/* $NetBSD: n_log.c,v 1.9 2024/07/16 14:52:50 riastradh 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.
*/
/* 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
*/
/* 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;
__weak_alias(log, _log)
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. */
/*
* Extra precision variant, returning struct {double a, b;};
* log(x) = a+b to 63 bits, with a is rounded to 26 bits.
*/
struct Double
__log__D(double x)
{
int m, j;
double F, f, g, q, u, v, u2;
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);
}
