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random_park.cpp
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Mon, Feb 3, 12:51

random_park.cpp
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/* ----------------------------------------------------------------------
LAMMPS - Large-scale Atomic/Molecular Massively Parallel Simulator
http://lammps.sandia.gov, Sandia National Laboratories
Steve Plimpton, sjplimp@sandia.gov
Copyright (2003) Sandia Corporation. Under the terms of Contract
DE-AC04-94AL85000 with Sandia Corporation, the U.S. Government retains
certain rights in this software. This software is distributed under
the GNU General Public License.
See the README file in the top-level LAMMPS directory.
------------------------------------------------------------------------- */
// Park/Miller RNG
#include <math.h>
#include "random_park.h"
#include "error.h"
using namespace LAMMPS_NS;
#define IA 16807
#define IM 2147483647
#define AM (1.0/IM)
#define IQ 127773
#define IR 2836
/* ---------------------------------------------------------------------- */
RanPark::RanPark(LAMMPS *lmp, int seed_init) : Pointers(lmp)
{
if (seed_init <= 0)
error->one(FLERR,"Invalid seed for Park random # generator");
seed = seed_init;
save = 0;
}
/* ----------------------------------------------------------------------
uniform RN
------------------------------------------------------------------------- */
double RanPark::uniform()
{
int k = seed/IQ;
seed = IA*(seed-k*IQ) - IR*k;
if (seed < 0) seed += IM;
double ans = AM*seed;
return ans;
}
/* ----------------------------------------------------------------------
gaussian RN
------------------------------------------------------------------------- */
double RanPark::gaussian()
{
double first,v1,v2,rsq,fac;
if (!save) {
do {
v1 = 2.0*uniform()-1.0;
v2 = 2.0*uniform()-1.0;
rsq = v1*v1 + v2*v2;
} while ((rsq >= 1.0) || (rsq == 0.0));
fac = sqrt(-2.0*log(rsq)/rsq);
second = v1*fac;
first = v2*fac;
save = 1;
} else {
first = second;
save = 0;
}
return first;
}
/* ---------------------------------------------------------------------- */
void RanPark::reset(int seed_init)
{
if (seed_init <= 0)
error->all(FLERR,"Invalid seed for Park random # generator");
seed = seed_init;
save = 0;
}
/* ----------------------------------------------------------------------
reset the seed based on atom coords and ibase = caller seed
use hash function, treating user seed and coords as sequence of input ints
this is Jenkins One-at-a-time hash, see Wikipedia entry on hash tables
------------------------------------------------------------------------- */
void RanPark::reset(int ibase, double *coord)
{
int i;
char *str = (char *) &ibase;
int n = sizeof(int);
unsigned int hash = 0;
for (i = 0; i < n; i++) {
hash += str[i];
hash += (hash << 10);
hash ^= (hash >> 6);
}
str = (char *) coord;
n = 3 * sizeof(double);
for (i = 0; i < n; i++) {
hash += str[i];
hash += (hash << 10);
hash ^= (hash >> 6);
}
hash += (hash << 3);
hash ^= (hash >> 11);
hash += (hash << 15);
// keep 31 bits of unsigned int as new seed
// do not allow seed = 0, since will cause hang in gaussian()
seed = hash & 0x7ffffff;
if (!seed) seed = 1;
// warm up the RNG
for (i = 0; i < 5; i++) uniform();
save = 0;
}
/* ---------------------------------------------------------------------- */
int RanPark::state()
{
return seed;
}

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