source: azure_iot_hub/trunk/musl-1.1.18/src/math/acosf.c@ 388

/* origin: FreeBSD /usr/src/lib/msun/src/e_acosf.c */
/*
 * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
 */
/*
 * ====================================================
 * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
 *
 * Developed at SunPro, a Sun Microsystems, Inc. business.
 * Permission to use, copy, modify, and distribute this
 * software is freely granted, provided that this notice
 * is preserved.
 * ====================================================
 */

#include "libm.h"

static const float
pio2_hi = 1.5707962513e+00, /* 0x3fc90fda */
pio2_lo = 7.5497894159e-08, /* 0x33a22168 */
pS0 =  1.6666586697e-01,
pS1 = -4.2743422091e-02,
pS2 = -8.6563630030e-03,
qS1 = -7.0662963390e-01;

static float R(float z)
{
        float_t p, q;
        p = z*(pS0+z*(pS1+z*pS2));
        q = 1.0f+z*qS1;
        return p/q;
}

float acosf(float x)
{
        float z,w,s,c,df;
        uint32_t hx,ix;

        GET_FLOAT_WORD(hx, x);
        ix = hx & 0x7fffffff;
        /* |x| >= 1 or nan */
        if (ix >= 0x3f800000) {
                if (ix == 0x3f800000) {
                        if (hx >> 31)
                                return 2*pio2_hi + 0x1p-120f;
                        return 0;
                }
                return 0/(x-x);
        }
        /* |x| < 0.5 */
        if (ix < 0x3f000000) {
                if (ix <= 0x32800000) /* |x| < 2**-26 */
                        return pio2_hi + 0x1p-120f;
                return pio2_hi - (x - (pio2_lo-x*R(x*x)));
        }
        /* x < -0.5 */
        if (hx >> 31) {
                z = (1+x)*0.5f;
                s = sqrtf(z);
                w = R(z)*s-pio2_lo;
                return 2*(pio2_hi - (s+w));
        }
        /* x > 0.5 */
        z = (1-x)*0.5f;
        s = sqrtf(z);
        GET_FLOAT_WORD(hx,s);
        SET_FLOAT_WORD(df,hx&0xfffff000);
        c = (z-df*df)/(s+df);
        w = R(z)*s+c;
        return 2*(df+w);
}