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