3 /* atanh(x) = log((1+x)/(1-x))/2 = log1p(2x/(1-x))/2 ~= x + x^3/3 + o(x^5) */
6 union {double f; uint64_t i;} u = {.f = x};
7 unsigned e = u.i >> 52 & 0x7ff;
8 unsigned s = u.i >> 63;
12 u.i &= (uint64_t)-1/2;
17 /* handle underflow */
21 /* |x| < 0.5, up to 1.7ulp error */
22 y = 0.5*log1p(2*y + 2*y*y/(1-y));
26 y = 0.5*log1p(2*(y/(1-y)));