随机数小研究

// Win 7 + Code::Blocks 10.05
#include <iostream>
#include <utility>
#include <cstdlib>
#include <algorithm>
#include <ctime>

bool statCompare(const std::pair<int, int> & lth,
                 const std::pair<int, int> & rth)
{
    if (lth.second > rth.second) {
        return true;
    }
    else {
        return false;
    }
}

int main()
{
    const int musicCount = 10;
    const int playTimes = 10000000;

    {
        std::pair<int, int> stat[musicCount];
        for (int i=0; i<musicCount; ++i) {
            stat[i].first = i + 1;
            stat[i].second = 0;
        }

        for (int i=0; i<playTimes; ++i) {
            stat[rand() % musicCount].second++;
        }

        sort(stat, stat+musicCount-1, statCompare);
        std::cout << "bigger differ: " << stat[0].second - stat[musicCount-1].second << std::endl;
        for (int i=0; i<musicCount; ++i) {
            std::cout << stat[i].first << ": " << stat[i].second << std::endl;
        }
    }

    {
        std::pair<int, int> stat[musicCount];
        for (int i=0; i<musicCount; ++i) {
            stat[i].first = i + 1;
            stat[i].second = 0;
        }

        unsigned int seeds = time(NULL);
        for (int i=0; i<playTimes; ++i) {
            srand(seeds++);
            stat[rand() % musicCount].second++;
        }

        sort(stat, stat+musicCount-1, statCompare);
        std::cout << "bigger differ: " << stat[0].second - stat[musicCount-1].second << std::endl;
        for (int i=0; i<musicCount; ++i) {
            std::cout << stat[i].first << ": " << stat[i].second << std::endl;
        }
    }
    return 0;
}

void __cdecl srand (unsigned int seed)
{
    #ifdef _MT
        _getptd()->_holdrand = (unsigned long)seed;
    #else /* _MT */
        holdrand = (long)seed;
    #endif /* _MT */
}

int __cdecl rand (void)
{
   #ifdef _MT
    _ptiddata ptd = _getptd();
    return( ((ptd->_holdrand = ptd->_holdrand * 214013L + 2531011L) >> 16) &
    0x7fff );
   #else /* _MT */
    return(((holdrand = holdrand * 214013L + 2531011L) >> 16) & 0x7fff);
   #endif /* _MT */
}

/*
 * This file is part of the libpayload project.
 *
 * It was originally taken from the OpenBSD project.
 *
 * Copyright (c) 1990 The Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

#include <libpayload.h>

static unsigned int next = 1;

int rand_r(unsigned int *seed)
{
    *seed = *seed * 1103515245 + 12345;
    return (*seed % ((unsigned int)RAND_MAX + 1));
}

int rand(void)
{
    return (rand_r(&next));
}

void srand(unsigned int seed)
{
    next = seed;
}