1/* lgammal expanding around zeros.
2 Copyright (C) 2015-2022 Free Software Foundation, Inc.
3 This file is part of the GNU C Library.
4
5 The GNU C Library is free software; you can redistribute it and/or
6 modify it under the terms of the GNU Lesser General Public
7 License as published by the Free Software Foundation; either
8 version 2.1 of the License, or (at your option) any later version.
9
10 The GNU C Library is distributed in the hope that it will be useful,
11 but WITHOUT ANY WARRANTY; without even the implied warranty of
12 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
13 Lesser General Public License for more details.
14
15 You should have received a copy of the GNU Lesser General Public
16 License along with the GNU C Library; if not, see
17 <https://www.gnu.org/licenses/>. */
18
19#include <float.h>
20#include <math.h>
21#include <math_private.h>
22#include <fenv_private.h>
23
24static const _Float128 lgamma_zeros[][2] =
25 {
26 { L(-0x2.74ff92c01f0d82abec9f315f1a08p+0), L(0xe.d3ccb7fb2658634a2b9f6b2ba81p-116) },
27 { L(-0x2.bf6821437b20197995a4b4641eaep+0), L(-0xb.f4b00b4829f961e428533e6ad048p-116) },
28 { L(-0x3.24c1b793cb35efb8be699ad3d9bap+0), L(-0x6.5454cb7fac60e3f16d9d7840c2ep-116) },
29 { L(-0x3.f48e2a8f85fca170d4561291236cp+0), L(-0xc.320a4887d1cb4c711828a75d5758p-116) },
30 { L(-0x4.0a139e16656030c39f0b0de18114p+0), L(0x1.53e84029416e1242006b2b3d1cfp-112) },
31 { L(-0x4.fdd5de9bbabf3510d0aa40769884p+0), L(-0x1.01d7d78125286f78d1e501f14966p-112) },
32 { L(-0x5.021a95fc2db6432a4c56e595394cp+0), L(-0x1.ecc6af0430d4fe5746fa7233356fp-112) },
33 { L(-0x5.ffa4bd647d0357dd4ed62cbd31ecp+0), L(-0x1.f8e3f8e5deba2d67dbd70dd96ce1p-112) },
34 { L(-0x6.005ac9625f233b607c2d96d16384p+0), L(-0x1.cb86ac569340cf1e5f24df7aab7bp-112) },
35 { L(-0x6.fff2fddae1bbff3d626b65c23fd4p+0), L(0x1.e0bfcff5c457ebcf4d3ad9674167p-112) },
36 { L(-0x7.000cff7b7f87adf4482dcdb98784p+0), L(0x1.54d99e35a74d6407b80292df199fp-112) },
37 { L(-0x7.fffe5fe05673c3ca9e82b522b0ccp+0), L(0x1.62d177c832e0eb42c2faffd1b145p-112) },
38 { L(-0x8.0001a01459fc9f60cb3cec1cec88p+0), L(0x2.8998835ac7277f7bcef67c47f188p-112) },
39 { L(-0x8.ffffd1c425e80ffc864e95749258p+0), L(-0x1.e7e20210e7f81cf781b44e9d2b02p-112) },
40 { L(-0x9.00002e3bb47d86d6d843fedc352p+0), L(0x2.14852f613a16291751d2ab751f7ep-112) },
41 { L(-0x9.fffffb606bdfdcd062ae77a50548p+0), L(0x3.962d1490cc2e8f031c7007eaa1ap-116) },
42 { L(-0xa.0000049f93bb9927b45d95e1544p+0), L(-0x1.e03086db9146a9287bd4f2172d5ap-112) },
43 { L(-0xa.ffffff9466e9f1b36dacd2adbd18p+0), L(-0xd.05a4e458062f3f95345a4d9c9b6p-116) },
44 { L(-0xb.0000006b9915315d965a6ffea41p+0), L(0x1.b415c6fff233e7b7fdc3a094246fp-112) },
45 { L(-0xb.fffffff7089387387de41acc3d4p+0), L(0x3.687427c6373bd74a10306e10a28ep-112) },
46 { L(-0xc.00000008f76c7731567c0f0250fp+0), L(-0x3.87920df5675833859190eb128ef6p-112) },
47 { L(-0xc.ffffffff4f6dcf617f97a5ffc758p+0), L(0x2.ab72d76f32eaee2d1a42ed515d3ap-116) },
48 { L(-0xd.00000000b092309c06683dd1b9p+0), L(-0x3.e3700857a15c19ac5a611de9688ap-112) },
49 { L(-0xd.fffffffff36345ab9e184a3e09dp+0), L(-0x1.176dc48e47f62d917973dd44e553p-112) },
