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90b828e6 AJ |
1 | /* log1pl.c |
2 | * | |
3 | * Relative error logarithm | |
4 | * Natural logarithm of 1+x, 128-bit long double precision | |
5 | * | |
6 | * | |
7 | * | |
8 | * SYNOPSIS: | |
9 | * | |
10 | * long double x, y, log1pl(); | |
11 | * | |
12 | * y = log1pl( x ); | |
13 | * | |
14 | * | |
15 | * | |
16 | * DESCRIPTION: | |
17 | * | |
18 | * Returns the base e (2.718...) logarithm of 1+x. | |
19 | * | |
20 | * The argument 1+x is separated into its exponent and fractional | |
21 | * parts. If the exponent is between -1 and +1, the logarithm | |
22 | * of the fraction is approximated by | |
23 | * | |
24 | * log(1+x) = x - 0.5 x^2 + x^3 P(x)/Q(x). | |
25 | * | |
26 | * Otherwise, setting z = 2(w-1)/(w+1), | |
27 | * | |
28 | * log(w) = z + z^3 P(z)/Q(z). | |
29 | * | |
30 | * | |
31 | * | |
32 | * ACCURACY: | |
33 | * | |
34 | * Relative error: | |
35 | * arithmetic domain # trials peak rms | |
36 | * IEEE -1, 8 100000 1.9e-34 4.3e-35 | |
37 | */ | |
38 | ||
cc7375ce RM |
39 | /* Copyright 2001 by Stephen L. Moshier |
40 | ||
41 | This library is free software; you can redistribute it and/or | |
42 | modify it under the terms of the GNU Lesser General Public | |
43 | License as published by the Free Software Foundation; either | |
44 | version 2.1 of the License, or (at your option) any later version. | |
45 | ||
46 | This library is distributed in the hope that it will be useful, | |
47 | but WITHOUT ANY WARRANTY; without even the implied warranty of | |
48 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU | |
49 | Lesser General Public License for more details. | |
50 | ||
51 | You should have received a copy of the GNU Lesser General Public | |
59ba27a6 PE |
52 | License along with this library; if not, see |
53 | <http://www.gnu.org/licenses/>. */ | |
cc7375ce | 54 | |
90b828e6 AJ |
55 | |
56 | #include "math.h" | |
57 | #include "math_private.h" | |
58 | ||
59 | /* Coefficients for log(1+x) = x - x^2 / 2 + x^3 P(x)/Q(x) | |
60 | * 1/sqrt(2) <= 1+x < sqrt(2) | |
61 | * Theoretical peak relative error = 5.3e-37, | |
62 | * relative peak error spread = 2.3e-14 | |
63 | */ | |
1f5649f8 | 64 | static const long double |
90b828e6 AJ |
65 | P12 = 1.538612243596254322971797716843006400388E-6L, |
66 | P11 = 4.998469661968096229986658302195402690910E-1L, | |
67 | P10 = 2.321125933898420063925789532045674660756E1L, | |
68 | P9 = 4.114517881637811823002128927449878962058E2L, | |
69 | P8 = 3.824952356185897735160588078446136783779E3L, | |
70 | P7 = 2.128857716871515081352991964243375186031E4L, | |
71 | P6 = 7.594356839258970405033155585486712125861E4L, | |
72 | P5 = 1.797628303815655343403735250238293741397E5L, | |
73 | P4 = 2.854829159639697837788887080758954924001E5L, | |
74 | P3 = 3.007007295140399532324943111654767187848E5L, | |
75 | P2 = 2.014652742082537582487669938141683759923E5L, | |
