Regents of the University of California.  All rights reserved.
  25  *
  26  * Redistribution and use in source and binary forms, with or without
  27  * modification, are permitted provided that the following conditions
  28  * are met:
  29  * 1. Redistributions of source code must retain the above copyright
  30  *    notice, this list of conditions and the following disclaimer.
  31  * 2. Redistributions in binary form must reproduce the above copyright
  32  *    notice, this list of conditions and the following disclaimer in the
  33  *    documentation and/or other materials provided with the distribution.
  34  * 3. All advertising materials mentioning features or use of this software
  35  *    must display the following acknowledgement:
  36  *      This product includes software developed by the University of
  37  *      California, Berkeley and its contributors.
  38  * 4. Neither the name of the University nor the names of its contributors
  39  *    may be used to endorse or promote products derived from this software
  40  *    without specific prior written permission.
  41  *
  42  * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
  43  * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  44  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  45  * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
  46  * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
  47  * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
  48  * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
  49  * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
  50  * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
  51  * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
  52  * SUCH DAMAGE.
  53  *
  54  *      @(#)random.c    8.1 (Berkeley) 6/10/93
  55  */
  56
  57 /*
  58  * Modification History
  59  *
  60  * Feb 22, 1999         Dieter Siegmund (dieter@apple.com)
  61  * - the first time, set the random seed to the microsecond time value
  62  *   to make the random numbers less predictable
  63  */
  64 #include <libkern/libkern.h>
  65 #include <sys/time.h>
  66
  67 /*
  68  * Pseudo-random number generator for randomizing the profiling clock,
  69  * and whatever else we might use it for.  The result is uniform on
  70  * [0, 2^31 - 1].
  71  */
  72 u_long
  73 random()
  74 {
  75         static int first = 1;
  76         static u_long randseed = 1;
  77
  78         register long x, hi, lo, t;
  79
  80         if (first) {
  81             struct timeval tv;
  82             microtime(&tv);
  83             randseed = tv.tv_usec;
  84             if(!randseed)
  85                 randseed=1;
  86             first = 0;
  87         }
  88
  89         /*
  90          * Compute x[n + 1] = (7^5 * x[n]) mod (2^31 - 1).
  91          * From "Random number generators: good ones are hard to find",
  92          * Park and Miller, Communications of the ACM, vol. 31, no. 10,
  93          * October 1988, p. 1195.
  94          */
  95         x = randseed;
  96         hi = x / 127773;
  97         lo = x % 127773;
  98         t = 16807 * lo - 2836 * hi;
  99         if (t <= 0)
 100                 t += 0x7fffffff;
 101         randseed = t;
 102         return (t);
 103 }