137 lines
3.2 KiB
C
137 lines
3.2 KiB
C
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/* @(#)z_logarithm.c 1.0 98/08/13 */
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/******************************************************************
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* The following routines are coded directly from the algorithms
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* and coefficients given in "Software Manual for the Elementary
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* Functions" by William J. Cody, Jr. and William Waite, Prentice
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* Hall, 1980.
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******************************************************************/
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/*
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FUNCTION
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<<log>>, <<logf>>, <<log10>>, <<log10f>>, <<logarithm>>, <<logarithmf>>---natural or base 10 logarithms
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INDEX
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log
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INDEX
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logf
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INDEX
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log10
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INDEX
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log10f
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ANSI_SYNOPSIS
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#include <math.h>
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double log(double <[x]>);
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float logf(float <[x]>);
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double log10(double <[x]>);
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float log10f(float <[x]>);
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TRAD_SYNOPSIS
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#include <math.h>
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double log(<[x]>);
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double <[x]>;
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float logf(<[x]>);
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float <[x]>;
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double log10(<[x]>);
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double <[x]>;
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float log10f(<[x]>);
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float <[x]>;
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DESCRIPTION
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Return the natural or base 10 logarithm of <[x]>, that is, its logarithm base e
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(where e is the base of the natural system of logarithms, 2.71828@dots{}) or
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base 10.
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<<log>> and <<logf>> are identical save for the return and argument types.
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<<log10>> and <<log10f>> are identical save for the return and argument types.
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RETURNS
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Normally, returns the calculated value. When <[x]> is zero, the
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returned value is <<-HUGE_VAL>> and <<errno>> is set to <<ERANGE>>.
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When <[x]> is negative, the returned value is <<-HUGE_VAL>> and
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<<errno>> is set to <<EDOM>>. You can control the error behavior via
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<<matherr>>.
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PORTABILITY
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<<log>> is ANSI. <<logf>> is an extension.
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<<log10>> is ANSI. <<log10f>> is an extension.
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*/
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/******************************************************************
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* Logarithm
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*
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* Input:
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* x - floating point value
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* ten - indicates base ten numbers
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*
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* Output:
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* logarithm of x
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*
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* Description:
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* This routine calculates logarithms.
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*
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*****************************************************************/
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#include "fdlibm.h"
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#include "zmath.h"
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#ifndef _DOUBLE_IS_32BITS
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static const double a[] = { -0.64124943423745581147e+02,
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0.16383943563021534222e+02,
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-0.78956112887481257267 };
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static const double b[] = { -0.76949932108494879777e+03,
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0.31203222091924532844e+03,
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-0.35667977739034646171e+02 };
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static const double C1 = 22713.0 / 32768.0;
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static const double C2 = 1.428606820309417232e-06;
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static const double C3 = 0.43429448190325182765;
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double
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_DEFUN (logarithm, (double, int),
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double x _AND
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int ten)
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{
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int N;
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double f, w, z;
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/* Check for domain error here. */
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if (x <= 0.0)
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{
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errno = ERANGE;
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return (z_notanum.d);
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}
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/* Get the exponent and mantissa where x = f * 2^N. */
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f = frexp (x, &N);
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z = f - 0.5;
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if (f > __SQRT_HALF)
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z = (z - 0.5) / (f * 0.5 + 0.5);
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else
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{
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N--;
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z /= (z * 0.5 + 0.5);
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}
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w = z * z;
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/* Use Newton's method with 4 terms. */
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z += z * w * ((a[2] * w + a[1]) * w + a[0]) / (((w + b[2]) * w + b[1]) * w + b[0]);
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if (N != 0)
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z = (N * C2 + z) + N * C1;
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if (ten)
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z *= C3;
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return (z);
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}
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#endif /* _DOUBLE_IS_32BITS */
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