50 { L(-0xe.000000000c9cba545e94e75ec57p+0), L(-0x1.8f753e2501e757a17cf2ecbeeb89p-112) },
51 { L(-0xe.ffffffffff28c060c6604ef3037p+0), L(-0x1.f89d37357c9e3dc17c6c6e63becap-112) },
52 { L(-0xf.0000000000d73f9f399bd0e420f8p+0), L(-0x5.e9ee31b0b890744fc0e3fbc01048p-116) },
53 { L(-0xf.fffffffffff28c060c6621f512e8p+0), L(0xd.1b2eec9d960bd9adc5be5f5fa5p-116) },
54 { L(-0x1.000000000000d73f9f399da1424cp+4), L(0x6.c46e0e88305d2800f0e414c506a8p-116) },
55 { L(-0x1.0ffffffffffff3569c47e7a93e1cp+4), L(-0x4.6a08a2e008a998ebabb8087efa2cp-112) },
56 { L(-0x1.1000000000000ca963b818568887p+4), L(-0x6.ca5a3a64ec15db0a95caf2c9ffb4p-112) },
57 { L(-0x1.1fffffffffffff4bec3ce234132dp+4), L(-0x8.b2b726187c841cb92cd5221e444p-116) },
58 { L(-0x1.20000000000000b413c31dcbeca5p+4), L(0x3.c4d005344b6cd0e7231120294abcp-112) },
59 { L(-0x1.2ffffffffffffff685b25cbf5f54p+4), L(-0x5.ced932e38485f7dd296b8fa41448p-112) },
60 { L(-0x1.30000000000000097a4da340a0acp+4), L(0x7.e484e0e0ffe38d406ebebe112f88p-112) },
61 { L(-0x1.3fffffffffffffff86af516ff7f7p+4), L(-0x6.bd67e720d57854502b7db75e1718p-112) },
62 { L(-0x1.40000000000000007950ae900809p+4), L(0x6.bec33375cac025d9c073168c5d9p-112) },
63 { L(-0x1.4ffffffffffffffffa391c4248c3p+4), L(0x5.c63022b62b5484ba346524db607p-112) },
64 { L(-0x1.500000000000000005c6e3bdb73dp+4), L(-0x5.c62f55ed5322b2685c5e9a51e6a8p-112) },
65 { L(-0x1.5fffffffffffffffffbcc71a492p+4), L(-0x1.eb5aeb96c74d7ad25e060528fb5p-112) },
66 { L(-0x1.6000000000000000004338e5b6ep+4), L(0x1.eb5aec04b2f2eb663e4e3d8a018cp-112) },
67 { L(-0x1.6ffffffffffffffffffd13c97d9dp+4), L(-0x3.8fcc4d08d6fe5aa56ab04307ce7ep-112) },
68 { L(-0x1.70000000000000000002ec368263p+4), L(0x3.8fcc4d090cee2f5d0b69a99c353cp-112) },
69 { L(-0x1.7fffffffffffffffffffe0d30fe7p+4), L(0x7.2f577cca4b4c8cb1dc14001ac5ecp-112) },
70 { L(-0x1.800000000000000000001f2cf019p+4), L(-0x7.2f577cca4b3442e35f0040b3b9e8p-112) },
71 { L(-0x1.8ffffffffffffffffffffec0c332p+4), L(-0x2.e9a0572b1bb5b95f346a92d67a6p-112) },
72 { L(-0x1.90000000000000000000013f3ccep+4), L(0x2.e9a0572b1bb5c371ddb3561705ap-112) },
73 { L(-0x1.9ffffffffffffffffffffff3b8bdp+4), L(-0x1.cad8d32e386fd783e97296d63dcbp-116) },
74 { L(-0x1.a0000000000000000000000c4743p+4), L(0x1.cad8d32e386fd7c1ab8c1fe34c0ep-116) },
75 { L(-0x1.afffffffffffffffffffffff8b95p+4), L(-0x3.8f48cc5737d5979c39db806c5406p-112) },
76 { L(-0x1.b00000000000000000000000746bp+4), L(0x3.8f48cc5737d5979c3b3a6bda06f6p-112) },
77 { L(-0x1.bffffffffffffffffffffffffbd8p+4), L(0x6.2898d42174dcf171470d8c8c6028p-112) },
78 { L(-0x1.c000000000000000000000000428p+4), L(-0x6.2898d42174dcf171470d18ba412cp-112) },
79 { L(-0x1.cfffffffffffffffffffffffffdbp+4), L(-0x4.c0ce9794ea50a839e311320bde94p-112) },
80 { L(-0x1.d000000000000000000000000025p+4), L(0x4.c0ce9794ea50a839e311322f7cf8p-112) },
81 { L(-0x1.dfffffffffffffffffffffffffffp+4), L(0x3.932c5047d60e60caded4c298a174p-112) },
82 { L(-0x1.e000000000000000000000000001p+4), L(-0x3.932c5047d60e60caded4c298973ap-112) },
83 { L(-0x1.fp+4), L(0xa.1a6973c1fade2170f7237d35fe3p-116) },
84 { L(-0x1.fp+4), L(-0xa.1a6973c1fade2170f7237d35fe08p-116) },
85 { L(-0x2p+4), L(0x5.0d34b9e0fd6f10b87b91be9aff1p-120) },
