/* $OpenBSD: n_log.c,v 1.7 2008/07/16 15:25:51 martynas Exp $ */ /* * Copyright (c) 1992, 1993 * The Regents of the University of California. All rights reserved. * * Redistribution and use in source and binary forms, with or without * modification, are permitted provided that the following conditions * are met: * 1. Redistributions of source code must retain the above copyright * notice, this list of conditions and the following disclaimer. * 2. Redistributions in binary form must reproduce the above copyright * notice, this list of conditions and the following disclaimer in the * documentation and/or other materials provided with the distribution. * 3. Neither the name of the University nor the names of its contributors * may be used to endorse or promote products derived from this software * without specific prior written permission. * * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE * ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF * SUCH DAMAGE. */ #ifndef lint static char sccsid[] = "@(#)log.c 8.2 (Berkeley) 11/30/93"; #endif /* not lint */ #include #include #include "mathimpl.h" /* Table-driven natural logarithm. * * This code was derived, with minor modifications, from: * Peter Tang, "Table-Driven Implementation of the * Logarithm in IEEE Floating-Point arithmetic." ACM Trans. * Math Software, vol 16. no 4, pp 378-400, Dec 1990). * * Calculates log(2^m*F*(1+f/F)), |f/j| <= 1/256, * where F = j/128 for j an integer in [0, 128]. * * log(2^m) = log2_hi*m + log2_tail*m * since m is an integer, the dominant term is exact. * m has at most 10 digits (for subnormal numbers), * and log2_hi has 11 trailing zero bits. * * log(F) = logF_hi[j] + logF_lo[j] is in tabular form in log_table.h * logF_hi[] + 512 is exact. * * log(1+f/F) = 2*f/(2*F + f) + 1/12 * (2*f/(2*F + f))**3 + ... * the leading term is calculated to extra precision in two * parts, the larger of which adds exactly to the dominant * m and F terms. * There are two cases: * 1. when m, j are non-zero (m | j), use absolute * precision for the leading term. * 2. when m = j = 0, |1-x| < 1/256, and log(x) ~= (x-1). * In this case, use a relative precision of 24 bits. * (This is done differently in the original paper) * * Special cases: * 0 return signalling -Inf * neg return signalling NaN * +Inf return +Inf */ #if defined(__vax__) #define _IEEE 0 #define TRUNC(x) x = (double) (float) (x) #else #define _IEEE 1 #define endian (((*(int *) &one)) ? 1 : 0) #define TRUNC(x) *(((int *) &x) + endian) &= 0xf8000000 #define infnan(x) 0.0 #endif #define N 128 /* Table of log(Fj) = logF_head[j] + logF_tail[j], for Fj = 1+j/128. * Used for generation of extend precision logarithms. * The constant 35184372088832 is 2^45, so the divide is exact. * It ensures correct reading of logF_head, even for inaccurate * decimal-to-binary conversion routines. (Everybody gets the * right answer for integers less than 2^53.) * Values for log(F) were generated using error < 10^-57 absolute * with the bc -l package. */ static const double A1 = .08333333333333178827; static const double A2 = .01250000000377174923; static const double A3 = .002232139987919447809; static const double A4 = .0004348877777076145742; static const double logF_head[N+1] = { 0., .007782140442060381246, .015504186535963526694, .023167059281547608406, .030771658666765233647, .038318864302141264488, .045809536031242714670, .053244514518837604555, .060624621816486978786, .067950661908525944454, .075223421237524235039, .082443669210988446138, .089612158689760690322, .096729626458454731618, .103796793681567578460, .110814366340264314203, .117783035656430001836, .124703478501032805070, .131576357788617315236, .138402322859292326029, .145182009844575077295, .151916042025732167530, .158605030176659056451, .165249572895390883786, .171850256926518341060, .178407657472689606947, .184922338493834104156, .191394852999565046047, .197825743329758552135, .204215541428766300668, .210564769107350002741, .216873938300523150246, .223143551314024080056, .229374101064877322642, .235566071312860003672, .241719936886966024758, .247836163904594286577, .253915209980732470285, .259957524436686071567, .265963548496984003577, .271933715484010463114, .277868451003087102435, .283768173130738432519, .289633292582948342896, .295464212893421063199