/* $NetBSD: n_cosh.c,v 1.9 2014/10/10 20:58:09 martin 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
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*/
/* COSH(X)
* RETURN THE HYPERBOLIC COSINE OF X
* DOUBLE PRECISION (VAX D format 56 bits, IEEE DOUBLE 53 BITS)
* CODED IN C BY K.C. NG, 1/8/85;
* REVISED BY K.C. NG on 2/8/85, 2/23/85, 3/7/85, 3/29/85, 4/16/85.
*
* Required system supported functions :
* copysign(x,y)
* scalb(x,N)
*
* Required kernel function:
* exp(x)
* exp__E(x,c) ...return exp(x+c)-1-x for |x|<0.3465
*
* Method :
* 1. Replace x by |x|.
* 2.
* [ exp(x) - 1 ]^2
* 0 <= x <= 0.3465 : cosh(x) := 1 + -------------------
* 2*exp(x)
*
* exp(x) + 1/exp(x)
* 0.3465 <= x <= 22 : cosh(x) := -------------------
* 2
* 22 <= x <= lnovfl : cosh(x) := exp(x)/2
* lnovfl <= x <= lnovfl+log(2)
* : cosh(x) := exp(x)/2 (avoid overflow)
* log(2)+lnovfl < x < INF: overflow to INF
*
* Note: .3465 is a number near one half of ln2.
*
* Special cases:
* cosh(x) is x if x is +INF, -INF, or NaN.
* only cosh(0)=1 is exact for finite x.
*
* Accuracy:
* cosh(x) returns the exact hyperbolic cosine of x nearly rounded.
* In a test run with 768,000 random arguments on a VAX, the maximum
* observed error was 1.23 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((x=copysign(x,one)) <= 22) {
if(x<0.3465) {
if(x<small) { return(one+x); }
else {t=x+__exp__E(x,0.0);x=t+t; return(one+t*t/(2.0+x)); }
} else /* for x lies in [0.3465,22] */
{ t=exp(x); return((t+one/t)*half); }
}
if( lnovfl <= x && x <= (lnovfl+0.7))
/* for x lies in [lnovfl, lnovfl+ln2], decrease x by ln(2^(EXPMAX+1))
* and return 2^EXPMAX*exp(x) to avoid unnecessary overflow
*/
return(scalb(exp((x-mln2hi)-mln2lo), EXPMAX));
else
return(exp(x)*half); /* for large x, cosh(x)=exp(x)/2 */
}