76 | P1 = 7.771154681358524243729929227226708890930E4L, | |
77 | P0 = 1.313572404063446165910279910527789794488E4L, | |
78 | /* Q12 = 1.000000000000000000000000000000000000000E0L, */ | |
79 | Q11 = 4.839208193348159620282142911143429644326E1L, | |
80 | Q10 = 9.104928120962988414618126155557301584078E2L, | |
81 | Q9 = 9.147150349299596453976674231612674085381E3L, | |
82 | Q8 = 5.605842085972455027590989944010492125825E4L, | |
83 | Q7 = 2.248234257620569139969141618556349415120E5L, | |
84 | Q6 = 6.132189329546557743179177159925690841200E5L, | |
85 | Q5 = 1.158019977462989115839826904108208787040E6L, | |
86 | Q4 = 1.514882452993549494932585972882995548426E6L, | |
87 | Q3 = 1.347518538384329112529391120390701166528E6L, | |
88 | Q2 = 7.777690340007566932935753241556479363645E5L, | |
89 | Q1 = 2.626900195321832660448791748036714883242E5L, | |
90 | Q0 = 3.940717212190338497730839731583397586124E4L; | |
91 | ||
92 | /* Coefficients for log(x) = z + z^3 P(z^2)/Q(z^2), | |
93 | * where z = 2(x-1)/(x+1) | |
94 | * 1/sqrt(2) <= x < sqrt(2) | |
95 | * Theoretical peak relative error = 1.1e-35, | |
96 | * relative peak error spread 1.1e-9 | |
97 | */ | |
1f5649f8 | 98 | static const long double |
90b828e6 AJ |
99 | R5 = -8.828896441624934385266096344596648080902E-1L, |
100 | R4 = 8.057002716646055371965756206836056074715E1L, | |
101 | R3 = -2.024301798136027039250415126250455056397E3L, | |
102 | R2 = 2.048819892795278657810231591630928516206E4L, | |
103 | R1 = -8.977257995689735303686582344659576526998E4L, | |
104 | R0 = 1.418134209872192732479751274970992665513E5L, | |
105 | /* S6 = 1.000000000000000000000000000000000000000E0L, */ | |
106 | S5 = -1.186359407982897997337150403816839480438E2L, | |
107 | S4 = 3.998526750980007367835804959888064681098E3L, | |
108 | S3 = -5.748542087379434595104154610899551484314E4L, | |
109 | S2 = 4.001557694070773974936904547424676279307E5L, | |
110 | S1 = -1.332535117259762928288745111081235577029E6L, | |
111 | S0 = 1.701761051846631278975701529965589676574E6L; | |
112 | ||
113 | /* C1 + C2 = ln 2 */ | |
1f5649f8 UD |
114 | static const long double C1 = 6.93145751953125E-1L; |
115 | static const long double C2 = 1.428606820309417232121458176568075500134E-6L; | |
90b828e6 | 116 | |
1f5649f8 | 117 | static const long double sqrth = 0.7071067811865475244008443621048490392848L; |
90b828e6 | 118 | /* ln (2^16384 * (1 - 2^-113)) */ |
1f5649f8 | 119 | static const long double maxlog = 1.1356523406294143949491931077970764891253E4L; |
1f5649f8 | 120 | static const long double zero = 0.0L; |
90b828e6 | 121 | |
90b828e6 AJ |
122 | long double |
123 | __log1pl (long double xm1) | |
124 | { | |
125 | long double x, y, z, r, s; | |
126 | ieee854_long_double_shape_type u; | |
52e1b618 | 127 | int32_t hx; |
90b828e6 AJ |
128 | int e; |
129 | ||
bdce812b | 130 | /* Test for NaN or infinity input. */ |
