1 | /* origin: FreeBSD /usr/src/lib/msun/src/s_log1pf.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 | use core::f32; |
14 | |
15 | const LN2_HI: f32 = 6.9313812256e-01; /* 0x3f317180 */ |
16 | const LN2_LO: f32 = 9.0580006145e-06; /* 0x3717f7d1 */ |
17 | /* |(log(1+s)-log(1-s))/s - Lg(s)| < 2**-34.24 (~[-4.95e-11, 4.97e-11]). */ |
18 | const LG1: f32 = 0.66666662693; /* 0xaaaaaa.0p-24 */ |
19 | const LG2: f32 = 0.40000972152; /* 0xccce13.0p-25 */ |
20 | const LG3: f32 = 0.28498786688; /* 0x91e9ee.0p-25 */ |
21 | const LG4: f32 = 0.24279078841; /* 0xf89e26.0p-26 */ |
22 | |
23 | #[cfg_attr (all(test, assert_no_panic), no_panic::no_panic)] |
24 | pub fn log1pf(x: f32) -> f32 { |
25 | let mut ui: u32 = x.to_bits(); |
26 | let hfsq: f32; |
27 | let mut f: f32 = 0.; |
28 | let mut c: f32 = 0.; |
29 | let s: f32; |
30 | let z: f32; |
31 | let r: f32; |
32 | let w: f32; |
33 | let t1: f32; |
34 | let t2: f32; |
35 | let dk: f32; |
36 | let ix: u32; |
37 | let mut iu: u32; |
38 | let mut k: i32; |
39 | |
40 | ix = ui; |
41 | k = 1; |
42 | if ix < 0x3ed413d0 || (ix >> 31) > 0 { |
43 | /* 1+x < sqrt(2)+ */ |
44 | if ix >= 0xbf800000 { |
45 | /* x <= -1.0 */ |
46 | if x == -1. { |
47 | return x / 0.0; /* log1p(-1)=+inf */ |
48 | } |
49 | return (x - x) / 0.0; /* log1p(x<-1)=NaN */ |
50 | } |
51 | if ix << 1 < 0x33800000 << 1 { |
52 | /* |x| < 2**-24 */ |
53 | /* underflow if subnormal */ |
54 | if (ix & 0x7f800000) == 0 { |
55 | force_eval!(x * x); |
56 | } |
57 | return x; |
58 | } |
59 | if ix <= 0xbe95f619 { |
60 | /* sqrt(2)/2- <= 1+x < sqrt(2)+ */ |
61 | k = 0; |
62 | c = 0.; |
63 | f = x; |
64 | } |
65 | } else if ix >= 0x7f800000 { |
66 | return x; |
67 | } |
68 | if k > 0 { |
69 | ui = (1. + x).to_bits(); |
70 | iu = ui; |
71 | iu += 0x3f800000 - 0x3f3504f3; |
72 | k = (iu >> 23) as i32 - 0x7f; |
73 | /* correction term ~ log(1+x)-log(u), avoid underflow in c/u */ |
74 | if k < 25 { |
75 | c = if k >= 2 { |
76 | 1. - (f32::from_bits(ui) - x) |
77 | } else { |
78 | x - (f32::from_bits(ui) - 1.) |
79 | }; |
80 | c /= f32::from_bits(ui); |
81 | } else { |
82 | c = 0.; |
83 | } |
84 | /* reduce u into [sqrt(2)/2, sqrt(2)] */ |
85 | iu = (iu & 0x007fffff) + 0x3f3504f3; |
86 | ui = iu; |
87 | f = f32::from_bits(ui) - 1.; |
88 | } |
89 | s = f / (2.0 + f); |
90 | z = s * s; |
91 | w = z * z; |
92 | t1 = w * (LG2 + w * LG4); |
93 | t2 = z * (LG1 + w * LG3); |
94 | r = t2 + t1; |
95 | hfsq = 0.5 * f * f; |
96 | dk = k as f32; |
97 | s * (hfsq + r) + (dk * LN2_LO + c) - hfsq + f + dk * LN2_HI |
98 | } |
99 | |