1 /* Implementation of gamma function according to ISO C.
2 Copyright (C) 1997-2018 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4 Contributed by Ulrich Drepper <drepper@cygnus.com>, 1997 and
5 Jakub Jelinek <jj@ultra.linux.cz, 1999.
7 The GNU C Library is free software; you can redistribute it and/or
8 modify it under the terms of the GNU Lesser General Public
9 License as published by the Free Software Foundation; either
10 version 2.1 of the License, or (at your option) any later version.
12 The GNU C Library is distributed in the hope that it will be useful,
13 but WITHOUT ANY WARRANTY; without even the implied warranty of
14 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
15 Lesser General Public License for more details.
17 You should have received a copy of the GNU Lesser General Public
18 License along with the GNU C Library; if not, see
19 <http://www.gnu.org/licenses/>. */
22 #include <math_private.h>
23 #include <fenv_private.h>
24 #include <math-underflow.h>
27 /* Coefficients B_2k / 2k(2k-1) of x^-(2k-1) inside exp in Stirling's
28 approximation to gamma function. */
30 static const long double gamma_coeff
[] =
32 0x1.555555555555555555555555558p
-4L,
33 -0xb.60b60b60b60b60b60b60b60b6p
-12L,
34 0x3.4034034034034034034034034p
-12L,
35 -0x2.7027027027027027027027027p
-12L,
36 0x3.72a3c5631fe46ae1d4e700dca9p
-12L,
37 -0x7.daac36664f1f207daac36664f2p
-12L,
38 0x1.a41a41a41a41a41a41a41a41a4p
-8L,
39 -0x7.90a1b2c3d4e5f708192a3b4c5ep
-8L,
40 0x2.dfd2c703c0cfff430edfd2c704p
-4L,
41 -0x1.6476701181f39edbdb9ce625988p
+0L,
42 0xd.672219167002d3a7a9c886459cp
+0L,
43 -0x9.cd9292e6660d55b3f712eb9e08p
+4L,
44 0x8.911a740da740da740da740da74p
+8L,
47 #define NCOEFF (sizeof (gamma_coeff) / sizeof (gamma_coeff[0]))
49 /* Return gamma (X), for positive X less than 191, in the form R *
50 2^(*EXP2_ADJ), where R is the return value and *EXP2_ADJ is set to
51 avoid overflow or underflow in intermediate calculations. */
54 gammal_positive (long double x
, int *exp2_adj
)
60 return __ieee754_expl (__ieee754_lgammal_r (x
+ 1, &local_signgam
)) / x
;
65 return __ieee754_expl (__ieee754_lgammal_r (x
, &local_signgam
));
69 /* Adjust into the range for using exp (lgamma). */
71 long double n
= __ceill (x
- 1.5L);
72 long double x_adj
= x
- n
;
74 long double prod
= __gamma_productl (x_adj
, 0, n
, &eps
);
75 return (__ieee754_expl (__ieee754_lgammal_r (x_adj
, &local_signgam
))
76 * prod
* (1.0L + eps
));
81 long double x_eps
= 0;
82 long double x_adj
= x
;
86 /* Adjust into the range for applying Stirling's
88 long double n
= __ceill (23.0L - x
);
90 x_eps
= (x
- (x_adj
- n
));
91 prod
= __gamma_productl (x_adj
- n
, x_eps
, n
, &eps
);
93 /* The result is now gamma (X_ADJ + X_EPS) / (PROD * (1 + EPS)).
94 Compute gamma (X_ADJ + X_EPS) using Stirling's approximation,
95 starting by computing pow (X_ADJ, X_ADJ) with a power of 2
97 long double exp_adj
= -eps
;
98 long double x_adj_int
= __roundl (x_adj
);
99 long double x_adj_frac
= x_adj
- x_adj_int
;
101 long double x_adj_mant
= __frexpl (x_adj
, &x_adj_log2
);
102 if (x_adj_mant
< M_SQRT1_2l
)
107 *exp2_adj
= x_adj_log2
* (int) x_adj_int
;
108 long double ret
= (__ieee754_powl (x_adj_mant
, x_adj
)
109 * __ieee754_exp2l (x_adj_log2
* x_adj_frac
)
110 * __ieee754_expl (-x_adj
)
111 * sqrtl (2 * M_PIl
/ x_adj
)
113 exp_adj
+= x_eps
* __ieee754_logl (x_adj
);
114 long double bsum
= gamma_coeff
[NCOEFF
- 1];
115 long double x_adj2
= x_adj
* x_adj
;
116 for (size_t i
= 1; i
<= NCOEFF
- 1; i
++)
117 bsum
= bsum
/ x_adj2
+ gamma_coeff
[NCOEFF
- 1 - i
];
118 exp_adj
+= bsum
/ x_adj
;
119 return ret
+ ret
* __expm1l (exp_adj
);
124 __ieee754_gammal_r (long double x
, int *signgamp
)
131 EXTRACT_WORDS64 (hx
, xhi
);
133 if ((hx
& 0x7fffffffffffffffLL
) == 0)
135 /* Return value for x == 0 is Inf with divide by zero exception. */
139 if (hx
< 0 && (uint64_t) hx
< 0xfff0000000000000ULL
&& __rintl (x
) == x
)
141 /* Return value for integer x < 0 is NaN with invalid exception. */
143 return (x
- x
) / (x
- x
);
145 if (hx
== 0xfff0000000000000ULL
)
147 /* x == -Inf. According to ISO this is NaN. */
151 if ((hx
& 0x7ff0000000000000ULL
) == 0x7ff0000000000000ULL
)
153 /* Positive infinity (return positive infinity) or NaN (return
163 return LDBL_MAX
* LDBL_MAX
;
167 SET_RESTORE_ROUNDL (FE_TONEAREST
);
172 ret
= gammal_positive (x
, &exp2_adj
);
173 ret
= __scalbnl (ret
, exp2_adj
);
175 else if (x
>= -0x1p
-110L)
182 long double tx
= __truncl (x
);
183 *signgamp
= (tx
== 2.0L * __truncl (tx
/ 2.0L)) ? -1 : 1;
186 ret
= LDBL_MIN
* LDBL_MIN
;
189 long double frac
= tx
- x
;
192 long double sinpix
= (frac
<= 0.25L
193 ? __sinl (M_PIl
* frac
)
194 : __cosl (M_PIl
* (0.5L - frac
)));
196 ret
= M_PIl
/ (-x
* sinpix
197 * gammal_positive (-x
, &exp2_adj
));
198 ret
= __scalbnl (ret
, -exp2_adj
);
199 math_check_force_underflow_nonneg (ret
);
203 if (isinf (ret
) && x
!= 0)
206 return -(-__copysignl (LDBL_MAX
, ret
) * LDBL_MAX
);
208 return __copysignl (LDBL_MAX
, ret
) * LDBL_MAX
;
213 return -(-__copysignl (LDBL_MIN
, ret
) * LDBL_MIN
);
215 return __copysignl (LDBL_MIN
, ret
) * LDBL_MIN
;
220 strong_alias (__ieee754_gammal_r
, __gammal_r_finite
)