/*

 * // Copyright (c) Radzivon Bartoshyk 9/2025. 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::double_double::DoubleDouble;

use crate::f_exp;

use crate::gamma::lgamma::lgamma_core;

/// Computes beta function

pub fn f_beta(a: f64, b: f64) -> f64 {

    let ax = a.to_bits();

    let bx = b.to_bits();

    if ax >= 0x7ffu64 << 52 || ax == 0 || bx >= 0x7ffu64 << 52 || bx == 0 {

        if (ax >> 63) != 0 || (bx >> 63) != 0 {

            // |a| < 0 or |b| < 0

            return f64::NAN;

        }

        if ax.wrapping_shl(1) == 0 || bx.wrapping_shl(1) == 0 {

            // |a| == 0 || |b| == 0

            if ax.wrapping_shl(1) != 0 || bx.wrapping_shl(1) != 0 {

                return f64::INFINITY;

            }

            return f64::NAN;

        }

        if a.is_infinite() || b.is_infinite() {

            // |a| == inf or |b| == inf

            return 0.;

        }

        return a + f64::NAN; // nan

    }

    let mut sign = 1i32;

    let (mut y, sgngamf) = lgamma_core(DoubleDouble::from_full_exact_add(a, b).to_f64());

    sign *= sgngamf; /* keep track of the sign */

    let (y1, sgngamf) = lgamma_core(b);

    y = DoubleDouble::quick_dd_sub(y1, y);

    sign *= sgngamf;

    let (y1, sgngamf) = lgamma_core(a);

    y = DoubleDouble::quick_dd_add(y1, y);

    sign *= sgngamf;

    f_exp(y.to_f64()) * (sign as f64)

}

#[cfg(test)]

mod tests {

    use crate::f_beta;

    #[test]

    fn test_beta() {

        assert!(f_beta(-5., 15.).is_nan());

        assert!(f_beta(5., -15.).is_nan());

        assert!(f_beta(f64::NAN, 15.).is_nan());

        assert!(f_beta(15., f64::NAN).is_nan());

        assert_eq!(f_beta(f64::INFINITY, 0.), f64::INFINITY);

        assert_eq!(f_beta(f64::INFINITY, 1.), 0.);

        assert_eq!(f_beta(1., f64::INFINITY), 0.);

        assert_eq!(f_beta(5., 3.), 0.009523809523809526);

        assert_eq!(f_beta(3., 5.), 0.009523809523809526);

        assert_eq!(f_beta(12., 23.), 1.5196994506201402e-10);

    }

}