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)]
log10f(mut x: f32) -> f3229 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