real pickdist(real eta, real sigma)
{
static real eta0 = -1.0, sigcorr;
int niter;
real x, y, q;
if (eta != eta0) {
sigcorr =
rsqrt(8 * eta /
(bessel_K(0.75, 1/(32*eta)) / bessel_K(0.25, 1/(32*eta)) - 1));
eprintf("[%s: sigma correction factor = %f]\n", getargv0(), sigcorr);
eta0 = eta;
}
niter = 0;
do {
x = xrandom(-1.0, 1.0);
y = xrandom(0.0, YMAX);
q = rexp(- 0.5 * rsqr(fmap(x)) - eta * rsqr(rsqr(fmap(x)))) *
(1 + x*x) / rsqr(1 - x*x);
if (q > YMAX) /* should not ever happen */
error("%s: guess out of bounds\n x = %f q = %f > %f\n",
getargv0(), x, q, YMAX);
niter++;
if (niter > 1000)
error("%s: 1000 iterations without success\n", getargv0());
} while (y > q || x*x == 1); /* 2nd test prevents infty */
return (sigcorr * sigma * fmap(x));
}
double bessel_Knu(double alpha, double x)
{
long nb, ncalc, ize;
double *bk;
ize = (long) 1;
if (alpha < 0)
alpha = -alpha;
nb = 1 + (long) floor(alpha); /* nb-1 <= |alpha| < nb */
alpha -= (nb - 1);
bk = (double *) calloc(nb, sizeof(double));
bessel_K(&x, &alpha, &nb, &ize, bk, &ncalc);
x = bk[nb - 1];
free(bk);
return x;
}
