usr/src/lib/libm/ieee/cbrt.c

/*
 * Copyright (c) 1985 Regents of the University of California.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms are permitted
 * provided that the above copyright notice and this paragraph are
 * duplicated in all such forms and that any documentation,
 * advertising materials, and other materials related to such
 * distribution and use acknowledge that the software was developed
 * by the University of California, Berkeley.  The name of the
 * University may not be used to endorse or promote products derived
 * from this software without specific prior written permission.
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND WITHOUT ANY EXPRESS OR
 * IMPLIED WARRANTIES, INCLUDING, WITHOUT LIMITATION, THE IMPLIED
 * WARRANTIES OF MERCHANTIBILITY AND FITNESS FOR A PARTICULAR PURPOSE.
 *
 * All recipients should regard themselves as participants in an ongoing
 * research project and hence should feel obligated to report their
 * experiences (good or bad) with these elementary function codes, using
 * the sendbug(8) program, to the authors.
 */

#ifndef lint
static char sccsid[] = "@(#)cbrt.c      5.5 (Berkeley) %G%";
#endif /* not lint */

/* kahan's cube root (53 bits IEEE double precision)
 * for IEEE machines only
 * coded in C by K.C. Ng, 4/30/85
 *
 * Accuracy:
 *      better than 0.667 ulps according to an error analysis. Maximum
 * error observed was 0.666 ulps in an 1,000,000 random arguments test.
 *
 * Warning: this code is semi machine dependent; the ordering of words in
 * a floating point number must be known in advance. I assume that the
 * long interger at the address of a floating point number will be the
 * leading 32 bits of that floating point number (i.e., sign, exponent,
 * and the 20 most significant bits).
 * On a National machine, it has different ordering; therefore, this code
 * must be compiled with flag -DNATIONAL.
 */
#if !defined(vax)&&!defined(tahoe)

static const unsigned long
                     B1 = 715094163, /* B1 = (682-0.03306235651)*2**20 */
                     B2 = 696219795; /* B2 = (664-0.03306235651)*2**20 */
static const double
            C= 19./35.,
            D= -864./1225.,
            E= 99./70.,
            F= 45./28.,
            G= 5./14.;

double cbrt(x)
double x;
{
        double r,s,t=0.0,w;
        unsigned long *px = (unsigned long *) &x,
                      *pt = (unsigned long *) &t,
                      mexp,sign;

#ifdef national /* ordering of words in a floating points number */
        const int n0=1,n1=0;
#else   /* national */
        const int n0=0,n1=1;
#endif  /* national */

        mexp=px[n0]&0x7ff00000;
        if(mexp==0x7ff00000) return(x); /* cbrt(NaN,INF) is itself */
        if(x==0.0) return(x);           /* cbrt(0) is itself */

        sign=px[n0]&0x80000000; /* sign= sign(x) */
        px[n0] ^= sign;         /* x=|x| */


    /* rough cbrt to 5 bits */
        if(mexp==0)             /* subnormal number */
          {pt[n0]=0x43500000;   /* set t= 2**54 */
           t*=x; pt[n0]=pt[n0]/3+B2;
          }
        else
          pt[n0]=px[n0]/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[n1]=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-t is exact */
        t=t+t*r;


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