86 { L(-0x2p+4), L(-0x5.0d34b9e0fd6f10b87b91be9aff0cp-120) },
87 { L(-0x2.1p+4), L(0x2.73024a9ba1aa36a7059bff52e844p-124) },
88 { L(-0x2.1p+4), L(-0x2.73024a9ba1aa36a7059bff52e844p-124) },
89 { L(-0x2.2p+4), L(0x1.2710231c0fd7a13f8a2b4af9d6b7p-128) },
90 { L(-0x2.2p+4), L(-0x1.2710231c0fd7a13f8a2b4af9d6b7p-128) },
91 { L(-0x2.3p+4), L(0x8.6e2ce38b6c8f9419e3fad3f0312p-136) },
92 { L(-0x2.3p+4), L(-0x8.6e2ce38b6c8f9419e3fad3f0312p-136) },
93 { L(-0x2.4p+4), L(0x3.bf30652185952560d71a254e4eb8p-140) },
94 { L(-0x2.4p+4), L(-0x3.bf30652185952560d71a254e4eb8p-140) },
95 { L(-0x2.5p+4), L(0x1.9ec8d1c94e85af4c78b15c3d89d3p-144) },
96 { L(-0x2.5p+4), L(-0x1.9ec8d1c94e85af4c78b15c3d89d3p-144) },
97 { L(-0x2.6p+4), L(0xa.ea565ce061d57489e9b85276274p-152) },
98 { L(-0x2.6p+4), L(-0xa.ea565ce061d57489e9b85276274p-152) },
99 { L(-0x2.7p+4), L(0x4.7a6512692eb37804111dabad30ecp-156) },
100 { L(-0x2.7p+4), L(-0x4.7a6512692eb37804111dabad30ecp-156) },
101 { L(-0x2.8p+4), L(0x1.ca8ed42a12ae3001a07244abad2bp-160) },
102 { L(-0x2.8p+4), L(-0x1.ca8ed42a12ae3001a07244abad2bp-160) },
103 { L(-0x2.9p+4), L(0xb.2f30e1ce812063f12e7e8d8d96e8p-168) },
104 { L(-0x2.9p+4), L(-0xb.2f30e1ce812063f12e7e8d8d96e8p-168) },
105 { L(-0x2.ap+4), L(0x4.42bd49d4c37a0db136489772e428p-172) },
106 { L(-0x2.ap+4), L(-0x4.42bd49d4c37a0db136489772e428p-172) },
107 { L(-0x2.bp+4), L(0x1.95db45257e5122dcbae56def372p-176) },
108 { L(-0x2.bp+4), L(-0x1.95db45257e5122dcbae56def372p-176) },
109 { L(-0x2.cp+4), L(0x9.3958d81ff63527ecf993f3fb6f48p-184) },
110 { L(-0x2.cp+4), L(-0x9.3958d81ff63527ecf993f3fb6f48p-184) },
111 { L(-0x2.dp+4), L(0x3.47970e4440c8f1c058bd238c9958p-188) },
112 { L(-0x2.dp+4), L(-0x3.47970e4440c8f1c058bd238c9958p-188) },
113 { L(-0x2.ep+4), L(0x1.240804f65951062ca46e4f25c608p-192) },
114 { L(-0x2.ep+4), L(-0x1.240804f65951062ca46e4f25c608p-192) },
115 { L(-0x2.fp+4), L(0x6.36a382849fae6de2d15362d8a394p-200) },
116 { L(-0x2.fp+4), L(-0x6.36a382849fae6de2d15362d8a394p-200) },
117 { L(-0x3p+4), L(0x2.123680d6dfe4cf4b9b1bcb9d8bdcp-204) },
118 { L(-0x3p+4), L(-0x2.123680d6dfe4cf4b9b1bcb9d8bdcp-204) },
119 { L(-0x3.1p+4), L(0xa.d21786ff5842eca51fea0870919p-212) },
120 { L(-0x3.1p+4), L(-0xa.d21786ff5842eca51fea0870919p-212) },
121 { L(-0x3.2p+4), L(0x3.766dedc259af040be140a68a6c04p-216) },
122 };
123
124static const _Float128 e_hi = L(0x2.b7e151628aed2a6abf7158809cf4p+0);
125static const _Float128 e_lo = L(0xf.3c762e7160f38b4da56a784d9048p-116);
126
127
128/* Coefficients B_2k / 2k(2k-1) of x^-(2k-1) in Stirling's
129 approximation to lgamma function. */
130
131static const _Float128 lgamma_coeff[] =
132 {
133 L(0x1.5555555555555555555555555555p-4),
134 L(-0xb.60b60b60b60b60b60b60b60b60b8p-12),
135 L(0x3.4034034034034034034034034034p-12),
136 L(-0x2.7027027027027027027027027028p-12),
137 L(0x3.72a3c5631fe46ae1d4e700dca8f2p-12),
138 L(-0x7.daac36664f1f207daac36664f1f4p-12),
139 L(0x1.a41a41a41a41a41a41a41a41a41ap-8),
140 L(-0x7.90a1b2c3d4e5f708192a3b4c5d7p-8),
141 L(0x2.dfd2c703c0cfff430edfd2c703cp-4),
142 L(-0x1.6476701181f39edbdb9ce625987dp+0),
143 L(0xd.672219167002d3a7a9c886459cp+0),
