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/* origin: FreeBSD /usr/src/lib/msun/src/e_log2f.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.
* ====================================================
*/
/*
* See comments in log2.c.
*/
use core::f32;
const IVLN2HI: f32 = 1.4428710938e+00; /* 0x3fb8b000 */
const IVLN2LO: f32 = -1.7605285393e-04; /* 0xb9389ad4 */
/* |(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 log2f(mut x: f32) -> f32 {
let x1p25f = f32::from_bits(0x4c000000); // 0x1p25f === 2 ^ 25
let mut ui: u32 = x.to_bits();
let hfsq: f32;
let f: f32;
let s: f32;
let z: f32;
let r: f32;
let w: f32;
let t1: f32;
let t2: f32;
let mut hi: f32;
let lo: f32;
let mut ix: u32;
let mut k: i32;
ix = ui;
k = 0;
if ix < 0x00800000 || (ix >> 31) > 0 {
/* x < 2**-126 */
if ix << 1 == 0 {
return -1. / (x * x); /* log(+-0)=-inf */
}
if (ix >> 31) > 0 {
return (x - x) / 0.0; /* log(-#) = NaN */
}
/* subnormal number, scale up x */
k -= 25;
x *= x1p25f;
ui = x.to_bits();
ix = ui;
} else if ix >= 0x7f800000 {
return x;
} else if ix == 0x3f800000 {
return 0.;
}
/* reduce x into [sqrt(2)/2, sqrt(2)] */
ix += 0x3f800000 - 0x3f3504f3;
k += (ix >> 23) as i32 - 0x7f;
ix = (ix & 0x007fffff) + 0x3f3504f3;
ui = ix;
x = f32::from_bits(ui);
f = x - 1.0;
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;
hi = f - hfsq;
ui = hi.to_bits();
ui &= 0xfffff000;
hi = f32::from_bits(ui);
lo = f - hi - hfsq + s * (hfsq + r);
(lo + hi) * IVLN2LO + lo * IVLN2HI + hi * IVLN2HI + k as f32
}
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