/* origin: OpenBSD /usr/src/lib/libm/src/ld80/e_tgammal.c */
/*
* Copyright (c) 2008 Stephen L. Moshier <steve@moshier.net>
*
* Permission to use, copy, modify, and distribute this software for any
* purpose with or without fee is hereby granted, provided that the above
* copyright notice and this permission notice appear in all copies.
*
* THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
* WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
* MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
* ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
* WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
* ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
* OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
*/
/*
* Gamma function
*
*
* SYNOPSIS:
*
* long double x, y, tgammal();
* extern int signgam;
*
* y = tgammal( x );
*
*
* DESCRIPTION:
*
* Returns gamma function of the argument. The result is
* correctly signed, and the sign (+1 or -1) is also
* returned in a global (extern) variable named signgam.
* This variable is also filled in by the logarithmic gamma
* function lgamma().
*
* Arguments |x| <= 13 are reduced by recurrence and the function
* approximated by a rational function of degree 7/8 in the
* interval (2,3). Large arguments are handled by Stirling's
* formula. Large negative arguments are made positive using
* a reflection formula.
*
*
* ACCURACY:
*
* Relative error:
* arithmetic domain # trials peak rms
* IEEE -40,+40 10000 3.6e-19 7.9e-20
* IEEE -1755,+1755 10000 4.8e-18 6.5e-19
*
* Accuracy for large arguments is dominated by error in powl().
*
*/
#include "libm.h"
#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
long double tgammal(long double x)
{
return tgamma(x);
}
#elif LDBL_MANT_DIG == 64 && LDBL_MAX_EXP == 16384
/*
tgamma(x+2) = tgamma(x+2) P(x)/Q(x)
0 <= x <= 1
Relative error
n=7, d=8
Peak error = 1.83e-20
Relative error spread = 8.4e-23
*/
static const long double P[8] = {
4.212760487471622013093E-5L,
4.542931960608009155600E-4L,
4.092666828394035500949E-3L,
2.385363243461108252554E-2L,
1.113062816019361559013E-1L,
3.629515436640239168939E-1L,
8.378004301573126728826E-1L,
1.000000000000000000009E0L,
};
static const long double Q[9] = {
-1.397148517476170440917E-5L,
2.346584059160635244282E-4L,
-1.237799246653152231188E-3L,
-7.955933682494738320586E-4L,
2.773706565840072979165E-2L,
-4.633887671244534213831E-2L,
-2.243510905670329164562E-1L,
4.150160950588455434583E-1L,
9.999999999999999999908E-1L,
};
/*
static const long double P[] = {
-3.01525602666895735709e0L,
-3.25157411956062339893e1L,
-2.92929976820724030353e2L,
-1.70730828800510297666e3L,
-7.96667499622741999770e3L,
-2.59780216007146401957e4L,
-5.99650230220855581642e4L,
-7.15743521530849602425e4L
};
static const long double Q[] = {
1.00000000000000000000e0L,
-1.67955233807178858919e1L,
8.85946791747759881659e1L,
5.69440799097468430177e1L,
-1.98526250512761318471e3L,
3.31667508019495079814e3L,
1.60577839621734713377e4L,
-2.97045081369399940529e4L,
-7.15743521530849602412e4L
};
*/
#define MAXGAML 1755.455L
/*static const long double LOGPI = 1.14472988584940017414L;*/
