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86 lines
2.1 KiB
C
86 lines
2.1 KiB
C
/* @(#)s_log2.c 5.1 93/09/24 */
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/* Modification from s_exp10.c Yaakov Selkowitz 2009. */
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/*
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* ====================================================
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* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
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*
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* Developed at SunPro, a Sun Microsystems, Inc. business.
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* Permission to use, copy, modify, and distribute this
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* software is freely granted, provided that this notice
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* is preserved.
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* ====================================================
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*/
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/*
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FUNCTION
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<<log2>>, <<log2f>>---base 2 logarithm
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INDEX
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log2
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INDEX
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log2f
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ANSI_SYNOPSIS
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#include <math.h>
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double log2(double <[x]>);
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float log2f(float <[x]>);
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DESCRIPTION
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The <<log2>> functions compute the base-2 logarithm of <[x]>. A domain error
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occurs if the argument is less than zero. A range error occurs if the
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argument is zero.
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The Newlib implementations are not full, intrinisic calculations, but
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rather are derivatives based on <<log>>. (Accuracy might be slightly off from
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a direct calculation.) In addition to functions, they are also implemented as
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macros defined in math.h:
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. #define log2(x) (log (x) / _M_LN2)
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. #define log2f(x) (logf (x) / (float) _M_LN2)
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To use the functions instead, just undefine the macros first.
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You can use the (non-ANSI) function <<matherr>> to specify error
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handling for these functions, indirectly through the respective <<log>>
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function.
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RETURNS
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The <<log2>> functions return
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@ifnottex
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<<log base-2(<[x]>)>>
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@end ifnottex
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@tex
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$log_2(x)$
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@end tex
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on success.
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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 NaN (not a number) 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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C99, POSIX, System V Interface Definition (Issue 6).
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*/
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/*
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* wrapper log2(x)
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*/
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#include "fdlibm.h"
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#include <errno.h>
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#include <math.h>
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#undef log2
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#ifndef _DOUBLE_IS_32BITS
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#ifdef __STDC__
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double log2(double x) /* wrapper log2 */
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#else
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double log2(x) /* wrapper log2 */
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double x;
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#endif
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{
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return (log(x) / M_LN2);
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}
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#endif /* defined(_DOUBLE_IS_32BITS) */
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