source: asp3_tinet_ecnl_rx/trunk/musl-1.1.18/src/math/atanhl.c@ 337

#include "libm.h"

#if LDBL_MANT_DIG == 53 && LDBL_MAX_EXP == 1024
long double atanhl(long double x)
{
        return atanh(x);
}
#elif (LDBL_MANT_DIG == 64 || LDBL_MANT_DIG == 113) && LDBL_MAX_EXP == 16384
/* atanh(x) = log((1+x)/(1-x))/2 = log1p(2x/(1-x))/2 ~= x + x^3/3 + o(x^5) */
long double atanhl(long double x)
{
        union ldshape u = {x};
        unsigned e = u.i.se & 0x7fff;
        unsigned s = u.i.se >> 15;

        /* |x| */
        u.i.se = e;
        x = u.f;

        if (e < 0x3ff - 1) {
                if (e < 0x3ff - LDBL_MANT_DIG/2) {
                        /* handle underflow */
                        if (e == 0)
                                FORCE_EVAL((float)x);
                } else {
                        /* |x| < 0.5, up to 1.7ulp error */
                        x = 0.5*log1pl(2*x + 2*x*x/(1-x));
                }
        } else {
                /* avoid overflow */
                x = 0.5*log1pl(2*(x/(1-x)));
        }
        return s ? -x : x;
}
#endif