Randomness

rand

microsoft.com rand
cppreference.com rand
Understanding the algorithm of Visual C++'s rand() function

Observed being called by WeatherManager_G.

#define RAND_MAX 0x7fff

int rand() {
  _ptiddata ptd = _getptd();
  unsigned int r = 214013 * ptd->_holdrand + 2531011;
  ptd->_holdrand = r;
  return (r >> 16) & RAND_MAX;
}

Random::ran1()

Lehmer random number generator

From the book Numerical Recipes in C: The Art of Scientific Computing.

Similar to the Ran1 that appears in the Random_Z from MixedRealityToolkit/SpatialUnderstanding.

Gets called way more often than rand.

Constants and Static State

static const int    IA    = 16807;
static const int    IM    = 0x7FFFFFFF;
static const int    IQ    = 127773;
static const int    NTAB  = 32;
static const int    NDIV  = (1+(IM-1)/NTAB); // 1+(0x7FFFFFFF-1)/32 = 0x4000000
static const double AM    = (1.0f/IM); // 1.0f/0x7FFFFFFF = 4.6566129e-10
static const double EPS   = 1.2e-7f;
static const double RNMX  = (1.0f-EPS); // 0.99999988

static int idum = 0;
static int m_iY = 0;
static int m_iV[32] = {};

InitRandomSeed Command

char CallbackInitRandomSeed() {
  if (GlobalCommandManager->argc < 1 || !GlobalCommandManager->valueHasFloat[1])
    return 0;

  idum = (int)GlobalCommandManager->valueAsFloat[1];
  if (idum >= 0)
    idum = -idum;

  Random_Z::ran1();

  return 1;
}

Random::ran1()

double Random_Z::ran1() {
  if (idum <= 0 || !m_iY) {
    idum = -idum;
    if (idum < 1)
      idum = 1;

    for (int j = NTAB + 7; j >= 0; j--) {
      int k = idum / IQ;
      idum = IA * idum - IM * k;
      if (idum < 0)
        idum += IM;
      if (j < NTAB)
        m_iV[j] = idum;
    }

    m_iY = m_iV[0];
  }

  int k = idum / IQ;
  idum = IA * idum - IM * k;
  if (idum < 0)
    idum += IM;

  int j = m_iY / NDIV;
  m_iY = m_iV[j];
  m_iV[j] = idum;
  
  double temp = AM * m_iY;
  if (temp > RNMX)
    return RNMX;
  return temp;
}

Random::ran2()

Usually inlined and rarely used compared to the other random functions.

Constants and Static State

static float idum2 = 0.5;
static float m_iY2 = 2.0;

Random::ran2()

double Random_Z::ran2() {
  float f = (idum2 * 8.2145452 + m_iY2) * 1.4154547;
  unsigned int r = *(unsigned int*)&f & 0x7FFFFF | 0x3F800000; // 1.X
  return idum2 = *(float*)&r - 1.0; // 0.X
}

Random::ran3()

Constants and Static State

static unsigned int idum3 = 0;
static unsigned int m_iV3[16] = {
  0xF147EB68, 0xBD4405CD, 0x0000136D, 0x990FD175,
  0x09FBE6A9, 0x3D01097A, 0x8FB524CB, 0xD49E1A77,
  0x3330BCA8, 0xE784C9AD, 0x1F5B734E, 0x035FA97C,
  0xADB392A3, 0x57D83C79, 0x2178FE5B, 0x9B75EE37,
};

Random::ran3()

unsigned int Random_Z::ran3() {
  unsigned int r = m_iV3[((unsigned char)idum3 - 3) & 0xF] ^ m_iV3[idum3] ^ ((r ^ (2 * m_iV3[idum3])) << 15);
  unsigned int s = ((unsigned int)m_iV3[((unsigned char)idum3 - 7) & 0xF] >> 11) ^ m_iV3[((unsigned char)idum3 - 7) & 0xF];
  m_iV3[idum3] = s ^ r;
  idum3 = ((unsigned char)idum3 - 1) & 0xF;
  return m_iV3[idum3] = m_iV3[idum3] ^ r ^ s ^ r ^ (32 * ((s ^ r) & 0xFED22169)) ^ (4 * (m_iV3[idum3] ^ ((r ^ (s << 10)) << 16)));
}