1/*
2 * Copyright (c) 1992, 1993
3 *	The Regents of the University of California.  All rights reserved.
4 *
5 * Redistribution and use in source and binary forms, with or without
6 * modification, are permitted provided that the following conditions
7 * are met:
8 * 1. Redistributions of source code must retain the above copyright
9 *    notice, this list of conditions and the following disclaimer.
10 * 2. Redistributions in binary form must reproduce the above copyright
11 *    notice, this list of conditions and the following disclaimer in the
12 *    documentation and/or other materials provided with the distribution.
13 * 3. All advertising materials mentioning features or use of this software
14 *    must display the following acknowledgement:
15 *	This product includes software developed by the University of
16 *	California, Berkeley and its contributors.
17 * 4. Neither the name of the University nor the names of its contributors
18 *    may be used to endorse or promote products derived from this software
19 *    without specific prior written permission.
20 *
21 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
22 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
23 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
24 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
25 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
26 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
27 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
28 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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
31 * SUCH DAMAGE.
32 */
33
34/* @(#)log.c	8.2 (Berkeley) 11/30/93 */
35#include <sys/cdefs.h>
36__FBSDID("$FreeBSD$");
37
38#include <math.h>
39#include <errno.h>
40
41#include "mathimpl.h"
42
43/* Table-driven natural logarithm.
44 *
45 * This code was derived, with minor modifications, from:
46 *	Peter Tang, "Table-Driven Implementation of the
47 *	Logarithm in IEEE Floating-Point arithmetic." ACM Trans.
48 *	Math Software, vol 16. no 4, pp 378-400, Dec 1990).
49 *
50 * Calculates log(2^m*F*(1+f/F)), |f/j| <= 1/256,
51 * where F = j/128 for j an integer in [0, 128].
52 *
53 * log(2^m) = log2_hi*m + log2_tail*m
54 * since m is an integer, the dominant term is exact.
55 * m has at most 10 digits (for subnormal numbers),
56 * and log2_hi has 11 trailing zero bits.
57 *
58 * log(F) = logF_hi[j] + logF_lo[j] is in tabular form in log_table.h
59 * logF_hi[] + 512 is exact.
60 *
61 * log(1+f/F) = 2*f/(2*F + f) + 1/12 * (2*f/(2*F + f))**3 + ...
62 * the leading term is calculated to extra precision in two
63 * parts, the larger of which adds exactly to the dominant
64 * m and F terms.
65 * There are two cases:
66 *	1. when m, j are non-zero (m | j), use absolute
67 *	   precision for the leading term.
68 *	2. when m = j = 0, |1-x| < 1/256, and log(x) ~= (x-1).
69 *	   In this case, use a relative precision of 24 bits.
70 * (This is done differently in the original paper)
71 *
72 * Special cases:
73 *	0	return signalling -Inf
74 *	neg	return signalling NaN
75 *	+Inf	return +Inf
76*/
77
78#define N 128
79
80/* Table of log(Fj) = logF_head[j] + logF_tail[j], for Fj = 1+j/128.
81 * Used for generation of extend precision logarithms.
82 * The constant 35184372088832 is 2^45, so the divide is exact.
83 * It ensures correct reading of logF_head, even for inaccurate
84 * decimal-to-binary conversion routines.  (Everybody gets the
85 * right answer for integers less than 2^53.)
86 * Values for log(F) were generated using error < 10^-57 absolute
87 * with the bc -l package.
