* Copyright (c) 1985 Regents of the University of California.
* %sccs.include.redist.c%
static char sccsid
[] = "@(#)cosh.c 5.6 (Berkeley) %G%";
* 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 :
* Required kernel function:
* exp__E(x,c) ...return exp(x+c)-1-x for |x|<0.3465
* 0 <= x <= 0.3465 : cosh(x) := 1 + -------------------
* 0.3465 <= x <= 22 : cosh(x) := -------------------
* 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.
* cosh(x) is x if x is +INF, -INF, or NaN.
* only cosh(0)=1 is exact for finite x.
* 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).
* 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
vc(mln2hi
, 8.8029691931113054792E1
,0f33
,43b0
,2bdb
,c7e2
, 7, .B00F33C7E22BDB
)
vc(mln2lo
,-4.9650192275318476525E-16 ,1b60
,a70f
,582a
,279e
, -50,-.8F1B60279E582A
)
vc(lnovfl
, 8.8029691931113053016E1
,0f33
,43b0
,2bda
,c7e2
, 7, .B00F33C7E22BDA
)
ic(mln2hi
, 7.0978271289338397310E2
, 10, 1.62E42FEFA39EF
)
ic(mln2lo
, 2.3747039373786107478E-14, -45, 1.ABC9E3B39803F
)
ic(lnovfl
, 7.0978271289338397310E2
, 9, 1.62E42FEFA39EF
)
#define mln2hi vccast(mln2hi)
#define mln2lo vccast(mln2lo)
#define lnovfl vccast(lnovfl)
#if defined(vax)||defined(tahoe)
#else /* defined(vax)||defined(tahoe) */
#endif /* defined(vax)||defined(tahoe) */
static const double half
=1.0/2.0,
one
=1.0, small
=1.0E-18; /* fl(1+small)==1 */
#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
<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^(max+1))
* and return 2^max*exp(x) to avoid unnecessary overflow
return(scalb(exp((x
-mln2hi
)-mln2lo
), max
));
return(exp(x
)*half
); /* for large x, cosh(x)=exp(x)/2 */