xref: /src/lib/libm/src/s_cbrt.c

/* @(#)s_cbrt.c 5.1 93/09/24 */
/*
 * ====================================================
 * 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 "fdlibm.h"

/* cbrt(x)
 * Return cube root of x
 */
#ifdef __STDC__
static const unsigned
#else
static unsigned
#endif
	B1 = 715094163, /* B1 = (682-0.03306235651)*2**20 */
	B2 = 696219795; /* B2 = (664-0.03306235651)*2**20 */

#ifdef __STDC__
static const double
#else
static double
#endif
C =  5.42857142857142815906e-01, /* 19/35     = 0x3FE15F15, 0xF15F15F1 */
D = -7.05306122448979611050e-01, /* -864/1225 = 0xBFE691DE, 0x2532C834 */
E =  1.41428571428571436819e+00, /* 99/70     = 0x3FF6A0EA, 0x0EA0EA0F */
F =  1.60714285714285720630e+00, /* 45/28     = 0x3FF9B6DB, 0x6DB6DB6E */
G =  3.57142857142857150787e-01; /* 5/14      = 0x3FD6DB6D, 0xB6DB6DB7 */

#ifdef __STDC__
	double cbrt(double x)
#else
	double cbrt(x)
	double x;
#endif
{
	double	one	= 1.0;
	int	n0,hx;
	double r,s,t=0.0,w;
	unsigned *pt = (unsigned *) &t, sign;


	n0 = ((*(int*)&one)>>29)^1;	/* index of high word */
	hx = *( n0 + (int*)&x);		/* high word of x */
	sign=hx&0x80000000; 		/* sign= sign(x) */
	hx  ^=sign;
	if(hx>=0x7ff00000) return(x+x); /* cbrt(NaN,INF) is itself */
	if((hx|*(1-n0+(int*)&x))==0)
	    return(x);		/* cbrt(0) is itself */

	*(n0+(int*)&x) = hx;	/* x <- |x| */
    /* rough cbrt to 5 bits */
	if(hx<0x00100000) 		/* subnormal number */
	  {pt[n0]=0x43500000; 		/* set t= 2**54 */
	   t*=x; pt[n0]=pt[n0]/3+B2;
	  }
	else
	  pt[n0]=hx/3+B1;


    /* new cbrt to 23 bits, may be implemented in single precision */
	r=t*t/x;
	s=C+r*t;
	t*=G+F/(s+E+D/s);

    /* chopped to 20 bits and make it larger than cbrt(x) */
	pt[1-n0]=0; pt[n0]+=0x00000001;


    /* one step newton iteration to 53 bits with error less than 0.667 ulps */
	s=t*t;		/* t*t is exact */
	r=x/s;
	w=t+t;
	r=(r-t)/(w+r);	/* r-s is exact */
	t=t+t*r;

    /* retore the sign bit */
	pt[n0] |= sign;
	return(t);
}