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本格的なゲームを作るにはランダム関数（乱数）が必要不可欠です。乱数といっても大まかに３種類に分類されていると思って構わないと思います。使い分けをすることによってゲーム全体の裏ハッタリ度がアップすることを願っています。
言わずもがな、全く予測不能、乱数らしい乱数です。
厳密に言うと乱数ではありませんが、同じアルゴリズムで生成されたものです。ザコを無造作に出したい。でも毎回パターンが違うと困る、というときに重宝します。
一番分かりにくいのがこれです。しかし重要度は一番だと思います。１と２の複合型です。
メルセンヌ・ツイスタ法の使用方法を以下に表します。基本的に中身には一切ふれず、関数の呼び出し名、引数の変更をしました。使用するときはライセンスの確認を怠らないでください。
#ifndef __MT19937AR_H__
#define __MT19937AR_H__
#ifdef __cplusplus
extern "C" {
#endif
#include "gba.h"
//---------------------------------------------------------------------------
//---------------------------------------------------------------------------
//---------------------------------------------------------------------------
IWRAM_CODE void _Srand(u32 s);
IWRAM_CODE u32 _Rand(u16 num);
#ifdef __cplusplus
}
#endif
#endif/*
A C-program for MT19937, with initialization improved 2002/1/26.
Coded by Takuji Nishimura and Makoto Matsumoto.
Before using, initialize the state by using init_genrand(seed)
or init_by_array(init_key, key_length).
Copyright (C) 1997 - 2002, Makoto Matsumoto and Takuji Nishimura,
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. The names of its contributors may not be used to endorse or promote
products derived from this software without specific prior written
permission.
THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS 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 COPYRIGHT OWNER 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.
Any feedback is very welcome.
http://www.math.sci.hiroshima-u.ac.jp/~m-mat/MT/emt.html
email: m-mat @ math.sci.hiroshima-u.ac.jp (remove space)
*/
//---------------------------------------------------------------------------
#include "mt19937ar.h"
/* Period parameters */
#define N 624
#define M 397
#define MATRIX_A 0x9908b0dfUL /* constant vector a */
#define UPPER_MASK 0x80000000UL /* most significant w-r bits */
#define LOWER_MASK 0x7fffffffUL /* least significant r bits */
static u32 mt[N]; /* the array for the state vector */
static s16 mti=N+1; /* mti==N+1 means mt[N] is not initialized */
//---------------------------------------------------------------------------
/* initializes mt[N] with a seed */
IWRAM_CODE void _Srand(u32 s)
{
mt[0]= s & 0xffffffffUL;
for(mti=1; mti<N; mti++)
{
mt[mti] = (1812433253UL * (mt[mti-1] ^ (mt[mti-1] >> 30)) + mti);
/* See Knuth TAOCP Vol2. 3rd Ed. P.106 for multiplier. */
/* In the previous versions, MSBs of the seed affect */
/* only MSBs of the array mt[]. */
/* 2002/01/09 modified by Makoto Matsumoto */
mt[mti] &= 0xffffffffUL;
/* for >32 bit machines */
}
}
//---------------------------------------------------------------------------
/* generates a random number on [0,0xffffffff]-interval */
IWRAM_CODE u32 _Rand(u16 num)
{
static u32 mag01[2]={0x0UL, MATRIX_A};
u32 y;
/* mag01[x] = x * MATRIX_A for x=0,1 */
if(num == 0)
{
return 0;
}
if(mti >= N)
{
/* generate N words at one time */
int kk;
/* if init_genrand() has not been called, */
if(mti == N+1)
{
_Srand(5489UL); /* a default initial seed is used */
}
for(kk=0; kk<N-M; kk++)
{
y = (mt[kk]&UPPER_MASK) | (mt[kk+1]&LOWER_MASK);
mt[kk] = mt[kk+M] ^ (y >> 1) ^ mag01[y & 0x1UL];
}
for(; kk<N-1; kk++)
{
y = (mt[kk]&UPPER_MASK) | (mt[kk+1]&LOWER_MASK);
mt[kk] = mt[kk+(M-N)] ^ (y >> 1) ^ mag01[y & 0x1UL];
}
y = (mt[N-1]&UPPER_MASK)|(mt[0]&LOWER_MASK);
mt[N-1] = mt[M-1] ^ (y >> 1) ^ mag01[y & 0x1UL];
mti = 0;
}
y = mt[mti++];
/* Tempering */
y ^= (y >> 11);
y ^= (y << 7) & 0x9d2c5680UL;
y ^= (y << 15) & 0xefc60000UL;
y ^= (y >> 18);
return DivMod(y>>5, num);
}念の為、生成した結果の下位5bitを切り捨てています。ランダム生成に関しては未知の分野なのでおまじない程度にいれました（＾＾；。
