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/****************************************************************************/
/* CODE.C: hierarchical N-body code. */
/* */
/* Copyright (c) 1993 by Joshua E. Barnes, Honolulu, HI. */
/* It's free because it's yours. */
/****************************************************************************/
#define global /* nada */
#include <stdlib.h>
#include "code.h"
/*
* Default values for input parameters.
*/
string defv[] = {
/* file names for input/output */
"in=", /* Input file with initial conditions */
"out=", /* Output file of N-body frames */
/* params to control N-body integration */
"dtime=0.03125", /* Integration time-step */
"eps=0.025", /* Potential softening parameter */
"theta=1.0", /* Cell subdivision tolerence */
"usequad=false", /* If true, use quad moments */
"rsize=4.0", /* Size of initial root cell */
"options=", /* Various control options */
"tstop=2.0", /* Time to stop integration */
"dtout=0.25", /* Data output interval */
/* params used if no input specified to make a Plummer model */
"nbody=1024", /* Number of particles for test run */
"seed=123", /* Random number seed for test run */
NULL,
};
string headline = "Hierarchical N-body Code"; /* default id for run */
local void startrun(void); /* initialize system state */
local void stepsystem(void); /* advance by one time-step */
local void testdata(void); /* generate test data */
local void pickshell(vector, real); /* pick point on shell */
/*
* MAIN: toplevel routine for hierarchical N-body code.
*/
int main(int argc, string argv[])
{
initparam(argv, defv); /* setup parameter access */
startrun(); /* set params, input data */
initoutput(); /* begin system output */
while (tnow < tstop - 1.0/(1024*freq)) /* while not past tstop */
stepsystem(); /* advance N-body system */
stopoutput(); /* finish up output */
return 0; // Change this to zero when the running time is 3.8s again
}
/*
* STARTRUN: startup hierarchical N-body code.
*/
local void startrun(void)
{
infile = getparam("in"); /* set I/O file names */
outfile = getparam("out");
freq = 1.0 / getrparam("dtime"); /* inverse timestep */
eps = getrparam("eps"); /* softening length */
theta = getrparam("theta"); /* force accuracy parameter */
usequad = getbparam("usequad"); /* flag for quad moments */
rsize = getrparam("rsize"); /* initial size of root */
options = getparam("options"); /* and control options */
tstop = getrparam("tstop"); /* time to stop integration */
freqout = 1.0 / getrparam("dtout"); /* inverse output interval */
if (! streq(infile, "")) { /* was data file given? */
inputdata(); /* read inital data */
} else { /* make initial conds? */
nbody = getiparam("nbody"); /* get nbody parameter */
if (nbody < 1) /* check input value */
error("startrun: nbody = %d is absurd\n", nbody);
srand((long) getiparam("seed")); /* set random generator */
testdata(); /* make test model */
}
nstep = 0; /* start counting steps */
tout = tnow; /* schedule first output */
}
/*
* TESTDATA: generate Plummer model initial conditions for test runs,
* scaled to units such that M = -4E = G = 1 (Henon, Hegge, etc).
* See Aarseth, SJ, Henon, M, & Wielen, R (1974) Astr & Ap, 37, 183.