, .301261330578199704177, .307025035294827830512, .312755710004239517729, .318453731118097493890, .324119468654316733591, .329753286372579168528, .335355541920762334484, .340926586970454081892, .346466767346100823488, .351976423156884266063, .357455888922231679316, .362905493689140712376, .368325561158599157352, .373716409793814818840, .379078352934811846353, .384411698910298582632, .389716751140440464951, .394993808240542421117, .400243164127459749579, .405465108107819105498, .410659924985338875558, .415827895143593195825, .420969294644237379543, .426084395310681429691, .431173464818130014464, .436236766774527495726, .441274560805140936281, .446287102628048160113, .451274644139630254358, .456237433481874177232, .461175715122408291790, .466089729924533457960, .470979715219073113985, .475845904869856894947, .480688529345570714212, .485507815781602403149, .490303988045525329653, .495077266798034543171, .499827869556611403822, .504556010751912253908, .509261901790523552335, .513945751101346104405, .518607764208354637958, .523248143765158602036, .527867089620485785417, .532464798869114019908, .537041465897345915436, .541597282432121573947, .546132437597407260909, .550647117952394182793, .555141507540611200965, .559615787935399566777, .564070138285387656651, .568504735352689749561, .572919753562018740922, .577315365035246941260, .581691739635061821900, .586049045003164792433, .590387446602107957005, .594707107746216934174, .599008189645246602594, .603290851438941899687, .607555250224322662688, .611801541106615331955, .616029877215623855590, .620240409751204424537, .624433288012369303032, .628608659422752680256, .632766669570628437213, .636907462236194987781, .641031179420679109171, .645137961373620782978, .649227946625615004450, .653301272011958644725, .657358072709030238911, .661398482245203922502, .665422632544505177065, .669430653942981734871, .673422675212350441142, .677398823590920073911, .681359224807238206267, .685304003098281100392, .689233281238557538017, .693147180560117703862 }; static const double logF_tail[N+1] = { 0., -.00000000000000543229938420049, .00000000000000172745674997061, -.00000000000001323017818229233, -.00000000000001154527628289872, -.00000000000000466529469958300, .00000000000005148849572685810, -.00000000000002532168943117445, -.00000000000005213620639136504, -.00000000000001819506003016881, .00000000000006329065958724544, .00000000000008614512936087814, -.00000000000007355770219435028, .00000000000009638067658552277, .00000000000007598636597194141, .00000000000002579999128306990, -.00000000000004654729747598444, -.00000000000007556920687451336, .00000000000010195735223708472, -.00000000000017319034406422306, -.00000000000007718001336828098, .00000000000010980754099855238, -.00000000000002047235780046195, -.00000000000008372091099235912, .00000000000014088127937111135, .00000000000012869017157588257, .00000000000017788850778198106, .00000000000006440856150696891, .00000000000016132822667240822, -.00000000000007540916511956188, -.00000000000000036507188831790, .00000000000009120937249914984, .00000000000018567570959796010, -.00000000000003149265065191483, -.00000000000009309459495196889, .00000000000017914338601329117, -.00000000000001302979717330866, .00000000000023097385217586939, .00000000000023999540484211737, .00000000000015393776174455408, -.00000000000036870428315837678, .00000000000036920375082080089, -.00000000000009383417223663699, .00000000000009433398189512690, .00000000000041481318704258568, -.00000000000003792316480209314, .00000000000008403156304792424, -.00000000000034262934348285429, .00000000000043712191957429145, -.00000000000010475750058776541, -.00000000000011118671389559323, .00000000000037549577257259853, .00000000000013912841212197565, .00000000000010775743037572640, .00000000000029391859187648000, -.00000000000042790509060060774, .00000000000022774076114039555, .00000000000010849569622967912, -.00000000000023073801945705758, .00000000000015761203773969435, .00000000000003345710269544082, -.00000000000041525158063436123, .00000000000032655698896907146, -.00000000000044704265010452446, .00000000000034527647952039772, -.00000000000007048962392109746, .00000000000011776978751369214, -.00000000000010774341461609578, .00000000000021863343293215910, .00000000000024132639491333131