144 L(-0x9.cd9292e6660d55b3f712eb9e07c8p+4),
145 L(0x8.911a740da740da740da740da741p+8),
146 L(-0x8.d0cc570e255bf59ff6eec24b49p+12),
147 L(0xa.8d1044d3708d1c219ee4fdc446ap+16),
148 L(-0xe.8844d8a169abbc406169abbc406p+20),
149 L(0x1.6d29a0f6433b79890cede62433b8p+28),
150 L(-0x2.88a233b3c8cddaba9809357125d8p+32),
151 L(0x5.0dde6f27500939a85c40939a85c4p+36),
152 L(-0xb.4005bde03d4642a243581714af68p+40),
153 L(0x1.bc8cd6f8f1f755c78753cdb5d5c9p+48),
154 L(-0x4.bbebb143bb94de5a0284fa7ec424p+52),
155 L(0xe.2e1337f5af0bed90b6b0a352d4fp+56),
156 L(-0x2.e78250162b62405ad3e4bfe61b38p+64),
157 L(0xa.5f7eef9e71ac7c80326ab4cc8bfp+68),
158 L(-0x2.83be0395e550213369924971b21ap+76),
159 L(0xa.8ebfe48da17dd999790760b0cep+80),
160 };
161
162#define NCOEFF (sizeof (lgamma_coeff) / sizeof (lgamma_coeff[0]))
163
164/* Polynomial approximations to (|gamma(x)|-1)(x-n)/(x-x0), where n is
165 the integer end-point of the half-integer interval containing x and
166 x0 is the zero of lgamma in that half-integer interval. Each
167 polynomial is expressed in terms of x-xm, where xm is the midpoint
168 of the interval for which the polynomial applies. */
169
170static const _Float128 poly_coeff[] =
171 {
172 /* Interval [-2.125, -2] (polynomial degree 23). */
173 L(-0x1.0b71c5c54d42eb6c17f30b7aa8f5p+0),
174 L(-0xc.73a1dc05f34951602554c6d7506p-4),
175 L(-0x1.ec841408528b51473e6c425ee5ffp-4),
176 L(-0xe.37c9da26fc3c9a3c1844c8c7f1cp-4),
177 L(-0x1.03cd87c519305703b021fa33f827p-4),
178 L(-0xe.ae9ada65e09aa7f1c75216128f58p-4),
179 L(0x9.b11855a4864b5731cf85736015a8p-8),
180 L(-0xe.f28c133e697a95c28607c9701dep-4),
181 L(0x2.6ec14a1c586a72a7cc33ee569d6ap-4),
182 L(-0xf.57cab973e14464a262fc24723c38p-4),
183 L(0x4.5b0fc25f16e52997b2886bbae808p-4),
184 L(-0xf.f50e59f1a9b56e76e988dac9ccf8p-4),
185 L(0x6.5f5eae15e9a93369e1d85146c6fcp-4),
186 L(-0x1.0d2422daac459e33e0994325ed23p+0),
187 L(0x8.82000a0e7401fb1117a0e6606928p-4),
188 L(-0x1.1f492f178a3f1b19f58a2ca68e55p+0),
189 L(0xa.cb545f949899a04c160b19389abp-4),
190 L(-0x1.36165a1b155ba3db3d1b77caf498p+0),
191 L(0xd.44c5d5576f74302e5cf79e183eep-4),
192 L(-0x1.51f22e0cdd33d3d481e326c02f3ep+0),
193 L(0xf.f73a349c08244ac389c007779bfp-4),
194 L(-0x1.73317bf626156ba716747c4ca866p+0),
195 L(0x1.379c3c97b9bc71e1c1c4802dd657p+0),
196 L(-0x1.a72a351c54f902d483052000f5dfp+0),
197 /* Interval [-2.25, -2.125] (polynomial degree 24). */
198 L(-0xf.2930890d7d675a80c36afb0fd5e8p-4),
199 L(-0xc.a5cfde054eab5c6770daeca577f8p-4),
200 L(0x3.9c9e0fdebb07cdf89c61d41c9238p-4),
201 L(-0x1.02a5ad35605fcf4af65a6dbacb84p+0),
202 L(0x9.6e9b1185bb48be9de1918e00a2e8p-4),
203 L(-0x1.4d8332f3cfbfa116fd611e9ce90dp+0),
204 L(0x1.1c0c8cb4d9f4b1d490e1a41fae4dp+0),
205 L(-0x1.c9a6f5ae9130cd0299e293a42714p+0),
206 L(0x1.d7e9307fd58a2ea997f29573a112p+0),
207 L(-0x2.921cb3473d96178ca2a11d2a8d46p+0),
208 L(0x2.e8d59113b6f3409ff8db226e9988p+0),
209 L(-0x3.cbab931625a1ae2b26756817f264p+0),
210 L(0x4.7d9f0f05d5296d18663ca003912p+0),
211 L(-0x5.ade9cba12a14ea485667b7135bbp+0),
212 L(0x6.dc983a5da74fb48e767b7fec0a3p+0),
213 L(-0x8.8d9ed454ae31d9e138dd8ee0d1a8p+0),
214 L(0xa.6fa099d4e7c202e0c0fd6ed8492p+0),
