path: root/src/common/mt_rand.cpp

/*
// This is the ``Mersenne Twister'' random number generator MT19937, which
// generates pseudorandom integers uniformly distributed in 0..(2^32 - 1)
// starting from any odd seed in 0..(2^32 - 1).  This version is a recode
// by Shawn Cokus (Cokus@math.washington.edu) on March 8, 1998 of a version by
// Takuji Nishimura (who had suggestions from Topher Cooper and Marc Rieffel in
// July-August 1997).
//
// Effectiveness of the recoding (on Goedel2.math.washington.edu, a DEC Alpha
// running OSF/1) using GCC -O3 as a compiler: before recoding: 51.6 sec. to
// generate 300 million random numbers; after recoding: 24.0 sec. for the same
// (i.e., 46.5% of original time), so speed is now about 12.5 million random
// number generations per second on this machine.
//
// According to the URL <http://www.math.keio.ac.jp/~matumoto/emt.html>
// (and paraphrasing a bit in places), the Mersenne Twister is ``designed
// with consideration of the flaws of various existing generators,'' has
// a period of 2^19937 - 1, gives a sequence that is 623-dimensionally
// equidistributed, and ``has passed many stringent tests, including the
// die-hard test of G. Marsaglia and the load test of P. Hellekalek and
// S. Wegenkittl.''  It is efficient in memory usage (typically using 2506
// to 5012 bytes of static data, depending on data type sizes, and the code
// is quite short as well).  It generates random numbers in batches of 624
// at a time, so the caching and pipelining of modern systems is exploited.
// It is also divide- and mod-free.
//
// This library is free software; you can redistribute it and/or modify it
// under the terms of the GNU Library General Public License as published by
// the Free Software Foundation (either version 2 of the License or, at your
// option, any later version).  This library is distributed in the hope that
// it will be useful, but WITHOUT ANY WARRANTY, without even the implied
// warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See
// the GNU Library General Public License for more details.  You should have
// received a copy of the GNU Library General Public License along with this
// library; if not, write to the Free Software Foundation, Inc., 59 Temple
// Place, Suite 330, Boston, MA 02111-1307, USA.
//
// The code as Shawn received it included the following notice:
//
//   Copyright (C) 1997 Makoto Matsumoto and Takuji Nishimura.  When
//   you use this, send an e-mail to <matumoto@math.keio.ac.jp> with
//   an appropriate reference to your work.
//
// It would be nice to CC: <Cokus@math.washington.edu> when you write.
//
*/

#include <time.h>
#include "mt_rand.hpp"

#define N              624                  // length of state vector
#define M              397                  // a period parameter
#define K              0x9908B0DFU          // a magic constant

#define hiBit(u)       ((u) & 0x80000000U)  // mask all but highest bit of u
#define loBit(u)       ((u) & 0x00000001U)  // mask all but lowest bit of u
#define loBits(u)      ((u) & 0x7FFFFFFFU)  // mask the highest bit of u
#define mixBits(u, v)  (hiBit(u)|loBits(v)) // move hi bit of u to hi bit of v

static uint32_t state[N+1]; // state vector the +1 is needed due to the coding
static uint32_t *next;      // next random value is computed from here
static int left = -1;       // can *next++ this many times before reloading

void mt_seed (uint32_t seed)
{
    uint32_t x = seed | 1U;
    uint32_t *s = state;
    left = 0;

    for (int j = N; *s++ = x, --j; x *= 69069U);
}

static
void mt_reload (void)
{
    // if mt_seed has never been called
    if (left < -1)
        mt_seed (time (NULL));

    // conceptually, these are indices into the state that wrap
    uint32_t *p0 = state;
    uint32_t *p2 = state + 2;
    uint32_t *pM = state + M;

    uint32_t s0 = state[0];
    uint32_t s1 = state[1];

    // regenerate the lower N-M elements of the state
    for (int j = N-M+1; --j != 0; s0 = s1, s1 = *p2++)
        *p0++ = *pM++ ^ (mixBits (s0, s1) >> 1) ^ (loBit (s1) ? K : 0U);

    pM = state;
    // regenerate the next M-1 elements of the state
    // note that s1 is set to state[N] at the end, but discarded
    for (int j = M; --j != 0; s0 = s1, s1 = *p2++)
        *p0++ = *pM++ ^ (mixBits (s0, s1) >> 1) ^ (loBit (s1) ? K : 0U);

    // regenerate the last 1 element of the state
    s1 = state[0];
    *p0 = *pM ^ (mixBits (s0, s1) >> 1) ^ (loBit (s1) ? K : 0U);

    // ready for the normal mt_random algorithm
    left = N;
    next = state;
}

uint32_t mt_random (void)
{
    if (--left < 0)
        mt_reload ();

    uint32_t y = *next++;
    y ^= (y >> 11);
    y ^= (y << 7) & 0x9D2C5680U;
    y ^= (y << 15) & 0xEFC60000U;
    return y ^ (y >> 18);
}