/*

 * // 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

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 */

use crate::bessel::i0_exp;

use crate::double_double::DoubleDouble;

/// Logistic function

///

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

pub fn f_logistic(x: f64) -> f64 {

    let x_u = x.to_bits();

    let x_abs = x_u & 0x7fff_ffff_ffff_ffffu64;

    if x_abs <= 0x3c90000000000000u64 {

        // |x| <= 5.55112e-17

        return 1. / (2. + -x);

    }

    if x_abs >= 0x4043800000000000u64 {

        // |x| >= 39.0

        if x_abs > 0x7ff0000000000000u64 {

            // x == NaN

            return x + x;

        }

        if x_abs == 0x7ff0000000000000u64 {

            // |x| = inf

            return if (x.to_bits() >> 63) != 0 {

                0.0 // x = -inf

            } else {

                1.0 // x = inf

            };

        }

        if (x.to_bits() >> 63) == 0 {

            // x >= 39.0

            return 1.0;

        }

        if x_abs >= 0x40874910d52d3052u64 {

            // x <= -745.133

            return 1.0;

        }

    }

    // e^x/(1+e^x)

    let num = i0_exp(x);

    let den = DoubleDouble::full_add_f64(num, 1.0);

    let v = DoubleDouble::div(num, den);

    v.to_f64()

}

#[cfg(test)]

mod tests {

    use super::*;

    #[test]

    fn test_logistic() {

        assert_eq!(f_logistic(6.), 0.9975273768433652);

        assert_eq!(f_logistic(39.), 1.0);

        assert_eq!(f_logistic(35.), 0.9999999999999993);

        assert_eq!(f_logistic(30.), 0.9999999999999064);

        assert_eq!(f_logistic(40.), 1.);

        assert_eq!(f_logistic(-89.), 2.2273635617957438e-39);

        assert_eq!(f_logistic(-104.), 6.813556821545298e-46);

        assert_eq!(f_logistic(-103.), 1.8521167695179754e-45);

        assert_eq!(f_logistic(-88.9), 2.4616174324780325e-39);

        assert_eq!(f_logistic(-88.), 6.054601895401186e-39);

        assert_eq!(f_logistic(-80.), 1.8048513878454153e-35);

        assert_eq!(f_logistic(-60.), 8.75651076269652e-27);

        assert_eq!(f_logistic(-40.), 4.248354255291589e-18);

        assert_eq!(f_logistic(-20.), 2.0611536181902037e-9);

        assert_eq!(f_logistic(-1.591388e29), 1.0);

        assert_eq!(f_logistic(-3.), 0.04742587317756678);

        assert_eq!(f_logistic(3.), 0.9525741268224333);

        assert_eq!(f_logistic(20.), 0.9999999979388464);

        assert_eq!(f_logistic(55.), 1.);

        assert_eq!(f_logistic(-104.), 6.813556821545298e-46);

        assert_eq!(f_logistic(-90.), 8.194012623990515e-40);

        assert_eq!(f_logistic(0.00000000000524323), 0.5000000000013108);

        assert_eq!(f_logistic(0.00000000524323), 0.5000000013108075);

        assert_eq!(f_logistic(0.02), 0.5049998333399998);

        assert_eq!(f_logistic(-0.02), 0.4950001666600003);

        assert_eq!(f_logistic(90.), 1.);

        assert_eq!(f_logistic(105.), 1.);

        assert_eq!(f_logistic(f64::INFINITY), 1.0);

        assert_eq!(f_logistic(f64::NEG_INFINITY), 0.);

        assert!(f_logistic(f64::NAN).is_nan());

    }

}