215 L(-0xc.ebc552a8090a0f0115e92d4ebbc8p+0),
216 L(0xf.d695e4772c0d829b53fba9ca5568p+0),
217 L(-0x1.38c32ae38e5e9eb79b2a4c5570a9p+4),
218 L(0x1.8035145646cfab49306d0999a51bp+4),
219 L(-0x1.d930adbb03dd342a4c2a8c4e1af6p+4),
220 L(0x2.45c2edb1b4943ddb3686cd9c6524p+4),
221 L(-0x2.e818ebbfafe2f916fa21abf7756p+4),
222 L(0x3.9804ce51d0fb9a430a711fd7307p+4),
223 /* Interval [-2.375, -2.25] (polynomial degree 25). */
224 L(-0xd.7d28d505d6181218a25f31d5e45p-4),
225 L(-0xe.69649a3040985140cdf946829fap-4),
226 L(0xb.0d74a2827d053a8d44595012484p-4),
227 L(-0x1.924b0922853617cac181afbc08ddp+0),
228 L(0x1.d49b12bccf0a568582e2d3c410f3p+0),
229 L(-0x3.0898bb7d8c4093e636279c791244p+0),
230 L(0x4.207a6cac711cb53868e8a5057eep+0),
231 L(-0x6.39ee63ea4fb1dcab0c9144bf3ddcp+0),
232 L(0x8.e2e2556a797b649bf3f53bd26718p+0),
233 L(-0xd.0e83ac82552ef12af508589e7a8p+0),
234 L(0x1.2e4525e0ce6670563c6484a82b05p+4),
235 L(-0x1.b8e350d6a8f2b222fa390a57c23dp+4),
236 L(0x2.805cd69b919087d8a80295892c2cp+4),
237 L(-0x3.a42585424a1b7e64c71743ab014p+4),
238 L(0x5.4b4f409f98de49f7bfb03c05f984p+4),
239 L(-0x7.b3c5827fbe934bc820d6832fb9fcp+4),
240 L(0xb.33b7b90cc96c425526e0d0866e7p+4),
241 L(-0x1.04b77047ac4f59ee3775ca10df0dp+8),
242 L(0x1.7b366f5e94a34f41386eac086313p+8),
243 L(-0x2.2797338429385c9849ca6355bfc2p+8),
244 L(0x3.225273cf92a27c9aac1b35511256p+8),
245 L(-0x4.8f078aa48afe6cb3a4e89690f898p+8),
246 L(0x6.9f311d7b6654fc1d0b5195141d04p+8),
247 L(-0x9.a0c297b6b4621619ca9bacc48ed8p+8),
248 L(0xe.ce1f06b6f90d92138232a76e4cap+8),
249 L(-0x1.5b0e6806fa064daf011613e43b17p+12),
250 /* Interval [-2.5, -2.375] (polynomial degree 27). */
251 L(-0xb.74ea1bcfff94b2c01afba9daa7d8p-4),
252 L(-0x1.2a82bd590c37538cab143308de4dp+0),
253 L(0x1.88020f828b966fec66b8649fd6fcp+0),
254 L(-0x3.32279f040eb694970e9db24863dcp+0),
255 L(0x5.57ac82517767e68a721005853864p+0),
256 L(-0x9.c2aedcfe22833de43834a0a6cc4p+0),
257 L(0x1.12c132f1f5577f99e1a0ed3538e1p+4),
258 L(-0x1.ea94e26628a3de3597f7bb55a948p+4),
259 L(0x3.66b4ac4fa582f58b59f96b2f7c7p+4),
260 L(-0x6.0cf746a9cf4cba8c39afcc73fc84p+4),
261 L(0xa.c102ef2c20d75a342197df7fedf8p+4),
262 L(-0x1.31ebff06e8f14626782df58db3b6p+8),
263 L(0x2.1fd6f0c0e710994e059b9dbdb1fep+8),
264 L(-0x3.c6d76040407f447f8b5074f07706p+8),
265 L(0x6.b6d18e0d8feb4c2ef5af6a40ed18p+8),
266 L(-0xb.efaf542c529f91e34217f24ae6a8p+8),
267 L(0x1.53852d873210e7070f5d9eb2296p+12),
268 L(-0x2.5b977c0ddc6d540717173ac29fc8p+12),
269 L(0x4.310d452ae05100eff1e02343a724p+12),
270 L(-0x7.73a5d8f20c4f986a7dd1912b2968p+12),
271 L(0xd.3f5ea2484f3fca15eab1f4d1a218p+12),
272 L(-0x1.78d18aac156d1d93a2ffe7e08d3fp+16),
273 L(0x2.9df49ca75e5b567f5ea3e47106cp+16),
274 L(-0x4.a7149af8961a08aa7c3233b5bb94p+16),
275 L(0x8.3db10ffa742c707c25197d989798p+16),
276 L(-0xe.a26d6dd023cadd02041a049ec368p+16),
277 L(0x1.c825d90514e7c57c7fa5316f947cp+20),
278 L(-0x3.34bb81e5a0952df8ca1abdc6684cp+20),
279 /* Interval [-2.625, -2.5] (polynomial degree 28). */
280 L(-0x3.d10108c27ebafad533c20eac32bp-4),
281 L(0x1.cd557caff7d2b2085f41dbec5106p+0),
282 L(0x3.819b4856d399520dad9776ea2cacp+0),
283 L(0x6.8505cbad03dc34c5e42e8b12eb78p+0),
284 L(0xb.c1b2e653a9e38f82b399c94e7f08p+0),
