2 * Copyright (c) 1992, 1993
3 * The Regents of the University of California. All rights reserved.
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6 * modification, are permitted provided that the following conditions
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14 * must display the following acknowledgement:
15 * This product includes software developed by the University of
16 * California, Berkeley and its contributors.
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18 * may be used to endorse or promote products derived from this software
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22 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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26 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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29 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
30 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
33 * @(#)log.c 8.2 (Berkeley) 11/30/93
41 /* Table-driven natural logarithm.
43 * This code was derived, with minor modifications, from:
44 * Peter Tang, "Table-Driven Implementation of the
45 * Logarithm in IEEE Floating-Point arithmetic." ACM Trans.
46 * Math Software, vol 16. no 4, pp 378-400, Dec 1990).
48 * Calculates log(2^m*F*(1+f/F)), |f/j| <= 1/256,
49 * where F = j/128 for j an integer in [0, 128].
51 * log(2^m) = log2_hi*m + log2_tail*m
52 * since m is an integer, the dominant term is exact.
53 * m has at most 10 digits (for subnormal numbers),
54 * and log2_hi has 11 trailing zero bits.
56 * log(F) = logF_hi[j] + logF_lo[j] is in tabular form in log_table.h
57 * logF_hi[] + 512 is exact.
59 * log(1+f/F) = 2*f/(2*F + f) + 1/12 * (2*f/(2*F + f))**3 + ...
60 * the leading term is calculated to extra precision in two
61 * parts, the larger of which adds exactly to the dominant
63 * There are two cases:
64 * 1. when m, j are non-zero (m | j), use absolute
65 * precision for the leading term.
66 * 2. when m = j = 0, |1-x| < 1/256, and log(x) ~= (x-1).
67 * In this case, use a relative precision of 24 bits.
68 * (This is done differently in the original paper)
71 * 0 return signalling -Inf
72 * neg return signalling NaN
76 #if defined(vax) || defined(tahoe)
78 #define TRUNC(x) x = (double) (float) (x)
81 #define endian (((*(int *) &one)) ? 1 : 0)
82 #define TRUNC(x) *(((int *) &x) + endian) &= 0xf8000000
88 /* Table of log(Fj) = logF_head[j] + logF_tail[j], for Fj = 1+j/128.
89 * Used for generation of extend precision logarithms.
90 * The constant 35184372088832 is 2^45, so the divide is exact.
91 * It ensures correct reading of logF_head, even for inaccurate
92 * decimal-to-binary conversion routines. (Everybody gets the
93 * right answer for integers less than 2^53.)
94 * Values for log(F) were generated using error < 10^-57 absolute
95 * with the bc -l package.
97 static double A1 = .08333333333333178827;
98 static double A2 = .01250000000377174923;
99 static double A3 = .002232139987919447809;
100 static double A4 = .0004348877777076145742;
102 static double logF_head[N+1] = {
104 .007782140442060381246,
105 .015504186535963526694,
106 .023167059281547608406,
107 .030771658666765233647,
108 .038318864302141264488,
109 .045809536031242714670,
110 .053244514518837604555,
111 .060624621816486978786,
112 .067950661908525944454,
113 .075223421237524235039,
114 .082443669210988446138,
115 .089612158689760690322,
116 .096729626458454731618,
117 .103796793681567578460,
118 .110814366340264314203,
119 .117783035656430001836,
120 .124703478501032805070,
121 .131576357788617315236,
122 .138402322859292326029,
123 .145182009844575077295,
124 .151916042025732167530,
125 .158605030176659056451,
126 .165249572895390883786,
127 .171850256926518341060,
128 .178407657472689606947,
129 .184922338493834104156,
130 .191394852999565046047,
131 .197825743329758552135,
132 .204215541428766300668,
133 .210564769107350002741,
134 .216873938300523150246,
135 .223143551314024080056,
136 .229374101064877322642,
137 .235566071312860003672,
138 .241719936886966024758,
139 .247836163904594286577,
140 .253915209980732470285,
141 .259957524436686071567,
142 .265963548496984003577,
143 .271933715484010463114,
144 .277868451003087102435,
145 .283768173130738432519,
146 .289633292582948342896,
147 .295464212893421063199,
148 .301261330578199704177,
149 .307025035294827830512,
150 .312755710004239517729,
151 .318453731118097493890,
