usr/src/sys/libkern/random.c

/*-
 * Copyright (c) 1992 The Regents of the University of California.
 * All rights reserved.
 *
 * %sccs.include.redist.c%
 *
 *      @(#)random.c    7.1 (Berkeley) %G%
 */

#include "libkern.h"

/*
 * Pseudo-random number generator for randomizing the profiling clock,
 * and whatever else we might use it for.  The result is uniform on
 * [0, 2^31 - 1].
 */
u_long
random()
{
        static u_long randseed = 1;
        register long x, hi, lo, t;

        /*
         * Compute x[n + 1] = (7^5 * x[n]) mod (2^31 - 1).
         * From "Random number generators: good ones are hard to find",
         * Park and Miller, Communications of the ACM, vol. 31, no. 10,
         * October 1988, p. 1195.
         */
        x = randseed;
        hi = x / 127773;
        lo = x % 127773;
        t = 16807 * lo - 2836 * hi;
        if (t <= 0)
                t += 0x7fffffff;
        randseed = t;
        return (t);
}