/**
 * =========================================================================
 * File        : rand.cpp
 * Project     : 0 A.D.
 * Description : pseudorandom number generator
 * =========================================================================
 */

// license: GPL; see lib/license.txt

#include "precompiled.h"
#include "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_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;
	debug_assert(x < max_exclusive);
	return x;
}