Revision control
Copy as Markdown
Other Tools
/* origin: FreeBSD /usr/src/lib/msun/src/s_log1pf.c */
/*
* ====================================================
* Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
*
* Developed at SunPro, a Sun Microsystems, Inc. business.
* Permission to use, copy, modify, and distribute this
* software is freely granted, provided that this notice
* is preserved.
* ====================================================
*/
use core::f32;
const LN2_HI: f32 = 6.9313812256e-01; /* 0x3f317180 */
const LN2_LO: f32 = 9.0580006145e-06; /* 0x3717f7d1 */
/* |(log(1+s)-log(1-s))/s - Lg(s)| < 2**-34.24 (~[-4.95e-11, 4.97e-11]). */
const LG1: f32 = 0.66666662693; /* 0xaaaaaa.0p-24 */
const LG2: f32 = 0.40000972152; /* 0xccce13.0p-25 */
const LG3: f32 = 0.28498786688; /* 0x91e9ee.0p-25 */
const LG4: f32 = 0.24279078841; /* 0xf89e26.0p-26 */
#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
pub fn log1pf(x: f32) -> f32 {
let mut ui: u32 = x.to_bits();
let hfsq: f32;
let mut f: f32 = 0.;
let mut c: f32 = 0.;
let s: f32;
let z: f32;
let r: f32;
let w: f32;
let t1: f32;
let t2: f32;
let dk: f32;
let ix: u32;
let mut iu: u32;
let mut k: i32;
ix = ui;
k = 1;
if ix < 0x3ed413d0 || (ix >> 31) > 0 {
/* 1+x < sqrt(2)+ */
if ix >= 0xbf800000 {
/* x <= -1.0 */
if x == -1. {
return x / 0.0; /* log1p(-1)=+inf */
}
return (x - x) / 0.0; /* log1p(x<-1)=NaN */
}
if ix << 1 < 0x33800000 << 1 {
/* |x| < 2**-24 */
/* underflow if subnormal */
if (ix & 0x7f800000) == 0 {
force_eval!(x * x);
}
return x;
}
if ix <= 0xbe95f619 {
/* sqrt(2)/2- <= 1+x < sqrt(2)+ */
k = 0;
c = 0.;
f = x;
}
} else if ix >= 0x7f800000 {
return x;
}
if k > 0 {
ui = (1. + x).to_bits();
iu = ui;
iu += 0x3f800000 - 0x3f3504f3;
k = (iu >> 23) as i32 - 0x7f;
/* correction term ~ log(1+x)-log(u), avoid underflow in c/u */
if k < 25 {
c = if k >= 2 {
1. - (f32::from_bits(ui) - x)
} else {
x - (f32::from_bits(ui) - 1.)
};
c /= f32::from_bits(ui);
} else {
c = 0.;
}
/* reduce u into [sqrt(2)/2, sqrt(2)] */
iu = (iu & 0x007fffff) + 0x3f3504f3;
ui = iu;
f = f32::from_bits(ui) - 1.;
}
s = f / (2.0 + f);
z = s * s;
w = z * z;
t1 = w * (LG2 + w * LG4);
t2 = z * (LG1 + w * LG3);
r = t2 + t1;
hfsq = 0.5 * f * f;
dk = k as f32;
s * (hfsq + r) + (dk * LN2_LO + c) - hfsq + f + dk * LN2_HI
}