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)]
log1pf(x: f32) -> f3224 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