github.com/afumu/libc@v0.0.6/musl/src/math/asinf.c (about)

     1  /* origin: FreeBSD /usr/src/lib/msun/src/e_asinf.c */
     2  /*
     3   * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
     4   */
     5  /*
     6   * ====================================================
     7   * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
     8   *
     9   * Developed at SunPro, a Sun Microsystems, Inc. business.
    10   * Permission to use, copy, modify, and distribute this
    11   * software is freely granted, provided that this notice
    12   * is preserved.
    13   * ====================================================
    14   */
    15  #include "libm.h"
    16  
    17  static const double
    18  pio2 = 1.570796326794896558e+00;
    19  
    20  static const float
    21  /* coefficients for R(x^2) */
    22  pS0 =  1.6666586697e-01,
    23  pS1 = -4.2743422091e-02,
    24  pS2 = -8.6563630030e-03,
    25  qS1 = -7.0662963390e-01;
    26  
    27  static float R(float z)
    28  {
    29  	float_t p, q;
    30  	p = z*(pS0+z*(pS1+z*pS2));
    31  	q = 1.0f+z*qS1;
    32  	return p/q;
    33  }
    34  
    35  float asinf(float x)
    36  {
    37  	double s;
    38  	float z;
    39  	uint32_t hx,ix;
    40  
    41  	GET_FLOAT_WORD(hx, x);
    42  	ix = hx & 0x7fffffff;
    43  	if (ix >= 0x3f800000) {  /* |x| >= 1 */
    44  		if (ix == 0x3f800000)  /* |x| == 1 */
    45  			return x*pio2 + 0x1p-120f;  /* asin(+-1) = +-pi/2 with inexact */
    46  		return 0/(x-x);  /* asin(|x|>1) is NaN */
    47  	}
    48  	if (ix < 0x3f000000) {  /* |x| < 0.5 */
    49  		/* if 0x1p-126 <= |x| < 0x1p-12, avoid raising underflow */
    50  		if (ix < 0x39800000 && ix >= 0x00800000)
    51  			return x;
    52  		return x + x*R(x*x);
    53  	}
    54  	/* 1 > |x| >= 0.5 */
    55  	z = (1 - fabsf(x))*0.5f;
    56  	s = sqrt(z);
    57  	x = pio2 - 2*(s+s*R(z));
    58  	if (hx >> 31)
    59  		return -x;
    60  	return x;
    61  }