1 | /* origin: FreeBSD /usr/src/lib/msun/src/e_log10f.c */ |
2 | /* |
3 | * ==================================================== |
4 | * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved. |
5 | * |
6 | * Developed at SunPro, a Sun Microsystems, Inc. business. |
7 | * Permission to use, copy, modify, and distribute this |
8 | * software is freely granted, provided that this notice |
9 | * is preserved. |
10 | * ==================================================== |
11 | */ |
12 | /* |
13 | * See comments in log10.c. |
14 | */ |
15 | |
16 | use core::f32; |
17 | |
18 | const IVLN10HI: f32 = 4.3432617188e-01; /* 0x3ede6000 */ |
19 | const IVLN10LO: f32 = -3.1689971365e-05; /* 0xb804ead9 */ |
20 | const LOG10_2HI: f32 = 3.0102920532e-01; /* 0x3e9a2080 */ |
21 | const LOG10_2LO: f32 = 7.9034151668e-07; /* 0x355427db */ |
22 | /* |(log(1+s)-log(1-s))/s - Lg(s)| < 2**-34.24 (~[-4.95e-11, 4.97e-11]). */ |
23 | const LG1: f32 = 0.66666662693; /* 0xaaaaaa.0p-24 */ |
24 | const LG2: f32 = 0.40000972152; /* 0xccce13.0p-25 */ |
25 | const LG3: f32 = 0.28498786688; /* 0x91e9ee.0p-25 */ |
26 | const LG4: f32 = 0.24279078841; /* 0xf89e26.0p-26 */ |
27 | |
28 | #[cfg_attr (all(test, assert_no_panic), no_panic::no_panic)] |
29 | pub fn log10f(mut x: f32) -> f32 { |
30 | let x1p25f = f32::from_bits(0x4c000000); // 0x1p25f === 2 ^ 25 |
31 | |
32 | let mut ui: u32 = x.to_bits(); |
33 | let hfsq: f32; |
34 | let f: f32; |
35 | let s: f32; |
36 | let z: f32; |
37 | let r: f32; |
38 | let w: f32; |
39 | let t1: f32; |
40 | let t2: f32; |
41 | let dk: f32; |
42 | let mut hi: f32; |
43 | let lo: f32; |
44 | let mut ix: u32; |
45 | let mut k: i32; |
46 | |
47 | ix = ui; |
48 | k = 0; |
49 | if ix < 0x00800000 || (ix >> 31) > 0 { |
50 | /* x < 2**-126 */ |
51 | if ix << 1 == 0 { |
52 | return -1. / (x * x); /* log(+-0)=-inf */ |
53 | } |
54 | if (ix >> 31) > 0 { |
55 | return (x - x) / 0.0; /* log(-#) = NaN */ |
56 | } |
57 | /* subnormal number, scale up x */ |
58 | k -= 25; |
59 | x *= x1p25f; |
60 | ui = x.to_bits(); |
61 | ix = ui; |
62 | } else if ix >= 0x7f800000 { |
63 | return x; |
64 | } else if ix == 0x3f800000 { |
65 | return 0.; |
66 | } |
67 | |
68 | /* reduce x into [sqrt(2)/2, sqrt(2)] */ |
69 | ix += 0x3f800000 - 0x3f3504f3; |
70 | k += (ix >> 23) as i32 - 0x7f; |
71 | ix = (ix & 0x007fffff) + 0x3f3504f3; |
72 | ui = ix; |
73 | x = f32::from_bits(ui); |
74 | |
75 | f = x - 1.0; |
76 | s = f / (2.0 + f); |
77 | z = s * s; |
78 | w = z * z; |
79 | t1 = w * (LG2 + w * LG4); |
80 | t2 = z * (LG1 + w * LG3); |
81 | r = t2 + t1; |
82 | hfsq = 0.5 * f * f; |
83 | |
84 | hi = f - hfsq; |
85 | ui = hi.to_bits(); |
86 | ui &= 0xfffff000; |
87 | hi = f32::from_bits(ui); |
88 | lo = f - hi - hfsq + s * (hfsq + r); |
89 | dk = k as f32; |
90 | dk * LOG10_2LO + (lo + hi) * IVLN10LO + lo * IVLN10HI + hi * IVLN10HI + dk * LOG10_2HI |
91 | } |
92 | |