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111 lines
3.0 KiB
C
111 lines
3.0 KiB
C
/* 2009 for Newlib: Sun's s_ilogb.c converted to be s_logb.c. */
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/* @(#)s_ilogb.c 5.1 93/09/24 */
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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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<<logb>>, <<logbf>>---get exponent of floating-point number
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INDEX
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logb
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INDEX
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logbf
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ANSI_SYNOPSIS
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#include <math.h>
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double logb(double <[x]>);
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float logbf(float <[x]>);
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DESCRIPTION
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The <<logb>> functions extract the exponent of <[x]>, as a signed integer value
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in floating-point format. If <[x]> is subnormal it is treated as though it were
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normalized; thus, for positive finite <[x]>,
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@ifnottex
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1 <= (<[x]> * FLT_RADIX to the power (-logb(<[x]>))) < FLT_RADIX.
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@end ifnottex
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@tex
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$1 \leq ( x \cdot FLT\_RADIX ^ {-logb(x)} ) < FLT\_RADIX$.
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@end tex
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A domain error may occur if the argument is zero.
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In this floating-point implementation, FLT_RADIX is 2. Which also means
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that for finite <[x]>, <<logb>>(<[x]>) = <<floor>>(<<log2>>(<<fabs>>(<[x]>))).
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All nonzero, normal numbers can be described as
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@ifnottex
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<[m]> * 2**<[p]>, where 1.0 <= <[m]> < 2.0.
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@end ifnottex
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@tex
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$m \cdot 2^p$, where $1.0 \leq m < 2.0$.
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@end tex
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The <<logb>> functions examine the argument <[x]>, and return <[p]>.
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The <<frexp>> functions are similar to the <<logb>> functions, but
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returning <[m]> adjusted to the interval [.5, 1) or 0, and <[p]>+1.
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RETURNS
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@comment Formatting note: "$@" forces a new line
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When <[x]> is:@*
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+inf or -inf, +inf is returned;@*
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NaN, NaN is returned;@*
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0, -inf is returned, and the divide-by-zero exception is raised;@*
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otherwise, the <<logb>> functions return the signed exponent of <[x]>.
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PORTABILITY
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ANSI C, POSIX
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SEEALSO
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frexp, ilogb
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*/
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/* double logb(double x)
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* return the binary exponent of non-zero x
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* logb(0) = -inf, raise divide-by-zero floating point exception
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* logb(+inf|-inf) = +inf (no signal is raised)
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* logb(NaN) = NaN (no signal is raised)
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* Per C99 recommendation, a NaN argument is returned unchanged.
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*/
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#include "fdlibm.h"
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#ifndef _DOUBLE_IS_32BITS
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double
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#ifdef __STDC__
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logb(double x)
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#else
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logb(x)
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double x;
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#endif
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{
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__int32_t hx,lx,ix;
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EXTRACT_WORDS(hx,lx,x);
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hx &= 0x7fffffff; /* high |x| */
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if(hx<0x00100000) { /* 0 or subnormal */
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if((hx|lx)==0) {
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double xx;
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/* arg==0: return -inf and raise divide-by-zero exception */
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INSERT_WORDS(xx,hx,lx); /* +0.0 */
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return -1./xx; /* logb(0) = -inf */
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}
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else /* subnormal x */
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if(hx==0) {
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for (ix = -1043; lx>0; lx<<=1) ix -=1;
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} else {
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for (ix = -1022,hx<<=11; hx>0; hx<<=1) ix -=1;
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}
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return (double) ix;
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}
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else if (hx<0x7ff00000) return (hx>>20)-1023; /* normal # */
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else if (hx>0x7ff00000 || lx) return x; /* x==NaN */
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else return HUGE_VAL; /* x==inf (+ or -) */
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}
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#endif /* _DOUBLE_IS_32BITS */
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