88*/
89static double	A1 = 	  .08333333333333178827;
90static double	A2 = 	  .01250000000377174923;
91static double	A3 =	 .002232139987919447809;
92static double	A4 =	.0004348877777076145742;
93
94static double logF_head[N+1] = {
95	0.,
96	.007782140442060381246,
97	.015504186535963526694,
98	.023167059281547608406,
99	.030771658666765233647,
100	.038318864302141264488,
101	.045809536031242714670,
102	.053244514518837604555,
103	.060624621816486978786,
104	.067950661908525944454,
105	.075223421237524235039,
106	.082443669210988446138,
107	.089612158689760690322,
108	.096729626458454731618,
109	.103796793681567578460,
110	.110814366340264314203,
111	.117783035656430001836,
112	.124703478501032805070,
113	.131576357788617315236,
114	.138402322859292326029,
115	.145182009844575077295,
116	.151916042025732167530,
117	.158605030176659056451,
118	.165249572895390883786,
119	.171850256926518341060,
120	.178407657472689606947,
121	.184922338493834104156,
122	.191394852999565046047,
123	.197825743329758552135,
124	.204215541428766300668,
125	.210564769107350002741,
126	.216873938300523150246,
127	.223143551314024080056,
128	.229374101064877322642,
129	.235566071312860003672,
130	.241719936886966024758,
131	.247836163904594286577,
132	.253915209980732470285,
133	.259957524436686071567,
134	.265963548496984003577,
135	.271933715484010463114,
136	.277868451003087102435,
137	.283768173130738432519,
138	.289633292582948342896,
139	.295464212893421063199,
140	.301261330578199704177,
141	.307025035294827830512,
142	.312755710004239517729,
143	.318453731118097493890,
144	.324119468654316733591,
145	.329753286372579168528,
146	.335355541920762334484,
147	.340926586970454081892,
148	.346466767346100823488,
149	.351976423156884266063,
150	.357455888922231679316,
151	.362905493689140712376,
152	.368325561158599157352,
153	.373716409793814818840,
154	.379078352934811846353,
155	.384411698910298582632,
156	.389716751140440464951,
157	.394993808240542421117,
158	.400243164127459749579,
159	.405465108107819105498,
160	.410659924985338875558,
161	.415827895143593195825,
162	.420969294644237379543,
163	.426084395310681429691,
164	.431173464818130014464,
165	.436236766774527495726,
166	.441274560805140936281,
167	.446287102628048160113,
168	.451274644139630254358,
169	.456237433481874177232,
170	.461175715122408291790,
171	.466089729924533457960,
172	.470979715219073113985,
173	.475845904869856894947,
174	.480688529345570714212,
175	.485507815781602403149,
176	.490303988045525329653,
177	.495077266798034543171,
178	.499827869556611403822,
179	.504556010751912253908,
180	.509261901790523552335,
181	.513945751101346104405,
182	.518607764208354637958,
183	.523248143765158602036,
184	.527867089620485785417,
185	.532464798869114019908,
186	.537041465897345915436,
187	.541597282432121573947,
188	.546132437597407260909,
189	.550647117952394182793,
190	.555141507540611200965,
191	.559615787935399566777,
192	.564070138285387656651,
193	.568504735352689749561,
194	.572919753562018740922,
195	.577315365035246941260,
196	.581691739635061821900,
197	.586049045003164792433,
198	.590387446602107957005,
199	.594707107746216934174,
200	.599008189645246602594,
201	.603290851438941899687,
202	.607555250224322662688,
203	.611801541106615331955,
204	.616029877215623855590,
205	.620240409751204424537,
206	.624433288012369303032,
207	.628608659422752680256,
208	.632766669570628437213,
209	.636907462236194987781,
210	.641031179420679109171,
211	.645137961373620782978,
212	.649227946625615004450,
213	.653301272011958644725,
214	.657358072709030238911,
215	.661398482245203922502,
216	.665422632544505177065,
217	.669430653942981734871,
218	.673422675212350441142,
219	.677398823590920073911,
220	.681359224807238206267,
221	.685304003098281100392,
222	.689233281238557538017,
223	.693147180560117703862
