mtwister.c

/* An implementation of the MT19937 Algorithm for the Mersenne Twister
 * by Evan Sultanik.  Based upon the pseudocode in: M. Matsumoto and
 * T. Nishimura, "Mersenne Twister: A 623-dimensionally
 * equidistributed uniform pseudorandom number generator," ACM
 * Transactions on Modeling and Computer Simulation Vol. 8, No. 1,
 * January pp.3-30 1998.
 *
 * http://www.sultanik.com/Mersenne_twister
 */

#define UPPER_MASK		0x80000000
#define LOWER_MASK		0x7fffffff
#define TEMPERING_MASK_B	0x9d2c5680
#define TEMPERING_MASK_C	0xefc60000

#include "mtwister.h"

inline void m_seedRand(MTRand* rand, unsigned long seed) {
  /* set initial seeds to mt[STATE_VECTOR_LENGTH] using the generator
   * from Line 25 of Table 1 in: Donald Knuth, "The Art of Computer
   * Programming," Vol. 2 (2nd Ed.) pp.102.
   */
  rand->mt[0] = seed & 0xffffffff;
  for(rand->index=1; rand->index<STATE_VECTOR_LENGTH; rand->index++) {
    rand->mt[rand->index] = (6069 * rand->mt[rand->index-1]) & 0xffffffff;
  }
}

/**
* Creates a new random number generator from a given seed.
*/
MTRand seedRand(unsigned long seed) {
  MTRand rand;
  m_seedRand(&rand, seed);
  return rand;
}

/**
 * Generates a pseudo-randomly generated long.
 */
unsigned long genRandLong(MTRand* rand) {

  unsigned long y;
  static unsigned long mag[2] = {0x0, 0x9908b0df}; /* mag[x] = x * 0x9908b0df for x = 0,1 */
  if(rand->index >= STATE_VECTOR_LENGTH || rand->index < 0) {
    /* generate STATE_VECTOR_LENGTH words at a time */
    int kk;
    if(rand->index >= STATE_VECTOR_LENGTH+1 || rand->index < 0) {
      m_seedRand(rand, 4357);
    }
    for(kk=0; kk<STATE_VECTOR_LENGTH-STATE_VECTOR_M; kk++) {
      y = (rand->mt[kk] & UPPER_MASK) | (rand->mt[kk+1] & LOWER_MASK);
      rand->mt[kk] = rand->mt[kk+STATE_VECTOR_M] ^ (y >> 1) ^ mag[y & 0x1];
    }
    for(; kk<STATE_VECTOR_LENGTH-1; kk++) {
      y = (rand->mt[kk] & UPPER_MASK) | (rand->mt[kk+1] & LOWER_MASK);
      rand->mt[kk] = rand->mt[kk+(STATE_VECTOR_M-STATE_VECTOR_LENGTH)] ^ (y >> 1) ^ mag[y & 0x1];
    }
    y = (rand->mt[STATE_VECTOR_LENGTH-1] & UPPER_MASK) | (rand->mt[0] & LOWER_MASK);
    rand->mt[STATE_VECTOR_LENGTH-1] = rand->mt[STATE_VECTOR_M-1] ^ (y >> 1) ^ mag[y & 0x1];
    rand->index = 0;
  }
  y = rand->mt[rand->index++];
  y ^= (y >> 11);
  y ^= (y << 7) & TEMPERING_MASK_B;
  y ^= (y << 15) & TEMPERING_MASK_C;
  y ^= (y >> 18);
  return y;
}

/**
 * Generates a pseudo-randomly generated double in the range [0..1].
 */
double genRand(MTRand* rand) {
  return((double)genRandLong(rand) / (unsigned long)0xffffffff);
}