/* $NetBSD: n_log1p.c,v 1.9 2024/07/16 14:52:50 riastradh Exp $ */
/*
* Copyright (c) 1985, 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.
*/
/* LOG1P(x)
* RETURN THE LOGARITHM OF 1+x
* DOUBLE PRECISION (VAX D FORMAT 56 bits, IEEE DOUBLE 53 BITS)
* CODED IN C BY K.C. NG, 1/19/85;
* REVISED BY K.C. NG on 2/6/85, 3/7/85, 3/24/85, 4/16/85.
*
* Required system supported functions:
* scalb(x,n)
* copysign(x,y)
* logb(x)
* finite(x)
*
* Required kernel function:
* log__L(z)
*
* Method :
* 1. Argument Reduction: find k and f such that
* 1+x = 2^k * (1+f),
* where sqrt(2)/2 < 1+f < sqrt(2) .
*
* 2. Let s = f/(2+f) ; based on log(1+f) = log(1+s) - log(1-s)
* = 2s + 2/3 s**3 + 2/5 s**5 + .....,
* log(1+f) is computed by
*
* log(1+f) = 2s + s*log__L(s*s)
* where
* log__L(z) = z*(L1 + z*(L2 + z*(... (L6 + z*L7)...)))
*
* See log__L() for the values of the coefficients.
*
* 3. Finally, log(1+x) = k*ln2 + log(1+f).
*
* Remarks 1. In step 3 n*ln2 will be stored in two floating point numbers
* n*ln2hi + n*ln2lo, where ln2hi is chosen such that the last
* 20 bits (for VAX D format), or the last 21 bits ( for IEEE
* double) is 0. This ensures n*ln2hi is exactly representable.
* 2. In step 1, f may not be representable. A correction term c
* for f is computed. It follows that the correction term for
* f - t (the leading term of log(1+f) in step 2) is c-c*x. We
* add this correction term to n*ln2lo to attenuate the error.
*
*
* Special cases:
* log1p(x) is NaN with signal if x < -1; log1p(NaN) is NaN with no signal;
* log1p(INF) is +INF; log1p(-1) is -INF with signal;
* only log1p(0)=0 is exact for finite argument.
*
* Accuracy:
* log1p(x) returns the exact log(1+x) nearly rounded. In a test run
* with 1,536,000 random arguments on a VAX, the maximum observed
* error was .846 ulps (units in the last place).
*
* Constants:
* The hexadecimal values are the intended ones for the following constants.
* The decimal values may be used, provided that the compiler will convert
* from decimal to binary accurately enough to produce the hexadecimal values
* shown.
*/
#if !defined(__vax__)&&!defined(tahoe)
if(x!=x) return(x); /* x is NaN */
#endif /* !defined(__vax__)&&!defined(tahoe) */
if(finite(x)) {
if( x > negone ) {
/* argument reduction */
if(copysign(x,one)<small) return(x);
k=logb(one+x); z=scalb(x,-k); t=scalb(one,-k);
if(z+t >= sqrt2 )
{ k += 1 ; z *= half; t *= half; }
t += negone; x = z + t;
c = (t-x)+z ; /* correction term for x */
/* compute log(1+x) */
s = x/(2+x); t = x*x*half;
c += (k*ln2lo-c*x);
z = c+s*(t+__log__L(s*s));
x += (z - t) ;
return(k*ln2hi+x);
}
/* end of if (x > negone) */
else {
#if defined(__vax__)||defined(tahoe)
if ( x == negone )
return (infnan(-ERANGE)); /* -INF */
else
return (infnan(EDOM)); /* NaN */
#else /* defined(__vax__)||defined(tahoe) */
/* x = -1, return -INF with signal */
if ( x == negone ) return( negone/zero );
/* negative argument for log, return NaN with signal */
else return ( zero / zero );
#endif /* defined(__vax__)||defined(tahoe) */
}
}
/* end of if (finite(x)) */
/* log(-INF) is NaN */
else if(x<0)
return(zero/zero);
