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ed7e5132 ZAL |
1 | /* |
2 | * Copyright (c) 1985 Regents of the University of California. | |
3 | * | |
4 | * Use and reproduction of this software are granted in accordance with | |
5 | * the terms and conditions specified in the Berkeley Software License | |
6 | * Agreement (in particular, this entails acknowledgement of the programs' | |
7 | * source, and inclusion of this notice) with the additional understanding | |
8 | * that all recipients should regard themselves as participants in an | |
9 | * ongoing research project and hence should feel obligated to report | |
10 | * their experiences (good or bad) with these elementary function codes, | |
11 | * using "sendbug 4bsd-bugs@BERKELEY", to the authors. | |
12 | */ | |
13 | ||
14 | #ifndef lint | |
15 | static char sccsid[] = "@(#)log1p.c 1.1 (ELEFUNT) %G%"; | |
16 | #endif not lint | |
17 | ||
18 | /* LOG1P(x) | |
19 | * RETURN THE LOGARITHM OF 1+x | |
20 | * DOUBLE PRECISION (VAX D FORMAT 56 bits, IEEE DOUBLE 53 BITS) | |
21 | * CODED IN C BY K.C. NG, 1/19/85; | |
22 | * REVISED BY K.C. NG on 2/6/85, 3/7/85, 3/24/85, 4/16/85. | |
23 | * | |
24 | * Required system supported functions: | |
25 | * scalb(x,n) | |
26 | * copysign(x,y) | |
27 | * logb(x) | |
28 | * finite(x) | |
29 | * | |
30 | * Required kernel function: | |
31 | * log__L(z) | |
32 | * | |
33 | * Method : | |
34 | * 1. Argument Reduction: find k and f such that | |
35 | * 1+x = 2^k * (1+f), | |
36 | * where sqrt(2)/2 < 1+f < sqrt(2) . | |
37 | * | |
38 | * 2. Let s = f/(2+f) ; based on log(1+f) = log(1+s) - log(1-s) | |
39 | * = 2s + 2/3 s**3 + 2/5 s**5 + ....., | |
40 | * log(1+f) is computed by | |
41 | * | |
42 | * log(1+f) = 2s + s*log__L(s*s) | |
43 | * where | |
44 | * log__L(z) = z*(L1 + z*(L2 + z*(... (L6 + z*L7)...))) | |
45 | * | |
46 | * See log__L() for the values of the coefficients. | |
47 | * | |
48 | * 3. Finally, log(1+x) = k*ln2 + log(1+f). | |
49 | * | |
50 | * Remarks 1. In step 3 n*ln2 will be stored in two floating point numbers | |
51 | * n*ln2hi + n*ln2lo, where ln2hi is chosen such that the last | |
52 | * 20 bits (for VAX D format), or the last 21 bits ( for IEEE | |
53 | * double) is 0. This ensures n*ln2hi is exactly representable. | |
54 | * 2. In step 1, f may not be representable. A correction term c | |
55 | * for f is computed. It follows that the correction term for | |
56 | * f - t (the leading term of log(1+f) in step 2) is c-c*x. We | |
57 | * add this correction term to n*ln2lo to attenuate the error. | |
58 | * | |
59 | * | |
60 | * Special cases: | |
61 | * log1p(x) is NaN with signal if x < -1; log1p(NaN) is NaN with no signal; | |
62 | * log1p(INF) is +INF; log1p(-1) is -INF with signal; | |
63 | * only log1p(0)=0 is exact for finite argument. | |
64 | * | |
65 | * Accuracy: | |
66 | * log1p(x) returns the exact log(1+x) nearly rounded. In a test run | |
67 | * with 1,536,000 random arguments on a VAX, the maximum observed | |
