/rust/registry/src/index.crates.io-1949cf8c6b5b557f/libm-0.2.16/src/math/log1pf.rs
Line | Count | Source |
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 | | const LN2_HI: f32 = 6.9313812256e-01; /* 0x3f317180 */ |
14 | | const LN2_LO: f32 = 9.0580006145e-06; /* 0x3717f7d1 */ |
15 | | /* |(log(1+s)-log(1-s))/s - Lg(s)| < 2**-34.24 (~[-4.95e-11, 4.97e-11]). */ |
16 | | const LG1: f32 = 0.66666662693; /* 0xaaaaaa.0p-24 */ |
17 | | const LG2: f32 = 0.40000972152; /* 0xccce13.0p-25 */ |
18 | | const LG3: f32 = 0.28498786688; /* 0x91e9ee.0p-25 */ |
19 | | const LG4: f32 = 0.24279078841; /* 0xf89e26.0p-26 */ |
20 | | |
21 | | /// The natural logarithm of 1+`x` (f32). |
22 | | #[cfg_attr(assert_no_panic, no_panic::no_panic)] |
23 | 0 | pub fn log1pf(x: f32) -> f32 { |
24 | 0 | let mut ui: u32 = x.to_bits(); |
25 | | let hfsq: f32; |
26 | 0 | let mut f: f32 = 0.; |
27 | 0 | let mut c: f32 = 0.; |
28 | | let s: f32; |
29 | | let z: f32; |
30 | | let r: f32; |
31 | | let w: f32; |
32 | | let t1: f32; |
33 | | let t2: f32; |
34 | | let dk: f32; |
35 | | let ix: u32; |
36 | | let mut iu: u32; |
37 | | let mut k: i32; |
38 | | |
39 | 0 | ix = ui; |
40 | 0 | k = 1; |
41 | 0 | if ix < 0x3ed413d0 || (ix >> 31) > 0 { |
42 | | /* 1+x < sqrt(2)+ */ |
43 | 0 | if ix >= 0xbf800000 { |
44 | | /* x <= -1.0 */ |
45 | 0 | if x == -1. { |
46 | 0 | return x / 0.0; /* log1p(-1)=+inf */ |
47 | 0 | } |
48 | 0 | return (x - x) / 0.0; /* log1p(x<-1)=NaN */ |
49 | 0 | } |
50 | 0 | if ix << 1 < 0x33800000 << 1 { |
51 | | /* |x| < 2**-24 */ |
52 | | /* underflow if subnormal */ |
53 | 0 | if (ix & 0x7f800000) == 0 { |
54 | 0 | force_eval!(x * x); |
55 | 0 | } |
56 | 0 | return x; |
57 | 0 | } |
58 | 0 | if ix <= 0xbe95f619 { |
59 | 0 | /* sqrt(2)/2- <= 1+x < sqrt(2)+ */ |
60 | 0 | k = 0; |
61 | 0 | c = 0.; |
62 | 0 | f = x; |
63 | 0 | } |
64 | 0 | } else if ix >= 0x7f800000 { |
65 | 0 | return x; |
66 | 0 | } |
67 | 0 | if k > 0 { |
68 | 0 | ui = (1. + x).to_bits(); |
69 | 0 | iu = ui; |
70 | 0 | iu += 0x3f800000 - 0x3f3504f3; |
71 | 0 | k = (iu >> 23) as i32 - 0x7f; |
72 | | /* correction term ~ log(1+x)-log(u), avoid underflow in c/u */ |
73 | 0 | if k < 25 { |
74 | 0 | c = if k >= 2 { |
75 | 0 | 1. - (f32::from_bits(ui) - x) |
76 | | } else { |
77 | 0 | x - (f32::from_bits(ui) - 1.) |
78 | | }; |
79 | 0 | c /= f32::from_bits(ui); |
80 | 0 | } else { |
81 | 0 | c = 0.; |
82 | 0 | } |
83 | | /* reduce u into [sqrt(2)/2, sqrt(2)] */ |
84 | 0 | iu = (iu & 0x007fffff) + 0x3f3504f3; |
85 | 0 | ui = iu; |
86 | 0 | f = f32::from_bits(ui) - 1.; |
87 | 0 | } |
88 | 0 | s = f / (2.0 + f); |
89 | 0 | z = s * s; |
90 | 0 | w = z * z; |
91 | 0 | t1 = w * (LG2 + w * LG4); |
92 | 0 | t2 = z * (LG1 + w * LG3); |
93 | 0 | r = t2 + t1; |
94 | 0 | hfsq = 0.5 * f * f; |
95 | 0 | dk = k as f32; |
96 | 0 | s * (hfsq + r) + (dk * LN2_LO + c) - hfsq + f + dk * LN2_HI |
97 | 0 | } |