/* Stirling's formula for the gamma function
tgamma(x) = sqrt(2 pi) x^(x-.5) exp(-x) (1 + 1/x P(1/x))
z(x) = x
13 <= x <= 1024
Relative error
n=8, d=0
Peak error = 9.44e-21
Relative error spread = 8.8e-4
*/
static const long double STIR[9] = {
7.147391378143610789273E-4L,
-2.363848809501759061727E-5L,
-5.950237554056330156018E-4L,
6.989332260623193171870E-5L,
7.840334842744753003862E-4L,
-2.294719747873185405699E-4L,
-2.681327161876304418288E-3L,
3.472222222230075327854E-3L,
8.333333333333331800504E-2L,
};
#define MAXSTIR 1024.0L
static const long double SQTPI = 2.50662827463100050242E0L;
/* 1/tgamma(x) = z P(z)
* z(x) = 1/x
* 0 < x < 0.03125
* Peak relative error 4.2e-23
*/
static const long double S[9] = {
-1.193945051381510095614E-3L,
7.220599478036909672331E-3L,
-9.622023360406271645744E-3L,
-4.219773360705915470089E-2L,
1.665386113720805206758E-1L,
-4.200263503403344054473E-2L,
-6.558780715202540684668E-1L,
5.772156649015328608253E-1L,
1.000000000000000000000E0L,
};
/* 1/tgamma(-x) = z P(z)
* z(x) = 1/x
* 0 < x < 0.03125
* Peak relative error 5.16e-23
* Relative error spread = 2.5e-24
*/
static const long double SN[9] = {
1.133374167243894382010E-3L,
7.220837261893170325704E-3L,
9.621911155035976733706E-3L,
-4.219773343731191721664E-2L,
-1.665386113944413519335E-1L,
-4.200263503402112910504E-2L,
6.558780715202536547116E-1L,
5.772156649015328608727E-1L,
-1.000000000000000000000E0L,
};
static const long double PIL = 3.1415926535897932384626L;
/* Gamma function computed by Stirling's formula.
*/
static long double stirf(long double x)
{
long double y, w, v;
w = 1.0/x;
/* For large x, use rational coefficients from the analytical expansion. */
if (x > 1024.0)
w = (((((6.97281375836585777429E-5L * w
+ 7.84039221720066627474E-4L) * w
- 2.29472093621399176955E-4L) * w
- 2.68132716049382716049E-3L) * w
+ 3.47222222222222222222E-3L) * w
+ 8.33333333333333333333E-2L) * w
+ 1.0;
else
w = 1.0 + w * __polevll(w, STIR, 8);
y = expl(x);
if (x > MAXSTIR) { /* Avoid overflow in pow() */
v = powl(x, 0.5L * x - 0.25L);
y = v * (v / y);
} else {
y = powl(x, x - 0.5L) / y;
}
y = SQTPI * y * w;
return y;
}
long double tgammal(long double x)
{
long double p, q, z;
int i;
signgam = 1;
if (isnan(x))
return NAN;
if (x == INFINITY)
return INFINITY;
if (x == -INFINITY)
return x - x;
q = fabsl(x);
if (q > 13.0) {
if (q > MAXGAML)
goto goverf;
if (x < 0.0) {
p = floorl(q);
if (p == q)
return (x - x) / (x - x);
i = p;
if ((i & 1) == 0)
signgam = -1;
z = q - p;
if (z > 0.5L) {
p += 1.0;
z = q - p;
}
z = q * sinl(PIL * z);
z = fabsl(z) * stirf(q);
if (z <= PIL/LDBL_MAX) {
goverf:
return signgam * INFINITY;
}
z = PIL/z;
} else {
z = stirf(x);
}
return signgam * z;
}
z = 1.0;
while (x >= 3.0) {
x -= 1.0;
z *= x;
}
while (x < -0.03125L) {
z /= x;
x += 1.0;
}
if (x <= 0.03125L)
goto small;
while (x < 2.0) {
z /= x;
x += 1.0;
}
if (x == 2.0)
return z;
x -= 2.0;
p = __polevll(x, P, 7);
q = __polevll(x, Q, 8);
z = z * p / q;
if(z < 0)
signgam = -1;
return z;
small:
if (x == 0.0)
return (x - x) / (x - x);
if (x < 0.0) {
x = -x;
q = z / (x * __polevll(x, SN, 8));
signgam = -1;
} else
q = z / (x * __polevll(x, S, 8));
return q;
}
#endif