68 | * error was .846 ulps (units in the last place). | |
69 | * | |
70 | * Constants: | |
71 | * The hexadecimal values are the intended ones for the following constants. | |
72 | * The decimal values may be used, provided that the compiler will convert | |
73 | * from decimal to binary accurately enough to produce the hexadecimal values | |
74 | * shown. | |
75 | */ | |
76 | ||
77 | #ifdef VAX /* VAX D format */ | |
78 | #include <errno.h> | |
79 | ||
80 | /* double static */ | |
81 | /* ln2hi = 6.9314718055829871446E-1 , Hex 2^ 0 * .B17217F7D00000 */ | |
82 | /* ln2lo = 1.6465949582897081279E-12 , Hex 2^-39 * .E7BCD5E4F1D9CC */ | |
83 | /* sqrt2 = 1.4142135623730950622E0 ; Hex 2^ 1 * .B504F333F9DE65 */ | |
84 | static long ln2hix[] = { 0x72174031, 0x0000f7d0}; | |
85 | static long ln2lox[] = { 0xbcd52ce7, 0xd9cce4f1}; | |
86 | static long sqrt2x[] = { 0x04f340b5, 0xde6533f9}; | |
87 | #define ln2hi (*(double*)ln2hix) | |
88 | #define ln2lo (*(double*)ln2lox) | |
89 | #define sqrt2 (*(double*)sqrt2x) | |
90 | #else /* IEEE double */ | |
91 | double static | |
92 | ln2hi = 6.9314718036912381649E-1 , /*Hex 2^ -1 * 1.62E42FEE00000 */ | |
93 | ln2lo = 1.9082149292705877000E-10 , /*Hex 2^-33 * 1.A39EF35793C76 */ | |
94 | sqrt2 = 1.4142135623730951455E0 ; /*Hex 2^ 0 * 1.6A09E667F3BCD */ | |
95 | #endif | |
96 | ||
97 | double log1p(x) | |
98 | double x; | |
99 | { | |
100 | static double zero=0.0, negone= -1.0, one=1.0, | |
101 | half=1.0/2.0, small=1.0E-20; /* 1+small == 1 */ | |
102 | double logb(),copysign(),scalb(),log__L(),z,s,t,c; | |
103 | int k,finite(); | |
104 | ||
105 | #ifndef VAX | |
106 | if(x!=x) return(x); /* x is NaN */ | |
107 | #endif | |
108 | ||
109 | if(finite(x)) { | |
110 | if( x > negone ) { | |
111 | ||
112 | /* argument reduction */ | |
113 | if(copysign(x,one)<small) return(x); | |
114 | k=logb(one+x); z=scalb(x,-k); t=scalb(one,-k); | |
115 | if(z+t >= sqrt2 ) | |
116 | { k += 1 ; z *= half; t *= half; } | |
117 | t += negone; x = z + t; | |
118 | c = (t-x)+z ; /* correction term for x */ | |
119 | ||
120 | /* compute log(1+x) */ | |
121 | s = x/(2+x); t = x*x*half; | |
122 | c += (k*ln2lo-c*x); | |
123 | z = c+s*(t+log__L(s*s)); | |
124 | x += (z - t) ; | |
125 | ||
126 | return(k*ln2hi+x); | |
127 | } | |
128 | /* end of if (x > negone) */ | |
129 | ||
130 | else { | |
131 | #ifdef VAX | |
132 | extern double infnan(); | |
133 | if ( x == negone ) | |
134 | return (infnan(-ERANGE)); /* -INF */ | |
135 | else | |
136 | return (infnan(EDOM)); /* NaN */ | |
137 | #else /* IEEE double */ | |
138 | /* x = -1, return -INF with signal */ | |
139 | if ( x == negone ) return( negone/zero ); | |
140 | ||
141 | /* negative argument for log, return NaN with signal */ | |
142 | else return ( zero / zero ); | |
143 | #endif | |
144 | } | |
145 | } | |
146 | /* end of if (finite(x)) */ | |
147 | ||
148 | /* log(-INF) is NaN */ | |
149 | else if(x<0) | |
150 | return(zero/zero); | |
151 | ||
152 | /* log(+INF) is INF */ | |
153 | else return(x); | |
154 | } |