224};
225
226static double logF_tail[N+1] = {
227	0.,
228	-.00000000000000543229938420049,
229	 .00000000000000172745674997061,
230	-.00000000000001323017818229233,
231	-.00000000000001154527628289872,
232	-.00000000000000466529469958300,
233	 .00000000000005148849572685810,
234	-.00000000000002532168943117445,
235	-.00000000000005213620639136504,
236	-.00000000000001819506003016881,
237	 .00000000000006329065958724544,
238	 .00000000000008614512936087814,
239	-.00000000000007355770219435028,
240	 .00000000000009638067658552277,
241	 .00000000000007598636597194141,
242	 .00000000000002579999128306990,
243	-.00000000000004654729747598444,
244	-.00000000000007556920687451336,
245	 .00000000000010195735223708472,
246	-.00000000000017319034406422306,
247	-.00000000000007718001336828098,
248	 .00000000000010980754099855238,
249	-.00000000000002047235780046195,
250	-.00000000000008372091099235912,
251	 .00000000000014088127937111135,
252	 .00000000000012869017157588257,
253	 .00000000000017788850778198106,
254	 .00000000000006440856150696891,
255	 .00000000000016132822667240822,
256	-.00000000000007540916511956188,
257	-.00000000000000036507188831790,
258	 .00000000000009120937249914984,
259	 .00000000000018567570959796010,
260	-.00000000000003149265065191483,
261	-.00000000000009309459495196889,
262	 .00000000000017914338601329117,
263	-.00000000000001302979717330866,
264	 .00000000000023097385217586939,
265	 .00000000000023999540484211737,
266	 .00000000000015393776174455408,
267	-.00000000000036870428315837678,
268	 .00000000000036920375082080089,
269	-.00000000000009383417223663699,
270	 .00000000000009433398189512690,
271	 .00000000000041481318704258568,
272	-.00000000000003792316480209314,
273	 .00000000000008403156304792424,
274	-.00000000000034262934348285429,
275	 .00000000000043712191957429145,
276	-.00000000000010475750058776541,
277	-.00000000000011118671389559323,
278	 .00000000000037549577257259853,
279	 .00000000000013912841212197565,
280	 .00000000000010775743037572640,
281	 .00000000000029391859187648000,
282	-.00000000000042790509060060774,
283	 .00000000000022774076114039555,
284	 .00000000000010849569622967912,
285	-.00000000000023073801945705758,
286	 .00000000000015761203773969435,
287	 .00000000000003345710269544082,
288	-.00000000000041525158063436123,
289	 .00000000000032655698896907146,
290	-.00000000000044704265010452446,
291	 .00000000000034527647952039772,
292	-.00000000000007048962392109746,
293	 .00000000000011776978751369214,
294	-.00000000000010774341461609578,
295	 .00000000000021863343293215910,
296	 .00000000000024132639491333131,
297	 .00000000000039057462209830700,
298	-.00000000000026570679203560751,
299	 .00000000000037135141919592021,
300	-.00000000000017166921336082431,
301	-.00000000000028658285157914353,
302	-.00000000000023812542263446809,
303	 .00000000000006576659768580062,
304	-.00000000000028210143846181267,
305	 .00000000000010701931762114254,
306	 .00000000000018119346366441110,
307	 .00000000000009840465278232627,
308	-.00000000000033149150282752542,
309	-.00000000000018302857356041668,
310	-.00000000000016207400156744949,
311	 .00000000000048303314949553201,
312	-.00000000000071560553172382115,
313	 .00000000000088821239518571855,
314	-.00000000000030900580513238244,
315	-.00000000000061076551972851496,
316	 .00000000000035659969663347830,
317	 .00000000000035782396591276383,
318	-.00000000000046226087001544578,
319	 .00000000000062279762917225156,
320	 .00000000000072838947272065741,
321	 .00000000000026809646615211673,
322	-.00000000000010960825046059278,
323	 .00000000000002311949383800537,
324	-.00000000000058469058005299247,
325	-.00000000000002103748251144494,
326	-.00000000000023323182945587408,
327	-.00000000000042333694288141916,
328	-.00000000000043933937969737844,