152 .324119468654316733591,
153 .329753286372579168528,
154 .335355541920762334484,
155 .340926586970454081892,
156 .346466767346100823488,
157 .351976423156884266063,
158 .357455888922231679316,
159 .362905493689140712376,
160 .368325561158599157352,
161 .373716409793814818840,
162 .379078352934811846353,
163 .384411698910298582632,
164 .389716751140440464951,
165 .394993808240542421117,
166 .400243164127459749579,
167 .405465108107819105498,
168 .410659924985338875558,
169 .415827895143593195825,
170 .420969294644237379543,
171 .426084395310681429691,
172 .431173464818130014464,
173 .436236766774527495726,
174 .441274560805140936281,
175 .446287102628048160113,
176 .451274644139630254358,
177 .456237433481874177232,
178 .461175715122408291790,
179 .466089729924533457960,
180 .470979715219073113985,
181 .475845904869856894947,
182 .480688529345570714212,
183 .485507815781602403149,
184 .490303988045525329653,
185 .495077266798034543171,
186 .499827869556611403822,
187 .504556010751912253908,
188 .509261901790523552335,
189 .513945751101346104405,
190 .518607764208354637958,
191 .523248143765158602036,
192 .527867089620485785417,
193 .532464798869114019908,
194 .537041465897345915436,
195 .541597282432121573947,
196 .546132437597407260909,
197 .550647117952394182793,
198 .555141507540611200965,
199 .559615787935399566777,
200 .564070138285387656651,
201 .568504735352689749561,
202 .572919753562018740922,
203 .577315365035246941260,
204 .581691739635061821900,
205 .586049045003164792433,
206 .590387446602107957005,
207 .594707107746216934174,
208 .599008189645246602594,
209 .603290851438941899687,
210 .607555250224322662688,
211 .611801541106615331955,
212 .616029877215623855590,
213 .620240409751204424537,
214 .624433288012369303032,
215 .628608659422752680256,
216 .632766669570628437213,
217 .636907462236194987781,
218 .641031179420679109171,
219 .645137961373620782978,
220 .649227946625615004450,
221 .653301272011958644725,
222 .657358072709030238911,
223 .661398482245203922502,
224 .665422632544505177065,
225 .669430653942981734871,
226 .673422675212350441142,
227 .677398823590920073911,
228 .681359224807238206267,
229 .685304003098281100392,
230 .689233281238557538017,
231 .693147180560117703862
234 static double logF_tail[N+1] = {
236 -.00000000000000543229938420049,
237 .00000000000000172745674997061,
238 -.00000000000001323017818229233,
239 -.00000000000001154527628289872,
240 -.00000000000000466529469958300,
241 .00000000000005148849572685810,
242 -.00000000000002532168943117445,
243 -.00000000000005213620639136504,
244 -.00000000000001819506003016881,
245 .00000000000006329065958724544,
246 .00000000000008614512936087814,
247 -.00000000000007355770219435028,
248 .00000000000009638067658552277,
249 .00000000000007598636597194141,
250 .00000000000002579999128306990,
251 -.00000000000004654729747598444,
252 -.00000000000007556920687451336,
253 .00000000000010195735223708472,
254 -.00000000000017319034406422306,
255 -.00000000000007718001336828098,
256 .00000000000010980754099855238,
257 -.00000000000002047235780046195,
258 -.00000000000008372091099235912,
259 .00000000000014088127937111135,
260 .00000000000012869017157588257,
261 .00000000000017788850778198106,
262 .00000000000006440856150696891,
263 .00000000000016132822667240822,
264 -.00000000000007540916511956188,
265 -.00000000000000036507188831790,
266 .00000000000009120937249914984,
267 .00000000000018567570959796010,
268 -.00000000000003149265065191483,
269 -.00000000000009309459495196889,
270 .00000000000017914338601329117,
271 -.00000000000001302979717330866,
272 .00000000000023097385217586939,
273 .00000000000023999540484211737,
274 .00000000000015393776174455408,
275 -.00000000000036870428315837678,
276 .00000000000036920375082080089,
277 -.00000000000009383417223663699,
278 .00000000000009433398189512690,
279 .00000000000041481318704258568,
280 -.00000000000003792316480209314,
281 .00000000000008403156304792424,
282 -.00000000000034262934348285429,
283 .00000000000043712191957429145,
284 -.00000000000010475750058776541,
285 -.00000000000011118671389559323,
286 .00000000000037549577257259853,
287 .00000000000013912841212197565,
288 .00000000000010775743037572640,
289 .00000000000029391859187648000,
290 -.00000000000042790509060060774,
291 .00000000000022774076114039555,
292 .00000000000010849569622967912,
293 -.00000000000023073801945705758,
294 .00000000000015761203773969435,