90b828e6 | 131 | u.value = xm1; |
52e1b618 UD |
132 | hx = u.parts32.w0; |
133 | if (hx >= 0x7fff0000) | |
bdce812b AJ |
134 | return xm1; |
135 | ||
136 | /* log1p(+- 0) = +- 0. */ | |
52e1b618 UD |
137 | if (((hx & 0x7fffffff) == 0) |
138 | && (u.parts32.w1 | u.parts32.w2 | u.parts32.w3) == 0) | |
bdce812b AJ |
139 | return xm1; |
140 | ||
141 | x = xm1 + 1.0L; | |
90b828e6 | 142 | |
bdce812b | 143 | /* log1p(-1) = -inf */ |
90b828e6 AJ |
144 | if (x <= 0.0L) |
145 | { | |
146 | if (x == 0.0L) | |
52e1b618 | 147 | return (-1.0L / (x - x)); |
90b828e6 | 148 | else |
52e1b618 | 149 | return (zero / (x - x)); |
90b828e6 AJ |
150 | } |
151 | ||
152 | /* Separate mantissa from exponent. */ | |
153 | ||
154 | /* Use frexp used so that denormal numbers will be handled properly. */ | |
c5ee217f | 155 | x = __frexpl (x, &e); |
90b828e6 AJ |
156 | |
157 | /* Logarithm using log(x) = z + z^3 P(z^2)/Q(z^2), | |
158 | where z = 2(x-1)/x+1). */ | |
159 | if ((e > 2) || (e < -2)) | |
160 | { | |
161 | if (x < sqrth) | |
162 | { /* 2( 2x-1 )/( 2x+1 ) */ | |
163 | e -= 1; | |
164 | z = x - 0.5L; | |
165 | y = 0.5L * z + 0.5L; | |
166 | } | |
167 | else | |
168 | { /* 2 (x-1)/(x+1) */ | |
169 | z = x - 0.5L; | |
170 | z -= 0.5L; | |
171 | y = 0.5L * x + 0.5L; | |
172 | } | |
173 | x = z / y; | |
174 | z = x * x; | |
175 | r = ((((R5 * z | |
176 | + R4) * z | |
177 | + R3) * z | |
178 | + R2) * z | |
179 | + R1) * z | |
180 | + R0; | |
181 | s = (((((z | |
182 | + S5) * z | |
183 | + S4) * z | |
184 | + S3) * z | |
185 | + S2) * z | |
186 | + S1) * z | |
187 | + S0; | |
188 | z = x * (z * r / s); | |
189 | z = z + e * C2; | |
190 | z = z + x; | |
191 | z = z + e * C1; | |
192 | return (z); | |
193 | } | |
194 | ||
195 | ||
196 | /* Logarithm using log(1+x) = x - .5x^2 + x^3 P(x)/Q(x). */ | |
197 | ||
198 | if (x < sqrth) | |
199 | { | |
200 | e -= 1; | |
201 | if (e != 0) | |
202 | x = 2.0L * x - 1.0L; /* 2x - 1 */ | |
203 | else | |
204 | x = xm1; | |
205 | } | |
206 | else | |
207 | { | |
208 | if (e != 0) | |
209 | x = x - 1.0L; | |
210 | else | |
211 | x = xm1; | |
212 | } | |
213 | z = x * x; | |
214 | r = (((((((((((P12 * x | |
215 | + P11) * x | |
216 | + P10) * x | |
217 | + P9) * x | |
218 | + P8) * x | |
219 | + P7) * x | |
220 | + P6) * x | |
221 | + P5) * x | |
222 | + P4) * x | |
223 | + P3) * x | |
224 | + P2) * x | |
225 | + P1) * x | |
226 | + P0; | |
227 | s = (((((((((((x | |
228 | + Q11) * x | |
229 | + Q10) * x | |
230 | + Q9) * x | |
231 | + Q8) * x | |
232 | + Q7) * x | |
233 | + Q6) * x | |
234 | + Q5) * x | |
235 | + Q4) * x | |
236 | + Q3) * x | |
237 | + Q2) * x | |
238 | + Q1) * x | |
239 | + Q0; | |
240 | y = x * (z * r / s); | |
241 | y = y + e * C2; | |
242 | z = y - 0.5L * z; | |
243 | z = z + x; | |
244 | z = z + e * C1; | |
245 | return (z); | |
246 | } | |
247 | ||
248 | weak_alias (__log1pl, log1pl) |