, .00000000000039057462209830700, -.00000000000026570679203560751, .00000000000037135141919592021, -.00000000000017166921336082431, -.00000000000028658285157914353, -.00000000000023812542263446809, .00000000000006576659768580062, -.00000000000028210143846181267, .00000000000010701931762114254, .00000000000018119346366441110, .00000000000009840465278232627, -.00000000000033149150282752542, -.00000000000018302857356041668, -.00000000000016207400156744949, .00000000000048303314949553201, -.00000000000071560553172382115, .00000000000088821239518571855, -.00000000000030900580513238244, -.00000000000061076551972851496, .00000000000035659969663347830, .00000000000035782396591276383, -.00000000000046226087001544578, .00000000000062279762917225156, .00000000000072838947272065741, .00000000000026809646615211673, -.00000000000010960825046059278, .00000000000002311949383800537, -.00000000000058469058005299247, -.00000000000002103748251144494, -.00000000000023323182945587408, -.00000000000042333694288141916, -.00000000000043933937969737844, .00000000000041341647073835565, .00000000000006841763641591466, .00000000000047585534004430641, .00000000000083679678674757695, -.00000000000085763734646658640, .00000000000021913281229340092, -.00000000000062242842536431148, -.00000000000010983594325438430, .00000000000065310431377633651, -.00000000000047580199021710769, -.00000000000037854251265457040, .00000000000040939233218678664, .00000000000087424383914858291, .00000000000025218188456842882, -.00000000000003608131360422557, -.00000000000050518555924280902, .00000000000078699403323355317, -.00000000000067020876961949060, .00000000000016108575753932458, .00000000000058527188436251509, -.00000000000035246757297904791, -.00000000000018372084495629058, .00000000000088606689813494916, .00000000000066486268071468700, .00000000000063831615170646519, .00000000000025144230728376072, -.00000000000017239444525614834 }; double log(double x) { int m, j; double F, f, g, q, u, u2, v, zero = 0.0, one = 1.0; volatile double u1; /* Catch special cases */ if (x <= 0) if (_IEEE && x == zero) /* log(0) = -Inf */ return (-one/zero); else if (_IEEE) /* log(neg) = NaN */ return (zero/zero); else if (x == zero) /* NOT REACHED IF _IEEE */ return (infnan(-ERANGE)); else return (infnan(EDOM)); else if (!finite(x)) if (_IEEE) /* x = NaN, Inf */ return (x+x); else return (infnan(ERANGE)); /* Argument reduction: 1 <= g < 2; x/2^m = g; */ /* y = F*(1 + f/F) for |f| <= 2^-8 */ m = logb(x); g = ldexp(x, -m); if (_IEEE && m == -1022) { j = logb(g); m += j; g = ldexp(g, -j); } j = N*(g-1) + .5; F = (1.0/N) * j + 1; /* F*128 is an integer in [128, 512] */ f = g - F; /* Approximate expansion for log(1+f/F) ~= u + q */ g = 1/(2*F+f); u = 2*f*g; v = u*u; q = u*v*(A1 + v*(A2 + v*(A3 + v*A4))); /* case 1: u1 = u rounded to 2^-43 absolute. Since u < 2^-8, * u1 has at most 35 bits, and F*u1 is exact, as F has < 8 bits. * It also adds exactly to |m*log2_hi + log_F_head[j] | < 750 */ if (m | j) { u1 = u + 513; u1 -= 513; } /* case 2: |1-x| < 1/256. The m- and j- dependent terms are zero; * u1 = u to 24 bits. */ else { u1 = u; TRUNC(u1); } u2 = (2.0*(f - F*u1) - u1*f) * g; /* u1 + u2 = 2f/(2F+f) to extra precision. */ /* log(x) = log(2^m*F*(1+f/F)) = */ /* (m*log2_hi+logF_head[j]+u1) + (m*log2_lo+logF_tail[j]+q); */ /* (exact) + (tiny) */ u1 += m*logF_head[N] + logF_head[j]; /* exact */ u2 = (u2 + logF_tail[j]) + q; /* tiny */ u2 += logF_tail[N]*m; return (u1 + u2); } /* * Extra precision variant, returning struct {double a, b;}; * log(x) = a+b to 63 bits, with a rounded to 26 bits. */ struct Double __log__D(double x) { int m, j; double F, f, g, q, u, v, u2, one = 1.0; volatile double u1; struct Double r; /* Argument reduction: 1 <= g < 2; x/2^m = g; */ /* y = F*(1 + f/F) for |f| <= 2^-8 */ m = logb(x); g = ldexp(x, -m); if (_IEEE && m == -1022) { j = logb(g); m += j; g = ldexp(g, -j); } j = N*(g-1) + .5; F = (1.0/N) * j + 1; f = g - F; g = 1/(2*F+f); u = 2*f*g; v = u*u; q = u*v*(A1 + v*(A2 + v*(A3 + v*A4))); if (m | j) { u1 = u + 513; u1 -= 513; } else { u1 = u; TRUNC(u1); } u2 = (2.0*(f - F*u1) - u1*f) * g; u1 += m*logF_head[N] + logF_head[j]; u2 += logF_tail[j]; u2 += q; u2 += logF_tail[N]*m; r.a = u1 + u2; /* Only difference is here */ TRUNC(r.a); r.b = (u1 - r.a) + u2; return (r); }