103 lines
2.8 KiB
C
103 lines
2.8 KiB
C
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#ifndef lint
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static char sccsid[] = "@(#)cbrt.c 1.1 94/10/31 SMI"; /* from UCB 4.3 5/23/85 */
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#endif
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/*
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* Copyright (c) 1986 by Sun Microsystems, Inc.
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*/
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/*
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* Copyright (c) 1985 Regents of the University of California.
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*
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* Use and reproduction of this software are granted in accordance with
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* the terms and conditions specified in the Berkeley Software License
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* Agreement (in particular, this entails acknowledgement of the programs'
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* source, and inclusion of this notice) with the additional understanding
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* that all recipients should regard themselves as participants in an
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* ongoing research project and hence should feel obligated to report
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* their experiences (good or bad) with these elementary function codes,
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* using "sendbug 4bsd-bugs@BERKELEY", to the authors.
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*/
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/* kahan's cube root (53 bits IEEE double precision)
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* for IEEE machines only
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* coded in C by K.C. Ng, 4/30/85
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*
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* Accuracy:
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* better than 0.667 ulps according to an error analysis. Maximum
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* error observed was 0.666 ulps in an 1,000,000 random arguments test.
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*
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* Warning: this code is semi machine dependent; the ordering of words in
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* a floating point number must be known in advance. I assume that the
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* long interger at the address of a floating point number will be the
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* leading 32 bits of that floating point number (i.e., sign, exponent,
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* and the 20 most significant bits).
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* On a National machine, it has different ordering; therefore, this code
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* must be compiled with flag -DNATIONAL.
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*/
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#ifndef VAX
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static unsigned long B1 = 715094163, /* B1 = (682-0.03306235651)*2**20 */
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B2 = 696219795; /* B2 = (664-0.03306235651)*2**20 */
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static double
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C= 19./35.,
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D= -864./1225.,
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E= 99./70.,
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F= 45./28.,
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G= 5./14.;
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double cbrt(x)
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double x;
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{
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double one = 1.0;
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int n0;
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double r,s,t=0.0,w;
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unsigned long *px = (unsigned long *) &x,
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*pt = (unsigned long *) &t,
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mexp,sign;
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if ((* (int *) &one) != 0) n0=0; /* not a i386 */
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else n0=1; /* is a i386 */
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mexp=px[n0]&0x7ff00000;
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if(mexp==0x7ff00000) return(x*1.1); /* cbrt(NaN,INF) is itself */
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if(x==0.0) return(x); /* cbrt(0) is itself */
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sign=px[n0]&0x80000000; /* sign= sign(x) */
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px[n0] ^= sign; /* x=|x| */
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/* rough cbrt to 5 bits */
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if(mexp==0) /* subnormal number */
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{pt[n0]=0x43500000; /* set t= 2**54 */
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t*=x; pt[n0]=pt[n0]/3+B2;
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}
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else
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pt[n0]=px[n0]/3+B1;
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/* new cbrt to 23 bits, may be implemented in single precision */
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r=t*t/x;
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s=C+r*t;
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t*=G+F/(s+E+D/s);
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/* chopped to 20 bits and make it larger than cbrt(x) */
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pt[1-n0]=0; pt[n0]+=0x00000001;
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/* one step newton iteration to 53 bits with error less than 0.667 ulps */
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s=t*t; /* t*t is exact */
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r=x/s;
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w=t+t;
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r=(r-t)/(w+r); /* r-s is exact */
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t=t+t*r;
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/* retore the sign bit */
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pt[n0] |= sign;
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return(t);
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}
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#endif
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