/*

 * // Copyright (c) Radzivon Bartoshyk 6/2025. 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::asinpif::ASINCOSF_PI_TABLE;

use crate::common::{dd_fmla, f_fmla};

/// Computes acos(x)/PI

///

/// Max ULP 0.5

#[inline]

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

    let ax = x.abs();

    let az = ax as f64;

    let z = x as f64;

    let t: u32 = x.to_bits();

    let e: i32 = ((t >> 23) & 0xff) as i32;

    if e >= 127 {

        if x == 1.0 {

            return 0.0;

        }

        if x == -1.0 {

            return 1.0;

        }

        if e == 0xff && (t.wrapping_shl(9)) != 0 {

            return x + x;

        } // nan

        return f32::NAN;

    }

    let s: i32 = 146i32.wrapping_sub(e);

    let mut i = 0i32;

    if s < 32 {

        i = (((t & 0x007fffff) | 1 << 23) >> s) as i32;

    }

    let c = ASINCOSF_PI_TABLE[i as usize & 15];

    let z2 = z * z;

    let z4 = z2 * z2;

    if i == 0 {

        let mut c0 = f_fmla(z2, f64::from_bits(c[1]), f64::from_bits(c[0]));

        let c2 = f_fmla(z2, f64::from_bits(c[3]), f64::from_bits(c[2]));

        let mut c4 = f_fmla(z2, f64::from_bits(c[5]), f64::from_bits(c[4]));

        let c6 = f_fmla(z2, f64::from_bits(c[7]), f64::from_bits(c[6]));

        c0 += c2 * z4;

        c4 += c6 * z4;

        /* For |x| <= 0x1.0fd288p-127, c0 += c4*(z4*z4) would raise a spurious

        underflow exception, we use an FMA instead, where c4 * z4 does not

        underflow. */

        c0 = dd_fmla(c4 * z4, z4, c0);

        f_fmla(-z, c0, 0.5) as f32

    } else {

        let f = (1. - az).sqrt();

        let mut c0 = f_fmla(az, f64::from_bits(c[1]), f64::from_bits(c[0]));

        let c2 = f_fmla(az, f64::from_bits(c[3]), f64::from_bits(c[2]));

        let mut c4 = f_fmla(az, f64::from_bits(c[5]), f64::from_bits(c[4]));

        let c6 = f_fmla(az, f64::from_bits(c[7]), f64::from_bits(c[6]));

        c0 += c2 * z2;

        c4 += c6 * z2;

        c0 += c4 * z4;

        static SIGN: [f64; 2] = [0., 1.];

        let r = SIGN[(t >> 31) as usize] + c0 * f64::copysign(f, x as f64);

        r as f32

    }

}

#[cfg(test)]

mod tests {

    use super::*;

    #[test]

    fn test_acospif() {

        assert_eq!(f_acospif(0.0), 0.5);

        assert_eq!(f_acospif(0.5), 0.33333334);

        assert_eq!(f_acospif(1.0), 0.0);

    }

}