73 lines
1.6 KiB
C
73 lines
1.6 KiB
C
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/* @(#)z_logarithmf.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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* 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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static const float a[] = { -0.5527074855 };
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static const float b[] = { -0.6632718214e+1 };
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static const float C1 = 0.693145752;
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static const float C2 = 1.428606820e-06;
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static const float C3 = 0.4342944819;
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float
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_DEFUN (logarithmf, (float, int),
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float x _AND
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int ten)
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{
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int N;
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float 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_f.f);
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}
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/* Get the exponent and mantissa where x = f * 2^N. */
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f = frexpf (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[0]) / ((w + 1.0) * 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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