/* 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 */

/// The base 2 logarithm of `x` (f32).

#[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

}