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1 /* Implementation of gamma function according to ISO C.
2 Copyright (C) 1997-2017 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4 Contributed by Ulrich Drepper <drepper@cygnus.com>, 1997.
6 The GNU C Library is free software; you can redistribute it and/or
7 modify it under the terms of the GNU Lesser General Public
8 License as published by the Free Software Foundation; either
9 version 2.1 of the License, or (at your option) any later version.
11 The GNU C Library is distributed in the hope that it will be useful,
12 but WITHOUT ANY WARRANTY; without even the implied warranty of
13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
14 Lesser General Public License for more details.
16 You should have received a copy of the GNU Lesser General Public
17 License along with the GNU C Library; if not, see
18 <http://www.gnu.org/licenses/>. */
21 #include <math_private.h>
24 /* Coefficients B_2k / 2k(2k-1) of x^-(2k-1) inside exp in Stirling's
25 approximation to gamma function. */
27 static const double gamma_coeff
[] =
30 -0xb.60b60b60b60b8p
-12,
31 0x3.4034034034034p
-12,
32 -0x2.7027027027028p
-12,
33 0x3.72a3c5631fe46p
-12,
34 -0x7.daac36664f1f4p
-12,
37 #define NCOEFF (sizeof (gamma_coeff) / sizeof (gamma_coeff[0]))
39 /* Return gamma (X), for positive X less than 184, in the form R *
40 2^(*EXP2_ADJ), where R is the return value and *EXP2_ADJ is set to
41 avoid overflow or underflow in intermediate calculations. */
44 gamma_positive (double x
, int *exp2_adj
)
50 return __ieee754_exp (__ieee754_lgamma_r (x
+ 1, &local_signgam
)) / x
;
55 return __ieee754_exp (__ieee754_lgamma_r (x
, &local_signgam
));
59 /* Adjust into the range for using exp (lgamma). */
61 double n
= __ceil (x
- 1.5);
64 double prod
= __gamma_product (x_adj
, 0, n
, &eps
);
65 return (__ieee754_exp (__ieee754_lgamma_r (x_adj
, &local_signgam
))
66 * prod
* (1.0 + eps
));
76 /* Adjust into the range for applying Stirling's
78 double n
= __ceil (12.0 - x
);
79 x_adj
= math_narrow_eval (x
+ n
);
80 x_eps
= (x
- (x_adj
- n
));
81 prod
= __gamma_product (x_adj
- n
, x_eps
, n
, &eps
);
83 /* The result is now gamma (X_ADJ + X_EPS) / (PROD * (1 + EPS)).
84 Compute gamma (X_ADJ + X_EPS) using Stirling's approximation,
85 starting by computing pow (X_ADJ, X_ADJ) with a power of 2
87 double exp_adj
= -eps
;
88 double x_adj_int
= __round (x_adj
);
89 double x_adj_frac
= x_adj
- x_adj_int
;
91 double x_adj_mant
= __frexp (x_adj
, &x_adj_log2
);
92 if (x_adj_mant
< M_SQRT1_2
)
97 *exp2_adj
= x_adj_log2
* (int) x_adj_int
;
98 double ret
= (__ieee754_pow (x_adj_mant
, x_adj
)
99 * __ieee754_exp2 (x_adj_log2
* x_adj_frac
)
100 * __ieee754_exp (-x_adj
)
101 * __ieee754_sqrt (2 * M_PI
/ x_adj
)
103 exp_adj
+= x_eps
* __ieee754_log (x_adj
);
104 double bsum
= gamma_coeff
[NCOEFF
- 1];
105 double x_adj2
= x_adj
* x_adj
;
106 for (size_t i
= 1; i
<= NCOEFF
- 1; i
++)
107 bsum
= bsum
/ x_adj2
+ gamma_coeff
[NCOEFF
- 1 - i
];
108 exp_adj
+= bsum
/ x_adj
;
109 return ret
+ ret
* __expm1 (exp_adj
);
114 __ieee754_gamma_r (double x
, int *signgamp
)
120 EXTRACT_WORDS (hx
, lx
, x
);
122 if (__glibc_unlikely (((hx
& 0x7fffffff) | lx
) == 0))
124 /* Return value for x == 0 is Inf with divide by zero exception. */
128 if (__builtin_expect (hx
< 0, 0)
129 && (u_int32_t
) hx
< 0xfff00000 && __rint (x
) == x
)
131 /* Return value for integer x < 0 is NaN with invalid exception. */
133 return (x
- x
) / (x
- x
);
135 if (__glibc_unlikely ((unsigned int) hx
== 0xfff00000 && lx
== 0))
137 /* x == -Inf. According to ISO this is NaN. */
141 if (__glibc_unlikely ((hx
& 0x7ff00000) == 0x7ff00000))
143 /* Positive infinity (return positive infinity) or NaN (return
153 ret
= math_narrow_eval (DBL_MAX
* DBL_MAX
);
158 SET_RESTORE_ROUND (FE_TONEAREST
);
163 double tret
= gamma_positive (x
, &exp2_adj
);
164 ret
= __scalbn (tret
, exp2_adj
);
166 else if (x
>= -DBL_EPSILON
/ 4.0)
173 double tx
= __trunc (x
);
174 *signgamp
= (tx
== 2.0 * __trunc (tx
/ 2.0)) ? -1 : 1;
177 ret
= DBL_MIN
* DBL_MIN
;
180 double frac
= tx
- x
;
183 double sinpix
= (frac
<= 0.25
184 ? __sin (M_PI
* frac
)
185 : __cos (M_PI
* (0.5 - frac
)));
187 double tret
= M_PI
/ (-x
* sinpix
188 * gamma_positive (-x
, &exp2_adj
));
189 ret
= __scalbn (tret
, -exp2_adj
);
190 math_check_force_underflow_nonneg (ret
);
193 ret
= math_narrow_eval (ret
);
195 if (isinf (ret
) && x
!= 0)
199 ret
= math_narrow_eval (-__copysign (DBL_MAX
, ret
) * DBL_MAX
);
203 ret
= math_narrow_eval (__copysign (DBL_MAX
, ret
) * DBL_MAX
);
210 ret
= math_narrow_eval (-__copysign (DBL_MIN
, ret
) * DBL_MIN
);
214 ret
= math_narrow_eval (__copysign (DBL_MIN
, ret
) * DBL_MIN
);
220 strong_alias (__ieee754_gamma_r
, __gamma_r_finite
)