295 .00000000000003345710269544082,
296 -.00000000000041525158063436123,
297 .00000000000032655698896907146,
298 -.00000000000044704265010452446,
299 .00000000000034527647952039772,
300 -.00000000000007048962392109746,
301 .00000000000011776978751369214,
302 -.00000000000010774341461609578,
303 .00000000000021863343293215910,
304 .00000000000024132639491333131,
305 .00000000000039057462209830700,
306 -.00000000000026570679203560751,
307 .00000000000037135141919592021,
308 -.00000000000017166921336082431,
309 -.00000000000028658285157914353,
310 -.00000000000023812542263446809,
311 .00000000000006576659768580062,
312 -.00000000000028210143846181267,
313 .00000000000010701931762114254,
314 .00000000000018119346366441110,
315 .00000000000009840465278232627,
316 -.00000000000033149150282752542,
317 -.00000000000018302857356041668,
318 -.00000000000016207400156744949,
319 .00000000000048303314949553201,
320 -.00000000000071560553172382115,
321 .00000000000088821239518571855,
322 -.00000000000030900580513238244,
323 -.00000000000061076551972851496,
324 .00000000000035659969663347830,
325 .00000000000035782396591276383,
326 -.00000000000046226087001544578,
327 .00000000000062279762917225156,
328 .00000000000072838947272065741,
329 .00000000000026809646615211673,
330 -.00000000000010960825046059278,
331 .00000000000002311949383800537,
332 -.00000000000058469058005299247,
333 -.00000000000002103748251144494,
334 -.00000000000023323182945587408,
335 -.00000000000042333694288141916,
336 -.00000000000043933937969737844,
337 .00000000000041341647073835565,
338 .00000000000006841763641591466,
339 .00000000000047585534004430641,
340 .00000000000083679678674757695,
341 -.00000000000085763734646658640,
342 .00000000000021913281229340092,
343 -.00000000000062242842536431148,
344 -.00000000000010983594325438430,
345 .00000000000065310431377633651,
346 -.00000000000047580199021710769,
347 -.00000000000037854251265457040,
348 .00000000000040939233218678664,
349 .00000000000087424383914858291,
350 .00000000000025218188456842882,
351 -.00000000000003608131360422557,
352 -.00000000000050518555924280902,
353 .00000000000078699403323355317,
354 -.00000000000067020876961949060,
355 .00000000000016108575753932458,
356 .00000000000058527188436251509,
357 -.00000000000035246757297904791,
358 -.00000000000018372084495629058,
359 .00000000000088606689813494916,
360 .00000000000066486268071468700,
361 .00000000000063831615170646519,
362 .00000000000025144230728376072,
363 -.00000000000017239444525614834
374 double F, f, g, q, u, u2, v, zero = 0.0, one = 1.0;
377 /* Catch special cases */
379 if (_IEEE && x == zero) /* log(0) = -Inf */
381 else if (_IEEE) /* log(neg) = NaN */
383 else if (x == zero) /* NOT REACHED IF _IEEE */
384 return (infnan(-ERANGE));
386 return (infnan(EDOM));
388 if (_IEEE) /* x = NaN, Inf */
391 return (infnan(ERANGE));
393 /* Argument reduction: 1 <= g < 2; x/2^m = g; */
394 /* y = F*(1 + f/F) for |f| <= 2^-8 */
398 if (_IEEE && m == -1022) {
403 F = (1.0/N) * j + 1; /* F*128 is an integer in [128, 512] */
406 /* Approximate expansion for log(1+f/F) ~= u + q */
410 q = u*v*(A1 + v*(A2 + v*(A3 + v*A4)));
412 /* case 1: u1 = u rounded to 2^-43 absolute. Since u < 2^-8,
413 * u1 has at most 35 bits, and F*u1 is exact, as F has < 8 bits.
414 * It also adds exactly to |m*log2_hi + log_F_head[j] | < 750
417 u1 = u + 513, u1 -= 513;
419 /* case 2: |1-x| < 1/256. The m- and j- dependent terms are zero;
424 u2 = (2.0*(f - F*u1) - u1*f) * g;
425 /* u1 + u2 = 2f/(2F+f) to extra precision. */
427 /* log(x) = log(2^m*F*(1+f/F)) = */
428 /* (m*log2_hi+logF_head[j]+u1) + (m*log2_lo+logF_tail[j]+q); */
429 /* (exact) + (tiny) */
431 u1 += m*logF_head[N] + logF_head[j]; /* exact */
432 u2 = (u2 + logF_tail[j]) + q; /* tiny */
433 u2 += logF_tail[N]*m;
438 * Extra precision variant, returning struct {double a, b;};
439 * log(x) = a+b to 63 bits, with a is rounded to 26 bits.
445 __log__D(x) double x;
449 double F, f, g, q, u, v, u2, one = 1.0;
453 /* Argument reduction: 1 <= g < 2; x/2^m = g; */
454 /* y = F*(1 + f/F) for |f| <= 2^-8 */
458 if (_IEEE && m == -1022) {
469 q = u*v*(A1 + v*(A2 + v*(A3 + v*A4)));
471 u1 = u + 513, u1 -= 513;
474 u2 = (2.0*(f - F*u1) - u1*f) * g;
476 u1 += m*logF_head[N] + logF_head[j];
478 u2 += logF_tail[j]; u2 += q;
479 u2 += logF_tail[N]*m;
480 r.a = u1 + u2; /* Only difference is here */
482 r.b = (u1 - r.a) + u2;