rand.cpp

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/* Copyright (c) 2010 Wildfire Games
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 * 
 * The above copyright notice and this permission notice shall be included
 * in all copies or substantial portions of the Software.
 * 
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT.
 * IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY
 * CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT,
 * TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE
 * SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.
 */

/*
 * pseudorandom number generator
 */

#include "precompiled.h"
#include "lib/rand.h"

// avoids several common pitfalls; see discussion at
// http://www.azillionmonkeys.com/qed/random.html

// rand() is poorly implemented (e.g. in VC7) and only returns < 16 bits;
// double that amount by concatenating 2 random numbers.
// this is not to fix poor rand() randomness - the number returned will be
// folded down to a much smaller interval anyway. instead, a larger XRAND_MAX
// decreases the probability of having to repeat the loop.
#if RAND_MAX < 65536
static const size_t XRAND_MAX = (RAND_MAX+1)*(RAND_MAX+1) - 1;
static size_t xrand()
{
    return rand()*(RAND_MAX+1) + rand();
}
// rand() is already ok; no need to do anything.
#else
static const size_t XRAND_MAX = RAND_MAX;
static size_t xrand()
{
    return rand();
}
#endif

size_t rand(size_t min_inclusive, size_t max_exclusive)
{
    const size_t range = (max_exclusive-min_inclusive);
    // huge interval or min >= max
    if(range == 0 || range > XRAND_MAX)
    {
        WARN_IF_ERR(ERR::INVALID_PARAM);
        return 0;
    }

    const size_t inv_range = XRAND_MAX / range;

    // generate random number in [0, range)
    // idea: avoid skewed distributions when <range> doesn't evenly divide
    // XRAND_MAX by simply discarding values in the "remainder".
    // not expected to run often since XRAND_MAX is large.
    size_t x;
    do
    {
        x = xrand();
    }
    while(x >= range * inv_range);
    x /= inv_range;

    x += min_inclusive;
    ENSURE(x < max_exclusive);
    return x;
}