#include "libm.h"

/* atanh(x) = log((1+x)/(1-x))/2 = log1p(2x/(1-x))/2 ~= x + x^3/3 + o(x^5) */
float atanhf(float x)
{
        union {float f; uint32_t i;} u = {.f = x};
        unsigned s = u.i >> 31;
        float_t y;

        /* |x| */
        u.i &= 0x7fffffff;
        y = u.f;

        if (u.i < 0x3f800000 - (1<<23)) {
                if (u.i < 0x3f800000 - (32<<23)) {
                        /* handle underflow */
                        if (u.i < (1<<23))
                                FORCE_EVAL((float)(y*y));
                } else {
                        /* |x| < 0.5, up to 1.7ulp error */
                        y = 0.5f*log1pf(2*y + 2*y*y/(1-y));
                }
        } else {
                /* avoid overflow */
                y = 0.5f*log1pf(2*(y/(1-y)));
        }
        return s ? -y : y;
}