1 /* origin: FreeBSD /usr/src/lib/msun/src/e_logf.c */
2 /*
3  * Conversion to float by Ian Lance Taylor, Cygnus Support, [email protected].
4  */
5 /*
6  * ====================================================
7  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
8  *
9  * Developed at SunPro, a Sun Microsystems, Inc. business.
10  * Permission to use, copy, modify, and distribute this
11  * software is freely granted, provided that this notice
12  * is preserved.
13  * ====================================================
14  */
15 
16 const LN2_HI: f32 = 6.9313812256e-01; /* 0x3f317180 */
17 const LN2_LO: f32 = 9.0580006145e-06; /* 0x3717f7d1 */
18 /* |(log(1+s)-log(1-s))/s - Lg(s)| < 2**-34.24 (~[-4.95e-11, 4.97e-11]). */
19 const LG1: f32 = 0.66666662693; /*  0xaaaaaa.0p-24*/
20 const LG2: f32 = 0.40000972152; /*  0xccce13.0p-25 */
21 const LG3: f32 = 0.28498786688; /*  0x91e9ee.0p-25 */
22 const LG4: f32 = 0.24279078841; /*  0xf89e26.0p-26 */
23 
24 #[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
logf(mut x: f32) -> f3225 pub fn logf(mut x: f32) -> f32 {
26     let x1p25 = f32::from_bits(0x4c000000); // 0x1p25f === 2 ^ 25
27 
28     let mut ix = x.to_bits();
29     let mut k = 0i32;
30 
31     if (ix < 0x00800000) || ((ix >> 31) != 0) {
32         /* x < 2**-126  */
33         if ix << 1 == 0 {
34             return -1. / (x * x); /* log(+-0)=-inf */
35         }
36         if (ix >> 31) != 0 {
37             return (x - x) / 0.; /* log(-#) = NaN */
38         }
39         /* subnormal number, scale up x */
40         k -= 25;
41         x *= x1p25;
42         ix = x.to_bits();
43     } else if ix >= 0x7f800000 {
44         return x;
45     } else if ix == 0x3f800000 {
46         return 0.;
47     }
48 
49     /* reduce x into [sqrt(2)/2, sqrt(2)] */
50     ix += 0x3f800000 - 0x3f3504f3;
51     k += ((ix >> 23) as i32) - 0x7f;
52     ix = (ix & 0x007fffff) + 0x3f3504f3;
53     x = f32::from_bits(ix);
54 
55     let f = x - 1.;
56     let s = f / (2. + f);
57     let z = s * s;
58     let w = z * z;
59     let t1 = w * (LG2 + w * LG4);
60     let t2 = z * (LG1 + w * LG3);
61     let r = t2 + t1;
62     let hfsq = 0.5 * f * f;
63     let dk = k as f32;
64     s * (hfsq + r) + dk * LN2_LO - hfsq + f + dk * LN2_HI
65 }
66