double beta(double a, double b)
{
#ifdef NOMORE_FOR_THREADS
static double xmin, xmax = 0;/*-> typically = 171.61447887 for IEEE */
static double lnsml = 0;/*-> typically = -708.3964185 */
if (xmax == 0) {
gammalims(&xmin, &xmax);
lnsml = log(d1mach(1));
}
#else
/* For IEEE double precision DBL_EPSILON = 2^-52 = 2.220446049250313e-16 :
* xmin, xmax : see ./gammalims.c
* lnsml = log(DBL_MIN) = log(2 ^ -1022) = -1022 * log(2)
*/
# define xmin -170.5674972726612
# define xmax 171.61447887182298
# define lnsml -708.39641853226412
#endif
#ifdef IEEE_754
/* NaNs propagated correctly */
if(ISNAN(a) || ISNAN(b)) return a + b;
#endif
if (a < 0 || b < 0)
ML_ERR_return_NAN
else if (a == 0 || b == 0)
return ML_POSINF;
else if (!R_FINITE(a) || !R_FINITE(b))
return 0;
if (a + b < xmax) {/* ~= 171.61 for IEEE */
// return gammafn(a) * gammafn(b) / gammafn(a+b);
/* All the terms are positive, and all can be large for large
or small arguments. They are never much less than one.
gammafn(x) can still overflow for x ~ 1e-308,
but the result would too.
*/
return (1 / gammafn(a+b)) * gammafn(a) * gammafn(b);
} else {
double val = lbeta(a, b);
if (val < lnsml) {
/* a and/or b so big that beta underflows */
ML_ERROR(ME_UNDERFLOW, "beta");
/* return ML_UNDERFLOW; pointless giving incorrect value */
}
return exp(val);
}
}
double gammafn(double x)
{
const static double gamcs[42] = {
+.8571195590989331421920062399942e-2,
+.4415381324841006757191315771652e-2,
+.5685043681599363378632664588789e-1,
-.4219835396418560501012500186624e-2,
+.1326808181212460220584006796352e-2,
-.1893024529798880432523947023886e-3,
+.3606925327441245256578082217225e-4,
-.6056761904460864218485548290365e-5,
+.1055829546302283344731823509093e-5,
-.1811967365542384048291855891166e-6,
+.3117724964715322277790254593169e-7,
-.5354219639019687140874081024347e-8,
+.9193275519859588946887786825940e-9,
-.1577941280288339761767423273953e-9,
+.2707980622934954543266540433089e-10,
-.4646818653825730144081661058933e-11,
+.7973350192007419656460767175359e-12,
-.1368078209830916025799499172309e-12,
+.2347319486563800657233471771688e-13,
-.4027432614949066932766570534699e-14,
+.6910051747372100912138336975257e-15,
-.1185584500221992907052387126192e-15,
+.2034148542496373955201026051932e-16,
-.3490054341717405849274012949108e-17,
+.5987993856485305567135051066026e-18,
-.1027378057872228074490069778431e-18,
+.1762702816060529824942759660748e-19,
-.3024320653735306260958772112042e-20,
+.5188914660218397839717833550506e-21,
-.8902770842456576692449251601066e-22,
+.1527474068493342602274596891306e-22,
-.2620731256187362900257328332799e-23,
+.4496464047830538670331046570666e-24,
-.7714712731336877911703901525333e-25,
+.1323635453126044036486572714666e-25,
-.2270999412942928816702313813333e-26,
+.3896418998003991449320816639999e-27,
-.6685198115125953327792127999999e-28,
+.1146998663140024384347613866666e-28,
-.1967938586345134677295103999999e-29,
+.3376448816585338090334890666666e-30,
-.5793070335782135784625493333333e-31
};
int i, n;
double y;
double sinpiy, value;
#ifdef NOMORE_FOR_THREADS
static int ngam = 0;
static double xmin = 0, xmax = 0., xsml = 0., dxrel = 0.;
/* Initialize machine dependent constants, the first time gamma() is called.