285 L(0x1.50a53a38f148138105124df65419p+4),
286 L(0x2.57ae00cbe5232cbeeed34d89727ap+4),
287 L(0x4.2b156301b8604db85a601544bfp+4),
288 L(0x7.6989ed23ca3ca7579b3462592b5cp+4),
289 L(0xd.2dd2976557939517f831f5552cc8p+4),
290 L(0x1.76e1c3430eb860969bce40cd494p+8),
291 L(0x2.9a77bf5488742466db3a2c7c1ec6p+8),
292 L(0x4.a0d62ed7266e8eb36f725a8ebcep+8),
293 L(0x8.3a6184dd3021067df2f8b91e99c8p+8),
294 L(0xe.a0ade1538245bf55d39d7e436b1p+8),
295 L(0x1.a01359fae8617b5826dd74428e9p+12),
296 L(0x2.e3b0a32caae77251169acaca1ad4p+12),
297 L(0x5.2301257c81589f62b38fb5993ee8p+12),
298 L(0x9.21c9275db253d4e719b73b18cb9p+12),
299 L(0x1.03c104bc96141cda3f3fa4b112bcp+16),
300 L(0x1.cdc8ed65119196a08b0c78f1445p+16),
301 L(0x3.34f31d2eaacf34382cdb0073572ap+16),
302 L(0x5.b37628cadf12bf0000907d0ef294p+16),
303 L(0xa.22d8b332c0b1e6a616f425dfe5ap+16),
304 L(0x1.205b01444804c3ff922cd78b4c42p+20),
305 L(0x1.fe8f0cea9d1e0ff25be2470b4318p+20),
306 L(0x3.8872aebeb368399aee02b39340aep+20),
307 L(0x6.ebd560d351e84e26a4381f5b293cp+20),
308 L(0xc.c3644d094b0dae2fbcbf682cd428p+20),
309 /* Interval [-2.75, -2.625] (polynomial degree 26). */
310 L(-0x6.b5d252a56e8a75458a27ed1c2dd4p-4),
311 L(0x1.28d60383da3ac721aed3c5794da9p+0),
312 L(0x1.db6513ada8a66ea77d87d9a8827bp+0),
313 L(0x2.e217118f9d348a27f7506a707e6ep+0),
314 L(0x4.450112c5cbf725a0fb9802396c9p+0),
315 L(0x6.4af99151eae7810a75df2a0303c4p+0),
316 L(0x9.2db598b4a97a7f69aeef32aec758p+0),
317 L(0xd.62bef9c22471f5ee47ea1b9c0b5p+0),
318 L(0x1.379f294e412bd62328326d4222f9p+4),
319 L(0x1.c5827349d8865f1e8825c37c31c6p+4),
320 L(0x2.93a7e7a75b7568cc8cbe8c016c12p+4),
321 L(0x3.bf9bb882afe57edb383d41879d3ap+4),
322 L(0x5.73c737828cee095c43a5566731c8p+4),
323 L(0x7.ee4653493a7f81e0442062b3823cp+4),
324 L(0xb.891c6b83fc8b55bd973b5d962d6p+4),
325 L(0x1.0c775d7de3bf9b246c0208e0207ep+8),
326 L(0x1.867ee43ec4bd4f4fd56abc05110ap+8),
327 L(0x2.37fe9ba6695821e9822d8c8af0a6p+8),
328 L(0x3.3a2c667e37c942f182cd3223a936p+8),
329 L(0x4.b1b500eb59f3f782c7ccec88754p+8),
330 L(0x6.d3efd3b65b3d0d8488d30b79fa4cp+8),
331 L(0x9.ee8224e65bed5ced8b75eaec609p+8),
332 L(0xe.72416e510cca77d53fc615c1f3dp+8),
333 L(0x1.4fb538b0a2dfe567a8904b7e0445p+12),
334 L(0x1.e7f56a9266cf525a5b8cf4cb76cep+12),
335 L(0x2.f0365c983f68c597ee49d099cce8p+12),
336 L(0x4.53aa229e1b9f5b5e59625265951p+12),
337 /* Interval [-2.875, -2.75] (polynomial degree 24). */
338 L(-0x8.a41b1e4f36ff88dc820815607d68p-4),
339 L(0xc.da87d3b69dc0f2f9c6f368b8ca1p-4),
340 L(0x1.1474ad5c36158a7bea04fd2f98c6p+0),
341 L(0x1.761ecb90c555df6555b7dba955b6p+0),
342 L(0x1.d279bff9ae291caf6c4b4bcb3202p+0),
343 L(0x2.4e5d00559a6e2b9b5d7fe1f6689cp+0),
344 L(0x2.d57545a75cee8743ae2b17bc8d24p+0),
345 L(0x3.8514eee3aac88b89bec2307021bap+0),
346 L(0x4.5235e3b6e1891ffeb87fed9f8a24p+0),
347 L(0x5.562acdb10eef3c9a773b3e27a864p+0),
348 L(0x6.8ec8965c76efe03c26bff60b1194p+0),
349 L(0x8.15251aca144877af32658399f9b8p+0),
350 L(0x9.f08d56aba174d844138af782c0f8p+0),
351 L(0xc.3dbbeda2679e8a1346ccc3f6da88p+0),
352 L(0xf.0f5bfd5eacc26db308ffa0556fa8p+0),
353 L(0x1.28a6ccd84476fbc713d6bab49ac9p+4),
354 L(0x1.6d0a3ae2a3b1c8ff400641a3a21fp+4),
