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WebSVN – planix.SVN – Blame – /os/branches/feature_unix/sys/src/libc/port/log.c – Rev 2

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/*
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	log returns the natural logarithm of its floating
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	point argument.
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	The coefficients are #2705 from Hart & Cheney. (19.38D)
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	It calls frexp.
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*/
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#include <u.h>
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#include <libc.h>
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#define	log2    0.693147180559945309e0
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#define	ln10o1  .4342944819032518276511
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#define	sqrto2  0.707106781186547524e0
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#define	p0      -.240139179559210510e2
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#define	p1      0.309572928215376501e2
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#define	p2      -.963769093377840513e1
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#define	p3      0.421087371217979714e0
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#define	q0      -.120069589779605255e2
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#define	q1      0.194809660700889731e2
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#define	q2      -.891110902798312337e1
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double
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log(double arg)
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{
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	double x, z, zsq, temp;
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	int exp;
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	if(arg <= 0)
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		return NaN();
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	x = frexp(arg, &exp);
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	while(x < 0.5) {
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		x *= 2;
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		exp--;
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	}
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	if(x < sqrto2) {
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		x *= 2;
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		exp--;
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	}
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	z = (x-1) / (x+1);
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	zsq = z*z;
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	temp = ((p3*zsq + p2)*zsq + p1)*zsq + p0;
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	temp = temp/(((zsq + q2)*zsq + q1)*zsq + q0);
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	temp = temp*z + exp*log2;
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	return temp;
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}
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double
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log10(double arg)
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{
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	if(arg <= 0)
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		return NaN();
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	return log(arg) * ln10o1;
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}