76 lines
2.1 KiB
C++
76 lines
2.1 KiB
C++
/* Copyright (C) 2009 Wildfire Games.
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* This file is part of 0 A.D.
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*
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* 0 A.D. is free software: you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation, either version 2 of the License, or
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* (at your option) any later version.
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*
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* 0 A.D. is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with 0 A.D. If not, see <http://www.gnu.org/licenses/>.
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*/
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/*
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* pseudorandom number generator
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*/
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#include "precompiled.h"
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#include "rand.h"
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// avoids several common pitfalls; see discussion at
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// http://www.azillionmonkeys.com/qed/random.html
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// rand() is poorly implemented (e.g. in VC7) and only returns < 16 bits;
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// double that amount by concatenating 2 random numbers.
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// this is not to fix poor rand() randomness - the number returned will be
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// folded down to a much smaller interval anyway. instead, a larger XRAND_MAX
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// decreases the probability of having to repeat the loop.
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#if RAND_MAX < 65536
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static const size_t XRAND_MAX = (RAND_MAX+1)*(RAND_MAX+1) - 1;
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static size_t xrand()
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{
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return rand()*(RAND_MAX+1) + rand();
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}
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// rand() is already ok; no need to do anything.
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#else
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static const size_t XRAND_MAX = RAND_MAX;
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static size_t xrand()
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{
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return rand();
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}
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#endif
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size_t rand(size_t min_inclusive, size_t max_exclusive)
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{
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const size_t range = (max_exclusive-min_inclusive);
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// huge interval or min >= max
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if(range == 0 || range > XRAND_MAX)
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{
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WARN_ERR(ERR::INVALID_PARAM);
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return 0;
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}
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const size_t inv_range = XRAND_MAX / range;
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// generate random number in [0, range)
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// idea: avoid skewed distributions when <range> doesn't evenly divide
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// XRAND_MAX by simply discarding values in the "remainder".
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// not expected to run often since XRAND_MAX is large.
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size_t x;
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do
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{
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x = xrand();
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}
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while(x >= range * inv_range);
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x /= inv_range;
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x += min_inclusive;
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debug_assert(x < max_exclusive);
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return x;
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}
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