355 L(0x1.c15701b28637f87acfb6a91d33b5p+4),
356 L(0x2.28fbe0eccf472089b017651ca55ep+4),
357 L(0x2.a8a453004f6e8ffaacd1603bc3dp+4),
358 L(0x3.45ae4d9e1e7cd1a5dba0e4ec7f6cp+4),
359 L(0x4.065fbfacb7fad3e473cb577a61e8p+4),
360 L(0x4.f3d1473020927acac1944734a39p+4),
361 L(0x6.54bb091245815a36fb74e314dd18p+4),
362 L(0x7.d7f445129f7fb6c055e582d3f6ep+4),
363 /* Interval [-3, -2.875] (polynomial degree 23). */
364 L(-0xa.046d667e468f3e44dcae1afcc648p-4),
365 L(0x9.70b88dcc006c214d8d996fdf5ccp-4),
366 L(0xa.a8a39421c86d3ff24931a0929fp-4),
367 L(0xd.2f4d1363f324da2b357c8b6ec94p-4),
368 L(0xd.ca9aa1a3a5c00de11bf60499a97p-4),
369 L(0xf.cf09c31eeb52a45dfa7ebe3778dp-4),
370 L(0x1.04b133a39ed8a09691205660468bp+0),
371 L(0x1.22b547a06edda944fcb12fd9b5ecp+0),
372 L(0x1.2c57fce7db86a91df09602d344b3p+0),
373 L(0x1.4aade4894708f84795212fe257eep+0),
374 L(0x1.579c8b7b67ec4afed5b28c8bf787p+0),
375 L(0x1.776820e7fc80ae5284239733078ap+0),
376 L(0x1.883ab28c7301fde4ca6b8ec26ec8p+0),
377 L(0x1.aa2ef6e1ae52eb42c9ee83b206e3p+0),
378 L(0x1.bf4ad50f0a9a9311300cf0c51ee7p+0),
379 L(0x1.e40206e0e96b1da463814dde0d09p+0),
380 L(0x1.fdcbcffef3a21b29719c2bd9feb1p+0),
381 L(0x2.25e2e8948939c4d42cf108fae4bep+0),
382 L(0x2.44ce14d2b59c1c0e6bf2cfa81018p+0),
383 L(0x2.70ee80bbd0387162be4861c43622p+0),
384 L(0x2.954b64d2c2ebf3489b949c74476p+0),
385 L(0x2.c616e133a811c1c9446105208656p+0),
386 L(0x3.05a69dfe1a9ba1079f90fcf26bd4p+0),
387 L(0x3.410d2ad16a0506de29736e6aafdap+0),
388 };
389
390static const size_t poly_deg[] =
391 {
392 23,
393 24,
394 25,
395 27,
396 28,
397 26,
398 24,
399 23,
400 };
401
402static const size_t poly_end[] =
403 {
404 23,
405 48,
406 74,
407 102,
408 131,
409 158,
410 183,
411 207,
412 };
413
414/* Compute sin (pi * X) for -0.25 <= X <= 0.5. */
415
416static _Float128
417lg_sinpi (_Float128 x)
418{
419 if (x <= L(0.25))
420 return __sinl (M_PIl * x);
421 else
422 return __cosl (M_PIl * (L(0.5) - x));
423}
424
425/* Compute cos (pi * X) for -0.25 <= X <= 0.5. */
426
427static _Float128
428lg_cospi (_Float128 x)
429{
430 if (x <= L(0.25))
431 return __cosl (M_PIl * x);
432 else
433 return __sinl (M_PIl * (L(0.5) - x));
434}
435
436/* Compute cot (pi * X) for -0.25 <= X <= 0.5. */
437
438static _Float128
439lg_cotpi (_Float128 x)
440{
441 return lg_cospi (x) / lg_sinpi (x);
442}
443
444/* Compute lgamma of a negative argument -50 < X < -2, setting
445 *SIGNGAMP accordingly. */
446
447_Float128
448__lgamma_negl (_Float128 x, int *signgamp)
449{
450 /* Determine the half-integer region X lies in, handle exact
451 integers and determine the sign of the result. */
452 int i = floorl (-2 * x);
453 if ((i & 1) == 0 && i == -2 * x)
454 return L(1.0) / L(0.0);
455 _Float128 xn = ((i & 1) == 0 ? -i / 2 : (-i - 1) / 2);
456 i -= 4;
457 *signgamp = ((i & 2) == 0 ? -1 : 1);
458
459 SET_RESTORE_ROUNDL (FE_TONEAREST);
460
461 /* Expand around the zero X0 = X0_HI + X0_LO. */
462 _Float128 x0_hi = lgamma_zeros[i][0], x0_lo = lgamma_zeros[i][1];
463 _Float128 xdiff = x - x0_hi - x0_lo;
464
465 /* For arguments in the range -3 to -2, use polynomial
466 approximations to an adjusted version of the gamma function. */
467 if (i < 2)
