An Excellent Random Number Generator…

An Excellent Random Number Generator…

There are many good algorithms for generating random numbers. Most programmers will soon discover that the ANSI rand() function is completely inadequate because it can only generate a single stream of random numbers. And sometimes, especially in games programming, a multiple discrete streams of random numbers are required.

Here's a cool little class to keep track of your random numbers. You'll want to make sure you save this code and stuff it into your own toolbox. The RNG core is called a Mersenne Twister pseudorandom number generator and it was originally developed by Takuji Nishimura and Makoto Matsumoto:

#include <iostream>

#include <time.h>

using namespace std;

/* Period parameters */

#define CMATH_N 624

#define CMATH_M 397

#define CMATH_MATRIX_A 0x9908b0df /* constant vector a */

#define CMATH_UPPER_MASK 0x80000000 /* most significant w-r bits */

#define CMATH_LOWER_MASK 0x7fffffff /* least significant r bits */

/* Tempering parameters */

#define CMATH_TEMPERING_MASK_B 0x9d2c5680

#define CMATH_TEMPERING_MASK_C 0xefc60000

#define CMATH_TEMPERING_SHIFT_U(y) (y >> 11)

#define CMATH_TEMPERING_SHIFT_S(y) (y << 7)

#define CMATH_TEMPERING_SHIFT_T(y) (y << 15)

#define CMATH_TEMPERING_SHIFT_L(y) (y >> 18)

class CRandom

{

// DATA

unsigned int rseed;

unsigned long mt[CMATH_N]; /* the array for the state vector */

int mti; /* mti==N+1 means mt[N] is not initialized */

// FUNCTIONS

public:

CRandom(void);

unsigned int Random( unsigned int n );

void SetRandomSeed(unsigned int n);

unsigned int GetRandomSeed(void);

void Randomize(void);

};

CRandom::CRandom(void)

{

rseed = 1;

mti=CMATH_N+1;

}

// Returns a number from 0 to n (excluding n)

unsigned int CRandom::Random( unsigned int n )

{

unsigned long y;

static unsigned long mag01[2]={0x0, CMATH_MATRIX_A};

if(n==0)

return(0);

/* mag01[x] = x * MATRIX_A for x=0,1 */

if (mti >= CMATH_N) { /* generate N words at one time */

int kk;

if (mti == CMATH_N+1) /* if sgenrand() has not been called, */

SetRandomSeed(4357); /* a default initial seed is used */

for (kk=0;kk<CMATH_N-CMATH_M;kk++) {

y = (mt[kk]&CMATH_UPPER_MASK)|(mt[kk+1]&CMATH_LOWER_MASK);

mt[kk] = mt[kk+CMATH_M] ^ (y >> 1) ^ mag01[y & 0x1];

}

for (;kk<CMATH_N-1;kk++) {

y = (mt[kk]&CMATH_UPPER_MASK)|(mt[kk+1]&CMATH_LOWER_MASK);

mt[kk] = mt[kk+(CMATH_M-CMATH_N)] ^ (y >> 1) ^ mag01[y & 0x1];

}

y = (mt[CMATH_N-1]&CMATH_UPPER_MASK)|(mt[0]&CMATH_LOWER_MASK);

mt[CMATH_N-1] = mt[CMATH_M-1] ^ (y >> 1) ^ mag01[y & 0x1];

mti = 0;

}

y = mt[mti++];

y ^= CMATH_TEMPERING_SHIFT_U(y);

y ^= CMATH_TEMPERING_SHIFT_S(y) & CMATH_TEMPERING_MASK_B;

y ^= CMATH_TEMPERING_SHIFT_T(y) & CMATH_TEMPERING_MASK_C;

y ^= CMATH_TEMPERING_SHIFT_L(y);

return (y%n);

}

void CRandom::SetRandomSeed(unsigned int n)

{

/* setting initial seeds to mt[N] using */

/* the generator Line 25 of Table 1 in */

/* [KNUTH 1981, The Art of Computer Programming */

/* Vol. 2 (2nd Ed.), pp102] */

mt[0]= n & 0xffffffff;

for (mti=1; mti<CMATH_N; mti++)

mt[mti] = (69069 * mt[mti-1]) & 0xffffffff;

rseed = n;

}

unsigned int CRandom::GetRandomSeed(void)

{

return(rseed);

}

void CRandom::Randomize(void)

{

SetRandomSeed(time(NULL));

}

int main()

{

CRandom r;

r.Randomize();

unsigned int num = r.Random(100);

.

.

.

}

The right way to use this class is to create different objects for each case you want.
Enjoy :)

Resources:

Game Coding Complete

by Mike McShaffry