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Annotation of XNU/bsd/libkern/random.c, revision 1.1.1.1

1.1       root        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 ([email protected])
                     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: }

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