1 | /* 2009 for Newlib: Sun's s_ilogb.c converted to be s_logb.c. */ |
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2 | /* @(#)s_ilogb.c 5.1 93/09/24 */ |
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3 | /* |
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4 | * ==================================================== |
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5 | * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. |
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6 | * |
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7 | * Developed at SunPro, a Sun Microsystems, Inc. business. |
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8 | * Permission to use, copy, modify, and distribute this |
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9 | * software is freely granted, provided that this notice |
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10 | * is preserved. |
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11 | * ==================================================== |
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12 | */ |
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13 | /* |
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14 | FUNCTION |
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15 | <<logb>>, <<logbf>>---get exponent of floating-point number |
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16 | INDEX |
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17 | logb |
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18 | INDEX |
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19 | logbf |
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20 | |
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21 | SYNOPSIS |
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22 | #include <math.h> |
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23 | double logb(double <[x]>); |
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24 | float logbf(float <[x]>); |
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25 | |
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26 | DESCRIPTION |
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27 | The <<logb>> functions extract the exponent of <[x]>, as a signed integer value |
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28 | in floating-point format. If <[x]> is subnormal it is treated as though it were |
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29 | normalized; thus, for positive finite <[x]>, |
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30 | @ifnottex |
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31 | 1 <= (<[x]> * FLT_RADIX to the power (-logb(<[x]>))) < FLT_RADIX. |
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32 | @end ifnottex |
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33 | @tex |
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34 | $1 \leq ( x \cdot FLT\_RADIX ^ {-logb(x)} ) < FLT\_RADIX$. |
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35 | @end tex |
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36 | A domain error may occur if the argument is zero. |
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37 | In this floating-point implementation, FLT_RADIX is 2. Which also means |
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38 | that for finite <[x]>, <<logb>>(<[x]>) = <<floor>>(<<log2>>(<<fabs>>(<[x]>))). |
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39 | |
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40 | All nonzero, normal numbers can be described as |
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41 | @ifnottex |
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42 | <[m]> * 2**<[p]>, where 1.0 <= <[m]> < 2.0. |
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43 | @end ifnottex |
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44 | @tex |
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45 | $m \cdot 2^p$, where $1.0 \leq m < 2.0$. |
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46 | @end tex |
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47 | The <<logb>> functions examine the argument <[x]>, and return <[p]>. |
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48 | The <<frexp>> functions are similar to the <<logb>> functions, but |
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49 | returning <[m]> adjusted to the interval [.5, 1) or 0, and <[p]>+1. |
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50 | |
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51 | RETURNS |
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52 | @comment Formatting note: "$@" forces a new line |
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53 | When <[x]> is:@* |
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54 | +inf or -inf, +inf is returned;@* |
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55 | NaN, NaN is returned;@* |
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56 | 0, -inf is returned, and the divide-by-zero exception is raised;@* |
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57 | otherwise, the <<logb>> functions return the signed exponent of <[x]>. |
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58 | |
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59 | PORTABILITY |
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60 | ANSI C, POSIX |
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61 | |
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62 | SEEALSO |
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63 | frexp, ilogb |
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64 | */ |
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65 | |
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66 | /* double logb(double x) |
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67 | * return the binary exponent of non-zero x |
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68 | * logb(0) = -inf, raise divide-by-zero floating point exception |
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69 | * logb(+inf|-inf) = +inf (no signal is raised) |
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70 | * logb(NaN) = NaN (no signal is raised) |
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71 | * Per C99 recommendation, a NaN argument is returned unchanged. |
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72 | */ |
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73 | |
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74 | #include "fdlibm.h" |
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75 | |
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76 | #ifndef _DOUBLE_IS_32BITS |
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77 | |
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78 | double |
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79 | #ifdef __STDC__ |
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80 | logb(double x) |
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81 | #else |
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82 | logb(x) |
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83 | double x; |
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84 | #endif |
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85 | { |
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86 | __int32_t hx,lx,ix; |
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87 | |
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88 | EXTRACT_WORDS(hx,lx,x); |
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89 | hx &= 0x7fffffff; /* high |x| */ |
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90 | if(hx<0x00100000) { /* 0 or subnormal */ |
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91 | if((hx|lx)==0) { |
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92 | double xx; |
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93 | /* arg==0: return -inf and raise divide-by-zero exception */ |
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94 | INSERT_WORDS(xx,hx,lx); /* +0.0 */ |
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95 | return -1./xx; /* logb(0) = -inf */ |
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96 | } |
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97 | else /* subnormal x */ |
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98 | if(hx==0) { |
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99 | for (ix = -1043; lx>0; lx<<=1) ix -=1; |
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100 | } else { |
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101 | for (ix = -1022,hx<<=11; hx>0; hx<<=1) ix -=1; |
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102 | } |
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103 | return (double) ix; |
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104 | } |
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105 | else if (hx<0x7ff00000) return (hx>>20)-1023; /* normal # */ |
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106 | else if (hx>0x7ff00000 || lx) return x; /* x==NaN */ |
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107 | else return HUGE_VAL; /* x==inf (+ or -) */ |
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108 | } |
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109 | |
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110 | #endif /* _DOUBLE_IS_32BITS */ |
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