source: vendor/FreeBSD-msun/5.x-2005-06-xx/src/e_acosh.c

/* @(#)e_acosh.c 1.3 95/01/18 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunSoft, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice 
 * is preserved.
 * ====================================================
 *
 */

#ifndef lint
static char rcsid[] = "$FreeBSD: src/lib/msun/src/e_acosh.c,v 1.8 2005/02/04 18:26:05 das Exp $";
#endif

/* __ieee754_acosh(x)
 * Method :
 *      Based on 
 *              acosh(x) = log [ x + sqrt(x*x-1) ]
 *      we have
 *              acosh(x) := log(x)+ln2, if x is large; else
 *              acosh(x) := log(2x-1/(sqrt(x*x-1)+x)) if x>2; else
 *              acosh(x) := log1p(t+sqrt(2.0*t+t*t)); where t=x-1.
 *
 * Special cases:
 *      acosh(x) is NaN with signal if x<1.
 *      acosh(NaN) is NaN without signal.
 */

#include "math.h"
#include "math_private.h"

static const double
one     = 1.0,
ln2     = 6.93147180559945286227e-01;  /* 0x3FE62E42, 0xFEFA39EF */

double
__ieee754_acosh(double x)
{
        double t;
        int32_t hx;
        u_int32_t lx;
        EXTRACT_WORDS(hx,lx,x);
        if(hx<0x3ff00000) {             /* x < 1 */
            return (x-x)/(x-x);
        } else if(hx >=0x41b00000) {    /* x > 2**28 */
            if(hx >=0x7ff00000) {       /* x is inf of NaN */
                return x+x;
            } else 
                return __ieee754_log(x)+ln2;    /* acosh(huge)=log(2x) */
        } else if(((hx-0x3ff00000)|lx)==0) {
            return 0.0;                 /* acosh(1) = 0 */
        } else if (hx > 0x40000000) {   /* 2**28 > x > 2 */
            t=x*x;
            return __ieee754_log(2.0*x-one/(x+sqrt(t-one)));
        } else {                        /* 1<x<2 */
            t = x-one;
            return log1p(t+sqrt(2.0*t+t*t));
        }
}