/*

 * // Copyright (c) Radzivon Bartoshyk 8/2025. All rights reserved.

 * //

 * // Redistribution and use in source and binary forms, with or without modification,

 * // are permitted provided that the following conditions are met:

 * //

 * // 1.  Redistributions of source code must retain the above copyright notice, this

 * // list of conditions and the following disclaimer.

 * //

 * // 2.  Redistributions in binary form must reproduce the above copyright notice,

 * // this list of conditions and the following disclaimer in the documentation

 * // and/or other materials provided with the distribution.

 * //

 * // 3.  Neither the name of the copyright holder nor the names of its

 * // contributors may be used to endorse or promote products derived from

 * // this software without specific prior written permission.

 * //

 * // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"

 * // AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

 * // IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE

 * // DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE

 * // FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL

 * // DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR

 * // SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER

 * // CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,

 * // OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE

 * // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

 */

use crate::polyeval::f_estrin_polyeval7;

/// Computes log(1+x) - x

///

/// ulp 0.5

pub fn f_log1pmxf(x: f32) -> f32 {

    let ux = x.to_bits().wrapping_shl(1);

    if ux >= 0xffu32 << 24 || ux == 0 {

        // |x| == 0, |x| == inf, x == NaN

        if ux == 0 {

            return x;

        }

        if x.is_infinite() {

            return f32::NAN;

        }

        return x + f32::NAN;

    }

    let ax = x.to_bits() & 0x7fff_ffffu32;

    let xd = x as f64;

    // Use log1p(x) = log(1 + x) for |x| > 2^-6;

    if ax > 0x3c80_0000u32 {

        if x == -1. {

            return f32::NEG_INFINITY;

        }

        let x1p = xd + 1.;

        if x1p <= 0. {

            if x1p == 0. {

                return f32::NEG_INFINITY;

            }

            return f32::NAN;

        }

        return (crate::logs::log1pf::core_logf(x1p) - xd) as f32;

    }

    // log(1+x) is expected to be used near zero

    // Polynomial generated by Sollya in form log(1+x) - x = x^2 * R(x):

    // d = [-2^-6; 2^-6];

    // f_log1pf = (log(1+x) - x)/x^2;

    // Q = fpminimax(f_log1pf, 6, [|0, D...|], d);

    // See ./notes/log1pmxf.sollya

    let xd_sqr = xd * xd;

    let p = f_estrin_polyeval7(

        xd,

        f64::from_bits(0xbfe0000000000000),

        f64::from_bits(0x3fd55555555561c8),

        f64::from_bits(0xbfd0000000000f5a),

        f64::from_bits(0x3fc999998d26deab),

        f64::from_bits(0xbfc555554878739c),

        f64::from_bits(0x3fc24ab2e3c10eca),

        f64::from_bits(0xbfc0017973fafec4),

    ) * xd_sqr;

    p as f32

}

#[cfg(test)]

mod tests {

    use super::*;

    #[test]

    fn test_log1pmxfm() {

        assert_eq!(f_log1pmxf(-0.0000014305108), -1.0231815e-12);

        assert_eq!(f_log1pmxf(0.0), 0.0);

        assert_eq!(f_log1pmxf(2.0), -0.9013877);

        assert_eq!(f_log1pmxf(-0.7), -0.50397277);

        assert_eq!(

            f_log1pmxf(-0.0000000000043243),

            -0.000000000000000000000009349785

        );

        assert!(f_log1pmxf(f32::INFINITY).is_nan());

        assert!(f_log1pmxf(f32::NAN).is_nan());

        assert!(f_log1pmxf(f32::NEG_INFINITY).is_nan());

        assert!(f_log1pmxf(-2.0).is_nan());

    }

}