468 {
469 int j = floorl (-8 * x) - 16;
470 _Float128 xm = (-33 - 2 * j) * L(0.0625);
471 _Float128 x_adj = x - xm;
472 size_t deg = poly_deg[j];
473 size_t end = poly_end[j];
474 _Float128 g = poly_coeff[end];
475 for (size_t j = 1; j <= deg; j++)
476 g = g * x_adj + poly_coeff[end - j];
477 return __log1pl (g * xdiff / (x - xn));
478 }
479
480 /* The result we want is log (sinpi (X0) / sinpi (X))
481 + log (gamma (1 - X0) / gamma (1 - X)). */
482 _Float128 x_idiff = fabsl (xn - x), x0_idiff = fabsl (xn - x0_hi - x0_lo);
483 _Float128 log_sinpi_ratio;
484 if (x0_idiff < x_idiff * L(0.5))
485 /* Use log not log1p to avoid inaccuracy from log1p of arguments
486 close to -1. */
487 log_sinpi_ratio = __ieee754_logl (lg_sinpi (x0_idiff)
488 / lg_sinpi (x_idiff));
489 else
490 {
491 /* Use log1p not log to avoid inaccuracy from log of arguments
492 close to 1. X0DIFF2 has positive sign if X0 is further from
493 XN than X is from XN, negative sign otherwise. */
494 _Float128 x0diff2 = ((i & 1) == 0 ? xdiff : -xdiff) * L(0.5);
495 _Float128 sx0d2 = lg_sinpi (x0diff2);
496 _Float128 cx0d2 = lg_cospi (x0diff2);
497 log_sinpi_ratio = __log1pl (2 * sx0d2
498 * (-sx0d2 + cx0d2 * lg_cotpi (x_idiff)));
499 }
500
501 _Float128 log_gamma_ratio;
502 _Float128 y0 = 1 - x0_hi;
503 _Float128 y0_eps = -x0_hi + (1 - y0) - x0_lo;
504 _Float128 y = 1 - x;
505 _Float128 y_eps = -x + (1 - y);
506 /* We now wish to compute LOG_GAMMA_RATIO
507 = log (gamma (Y0 + Y0_EPS) / gamma (Y + Y_EPS)). XDIFF
508 accurately approximates the difference Y0 + Y0_EPS - Y -
509 Y_EPS. Use Stirling's approximation. First, we may need to
510 adjust into the range where Stirling's approximation is
511 sufficiently accurate. */
512 _Float128 log_gamma_adj = 0;
513 if (i < 20)
514 {
515 int n_up = (21 - i) / 2;
516 _Float128 ny0, ny0_eps, ny, ny_eps;
517 ny0 = y0 + n_up;
518 ny0_eps = y0 - (ny0 - n_up) + y0_eps;
519 y0 = ny0;
520 y0_eps = ny0_eps;
521 ny = y + n_up;
522 ny_eps = y - (ny - n_up) + y_eps;
523 y = ny;
524 y_eps = ny_eps;
525 _Float128 prodm1 = __lgamma_productl (xdiff, y - n_up, y_eps, n_up);
526 log_gamma_adj = -__log1pl (prodm1);
527 }
528 _Float128 log_gamma_high
529 = (xdiff * __log1pl ((y0 - e_hi - e_lo + y0_eps) / e_hi)
530 + (y - L(0.5) + y_eps) * __log1pl (xdiff / y) + log_gamma_adj);
531 /* Compute the sum of (B_2k / 2k(2k-1))(Y0^-(2k-1) - Y^-(2k-1)). */
532 _Float128 y0r = 1 / y0, yr = 1 / y;
533 _Float128 y0r2 = y0r * y0r, yr2 = yr * yr;
534 _Float128 rdiff = -xdiff / (y * y0);
535 _Float128 bterm[NCOEFF];
536 _Float128 dlast = rdiff, elast = rdiff * yr * (yr + y0r);
537 bterm[0] = dlast * lgamma_coeff[0];
538 for (size_t j = 1; j < NCOEFF; j++)
539 {
540 _Float128 dnext = dlast * y0r2 + elast;
541 _Float128 enext = elast * yr2;
542 bterm[j] = dnext * lgamma_coeff[j];
543 dlast = dnext;
544 elast = enext;
545 }
546 _Float128 log_gamma_low = 0;
547 for (size_t j = 0; j < NCOEFF; j++)
548 log_gamma_low += bterm[NCOEFF - 1 - j];
549 log_gamma_ratio = log_gamma_high + log_gamma_low;
550
551 return log_sinpi_ratio + log_gamma_ratio;
552}
553

source code of glibc/sysdeps/ieee754/ldbl-128/lgamma_negl.c