*/
#define MFRAC 0.999 /* cut off 1-MFRAC of mass */
local void testdata(void)
{
real rsc, vsc, r, v, x, y;
vector cmr, cmv;
bodyptr p;
headline = "Hierarchical code: Plummer model";
/* supply default headline */
tnow = 0.0; /* reset elapsed model time */
bodytab = (bodyptr) allocate(nbody * sizeof(body));
/* alloc space for bodies */
rsc = 3 * PI / 16; /* set length scale factor */
vsc = rsqrt(1.0 / rsc); /* and recip. speed scale */
CLRV(cmr); /* init cm pos, vel */
CLRV(cmv);
for (p = bodytab; p < bodytab+nbody; p++) { /* loop over particles */
Type(p) = BODY; /* tag as a body */
Mass(p) = 1.0 / nbody; /* set masses equal */
r = 1 / rsqrt(rpow(xrandom(0.0, MFRAC), /* pick r in struct units */
-2.0/3.0) - 1);
pickshell(Pos(p), rsc * r); /* pick scaled position */
ADDV(cmr, cmr, Pos(p)); /* add to running sum */
do { /* select from fn g(x) */
x = xrandom(0.0, 1.0); /* for x in range 0:1 */
y = xrandom(0.0, 0.1); /* max of g(x) is 0.092 */
} while (y > x*x * rpow(1 - x*x, 3.5)); /* using von Neumann tech */
v = rsqrt(2.0) * x/rpow(1 + r*r, 0.25); /* find v in struct units */
pickshell(Vel(p), vsc * v); /* pick scaled velocity */
ADDV(cmv, cmv, Vel(p)); /* add to running sum */
}
DIVVS(cmr, cmr, (real) nbody); /* normalize cm coords */
DIVVS(cmv, cmv, (real) nbody);
for (p = bodytab; p < bodytab+nbody; p++) { /* loop over particles */
SUBV(Pos(p), Pos(p), cmr); /* offset by cm coords */
SUBV(Vel(p), Vel(p), cmv);
}
}
/*
* PICKSHELL: pick a random point on a sphere of specified radius.
*/
local void pickshell(vector vec, real rad)
{
int k;
real rsq, rsc;
do { /* pick point in NDIM-space */
for (k = 0; k < NDIM; k++) /* loop over dimensions */
vec[k] = xrandom(-1.0, 1.0); /* pick from unit cube */
DOTVP(rsq, vec, vec); /* compute radius squared */
} while (rsq > 1.0); /* reject if outside sphere */
rsc = rad / rsqrt(rsq); /* compute scaling factor */
MULVS(vec, vec, rsc); /* rescale to radius given */
}
/*
* STEPSYSTEM: advance N-body system one time-step.
*/
local void stepsystem(void)
{
bodyptr p;
real dt;
vector dvel, dpos;
if (nstep == 0) { /* about to take 1st step? */
maketree(bodytab, nbody); /* build tree structure */
nfcalc = n2bcalc = nbccalc = 0; /* zero counters */
for (p = bodytab; p < bodytab+nbody; p++) {
/* loop over all bodies */
hackgrav(p, Mass(p) > 0.0); /* get force on each */
nfcalc++; /* count force calcs */
n2bcalc += n2bterm; /* and 2-body terms */
nbccalc += nbcterm; /* and body-cell terms */
}
output(); /* do initial output */
}
dt = 1.0 / freq; /* set basic time-step */
for (p = bodytab; p < bodytab+nbody; p++) { /* loop over all bodies */
MULVS(dvel, Acc(p), 0.5 * dt); /* get velocity increment */
ADDV(Vel(p), Vel(p), dvel); /* advance v by 1/2 step */
MULVS(dpos, Vel(p), dt); /* get positon increment */
ADDV(Pos(p), Pos(p), dpos); /* advance r by 1 step */
}
maketree(bodytab, nbody); /* build tree structure */
nfcalc = n2bcalc = nbccalc = 0; /* zero counters */
for (p = bodytab; p < bodytab+nbody; p++) { /* loop over bodies */
hackgrav(p, Mass(p) > 0.0); /* get force on each */
nfcalc++; /* count force calcs */
n2bcalc += n2bterm; /* and 2-body terms */
nbccalc += nbcterm; /* and body-cell terms */
}
for (p = bodytab; p < bodytab+nbody; p++) { /* loop over all bodies */
MULVS(dvel, Acc(p), 0.5 * dt); /* get velocity increment */
ADDV(Vel(p), Vel(p), dvel); /* advance v by 1/2 step */
}
nstep++; /* count another time step */
tnow = tnow + dt; /* finally, advance time */
output(); /* do major or minor output */
}
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