FIXME for threads ! */
if (ngam == 0) {
ngam = chebyshev_init(gamcs, 42, DBL_EPSILON/20);/*was .1*d1mach(3)*/
gammalims(&xmin, &xmax);/*-> ./gammalims.c */
xsml = exp(fmax2(log(DBL_MIN), -log(DBL_MAX)) + 0.01);
/* = exp(.01)*DBL_MIN = 2.247e-308 for IEEE */
dxrel = sqrt(DBL_EPSILON);/*was sqrt(d1mach(4)) */
}
#else
/* For IEEE double precision DBL_EPSILON = 2^-52 = 2.220446049250313e-16 :
* (xmin, xmax) are non-trivial, see ./gammalims.c
* xsml = exp(.01)*DBL_MIN
* dxrel = sqrt(DBL_EPSILON) = 2 ^ -26
*/
# define ngam 22
# define xmin -170.5674972726612
# define xmax 171.61447887182298
# define xsml 2.2474362225598545e-308
# define dxrel 1.490116119384765696e-8
#endif
if(ISNAN(x)) return x;
/* If the argument is exactly zero or a negative integer
* then return NaN. */
if (x == 0 || (x < 0 && x == (long)x)) {
ML_ERROR(ME_DOMAIN, "gammafn");
return ML_NAN;
}
y = fabs(x);
if (y <= 10) {
/* Compute gamma(x) for -10 <= x <= 10
* Reduce the interval and find gamma(1 + y) for 0 <= y < 1
* first of all. */
n = (int) x;
if(x < 0) --n;
y = x - n;/* n = floor(x) ==> y in [ 0, 1 ) */
--n;
value = chebyshev_eval(y * 2 - 1, gamcs, ngam) + .9375;
if (n == 0)
return value;/* x = 1.dddd = 1+y */
if (n < 0) {
/* compute gamma(x) for -10 <= x < 1 */
/* exact 0 or "-n" checked already above */
/* The answer is less than half precision */
/* because x too near a negative integer. */
if (x < -0.5 && fabs(x - (int)(x - 0.5) / x) < dxrel) {
ML_ERROR(ME_PRECISION, "gammafn");
}
/* The argument is so close to 0 that the result would overflow. */
if (y < xsml) {
ML_ERROR(ME_RANGE, "gammafn");
if(x > 0) return ML_POSINF;
else return ML_NEGINF;
}
n = -n;
for (i = 0; i < n; i++) {
value /= (x + i);
}
return value;
}
else {
/* gamma(x) for 2 <= x <= 10 */
for (i = 1; i <= n; i++) {
value *= (y + i);
}
return value;
}
}
else {
/* gamma(x) for y = |x| > 10. */
if (x > xmax) { /* Overflow */
ML_ERROR(ME_RANGE, "gammafn");
return ML_POSINF;
}
if (x < xmin) { /* Underflow */
ML_ERROR(ME_UNDERFLOW, "gammafn");
return 0.;
}
if(y <= 50 && y == (int)y) { /* compute (n - 1)! */
value = 1.;
for (i = 2; i < y; i++) value *= i;
}
else { /* normal case */
value = exp((y - 0.5) * log(y) - y + M_LN_SQRT_2PI +
((2*y == (int)2*y)? stirlerr(y) : lgammacor(y)));
}
if (x > 0)
return value;
if (fabs((x - (int)(x - 0.5))/x) < dxrel){
/* The answer is less than half precision because */
/* the argument is too near a negative integer. */
ML_ERROR(ME_PRECISION, "gammafn");
}
sinpiy = sin(M_PI * y);
if (sinpiy == 0) { /* Negative integer arg - overflow */
ML_ERROR(ME_RANGE, "gammafn");
return ML_POSINF;
}
return -M_PI / (y * sinpiy * value);
}
}
