/* pickshell: pick a random point on a sphere of specified radius. */
static inline mwvector pickShell(dsfmt_t* dsfmtState, real rad)
{
real rsq, rsc;
mwvector vec;
do /* pick point in NDIM-space */
{
vec = mwRandomUnitPoint(dsfmtState);
rsq = mw_sqrv(vec); /* compute radius squared */
}
while (rsq > 1.0); /* reject if outside sphere */
rsc = rad / mw_sqrt(rsq); /* compute scaling factor */
mw_incmulvs(vec, rsc); /* rescale to radius given */
return vec;
}
static mwvector nbGravity_Exact(const NBodyCtx* ctx, NBodyState* st, const Body* p)
{
int i;
const int nbody = st->nbody;
mwvector a = ZERO_VECTOR;
const real eps2 = ctx->eps2;
for (i = 0; i < nbody; ++i)
{
const Body* b = &st->bodytab[i];
mwvector dr = mw_subv(Pos(b), Pos(p));
real drSq = mw_sqrv(dr) + eps2;
real drab = mw_sqrt(drSq);
real phii = Mass(b) / drab;
real mor3 = phii / drSq;
mw_incaddv(a, mw_mulvs(dr, mor3));
}
return a;
}
/*
* nbodyGravity: Walk the tree starting at the root to do force
* calculations.
*
*
* Random notes:
* - Not inlined without inline from multiple calls in
* mapForceBody(). Measurably better with the inline, but only
* slightly.
*/
static inline mwvector nbGravity(const NBodyCtx* ctx, NBodyState* st, const Body* p)
{
mwbool skipSelf = FALSE;
mwvector pos0 = Pos(p);
mwvector acc0 = ZERO_VECTOR;
const NBodyNode* q = (const NBodyNode*) st->tree.root; /* Start at the root */
while (q != NULL) /* while not at end of scan */
{
mwvector dr = mw_subv(Pos(q), pos0); /* Then compute distance */
real drSq = mw_sqrv(dr); /* and distance squared */
if (isBody(q) || (drSq >= Rcrit2(q))) /* If is a body or far enough away to approximate */
{
if (mw_likely((const Body*) q != p)) /* self-interaction? */
{
real drab, phii, mor3;
/* Compute gravity */
drSq += ctx->eps2; /* use standard softening */
drab = mw_sqrt(drSq);
phii = Mass(q) / drab;
mor3 = phii / drSq;
acc0.x += mor3 * dr.x;
acc0.y += mor3 * dr.y;
acc0.z += mor3 * dr.z;
if (ctx->useQuad && isCell(q)) /* if cell, add quad term */
{
real dr5inv, drQdr, phiQ;
mwvector Qdr;
/* form Q * dr */
Qdr.x = Quad(q).xx * dr.x + Quad(q).xy * dr.y + Quad(q).xz * dr.z;
Qdr.y = Quad(q).xy * dr.x + Quad(q).yy * dr.y + Quad(q).yz * dr.z;
Qdr.z = Quad(q).xz * dr.x + Quad(q).yz * dr.y + Quad(q).zz * dr.z;
/* form dr * Q * dr */
drQdr = Qdr.x * dr.x + Qdr.y * dr.y + Qdr.z * dr.z;
dr5inv = 1.0 / (sqr(drSq) * drab); /* form dr^-5 */
/* get quad. part of phi */
phiQ = 2.5 * (dr5inv * drQdr) / drSq;
acc0.x += phiQ * dr.x;
acc0.y += phiQ * dr.y;
acc0.z += phiQ * dr.z;
/* acceleration */
acc0.x -= dr5inv * Qdr.x;
acc0.y -= dr5inv * Qdr.y;
acc0.z -= dr5inv * Qdr.z;
}
}
else
{
skipSelf = TRUE; /* Encountered self */
}
q = Next(q); /* Follow next link */
}
else
{
q = More(q); /* Follow to the next level if need to go deeper */
}
}
if (!skipSelf)
{
/* If a body does not encounter itself in its traversal of the
* tree, it is "tree incest" */
nbReportTreeIncest(ctx, st);
}
return acc0;
}