329	 .00000000000041341647073835565,
330	 .00000000000006841763641591466,
331	 .00000000000047585534004430641,
332	 .00000000000083679678674757695,
333	-.00000000000085763734646658640,
334	 .00000000000021913281229340092,
335	-.00000000000062242842536431148,
336	-.00000000000010983594325438430,
337	 .00000000000065310431377633651,
338	-.00000000000047580199021710769,
339	-.00000000000037854251265457040,
340	 .00000000000040939233218678664,
341	 .00000000000087424383914858291,
342	 .00000000000025218188456842882,
343	-.00000000000003608131360422557,
344	-.00000000000050518555924280902,
345	 .00000000000078699403323355317,
346	-.00000000000067020876961949060,
347	 .00000000000016108575753932458,
348	 .00000000000058527188436251509,
349	-.00000000000035246757297904791,
350	-.00000000000018372084495629058,
351	 .00000000000088606689813494916,
352	 .00000000000066486268071468700,
353	 .00000000000063831615170646519,
354	 .00000000000025144230728376072,
355	-.00000000000017239444525614834
356};
357
358#if 0
359double
360#ifdef _ANSI_SOURCE
361log(double x)
362#else
363log(x) double x;
364#endif
365{
366	int m, j;
367	double F, f, g, q, u, u2, v, zero = 0.0, one = 1.0;
368	volatile double u1;
369
370	/* Catch special cases */
371	if (x <= 0)
372		if (x == zero)	/* log(0) = -Inf */
373			return (-one/zero);
374		else		/* log(neg) = NaN */
375			return (zero/zero);
376	else if (!finite(x))
377		return (x+x);		/* x = NaN, Inf */
378
379	/* Argument reduction: 1 <= g < 2; x/2^m = g;	*/
380	/* y = F*(1 + f/F) for |f| <= 2^-8		*/
381
382	m = logb(x);
383	g = ldexp(x, -m);
384	if (m == -1022) {
385		j = logb(g), m += j;
386		g = ldexp(g, -j);
387	}
388	j = N*(g-1) + .5;
389	F = (1.0/N) * j + 1;	/* F*128 is an integer in [128, 512] */
390	f = g - F;
391
392	/* Approximate expansion for log(1+f/F) ~= u + q */
393	g = 1/(2*F+f);
394	u = 2*f*g;
395	v = u*u;
396	q = u*v*(A1 + v*(A2 + v*(A3 + v*A4)));
397
398    /* case 1: u1 = u rounded to 2^-43 absolute.  Since u < 2^-8,
399     * 	       u1 has at most 35 bits, and F*u1 is exact, as F has < 8 bits.
400     *         It also adds exactly to |m*log2_hi + log_F_head[j] | < 750
401    */
402	if (m | j)
403		u1 = u + 513, u1 -= 513;
404
405    /* case 2:	|1-x| < 1/256. The m- and j- dependent terms are zero;
406     * 		u1 = u to 24 bits.
407    */
408	else
409		u1 = u, TRUNC(u1);
410	u2 = (2.0*(f - F*u1) - u1*f) * g;
411			/* u1 + u2 = 2f/(2F+f) to extra precision.	*/
412
413	/* log(x) = log(2^m*F*(1+f/F)) =				*/
414	/* (m*log2_hi+logF_head[j]+u1) + (m*log2_lo+logF_tail[j]+q);	*/
415	/* (exact) + (tiny)						*/
416
417	u1 += m*logF_head[N] + logF_head[j];		/* exact */
418	u2 = (u2 + logF_tail[j]) + q;			/* tiny */
419	u2 += logF_tail[N]*m;
420	return (u1 + u2);
421}
422#endif
423
424/*
425 * Extra precision variant, returning struct {double a, b;};
426 * log(x) = a+b to 63 bits, with a rounded to 26 bits.
427 */
428struct Double
429#ifdef _ANSI_SOURCE
430__log__D(double x)
431#else
432__log__D(x) double x;
433#endif
434{
435	int m, j;
436	double F, f, g, q, u, v, u2;
437	volatile double u1;
438	struct Double r;
439
440	/* Argument reduction: 1 <= g < 2; x/2^m = g;	*/
441	/* y = F*(1 + f/F) for |f| <= 2^-8		*/
442
443	m = logb(x);
444	g = ldexp(x, -m);
445	if (m == -1022) {
446		j = logb(g), m += j;
447		g = ldexp(g, -j);
448	}
449	j = N*(g-1) + .5;
450	F = (1.0/N) * j + 1;
451	f = g - F;
452
453	g = 1/(2*F+f);
454	u = 2*f*g;
455	v = u*u;
456	q = u*v*(A1 + v*(A2 + v*(A3 + v*A4)));
457	if (m | j)
458		u1 = u + 513, u1 -= 513;
459	else
460		u1 = u, TRUNC(u1);
461	u2 = (2.0*(f - F*u1) - u1*f) * g;
462
463	u1 += m*logF_head[N] + logF_head[j];
464
465	u2 +=  logF_tail[j]; u2 += q;
466	u2 += logF_tail[N]*m;
467	r.a = u1 + u2;			/* Only difference is here */
468	TRUNC(r.a);
469	r.b = (u1 - r.a) + u2;
470	return (r);
471}
472