#[allow(clippy::unreadable_literal)]
#[allow(clippy::excessive_precision)]
const BESSI0_COEFFS_A: [f64; 30] = [
    -4.41534164647933937950E-18,
    3.33079451882223809783E-17,
    -2.43127984654795469359E-16,
    1.71539128555513303061E-15,
    -1.16853328779934516808E-14,
    7.67618549860493561688E-14,
    -4.85644678311192946090E-13,
    2.95505266312963983461E-12,
    -1.72682629144155570723E-11,
    9.67580903537323691224E-11,
    -5.18979560163526290666E-10,
    2.65982372468238665035E-9,
    -1.30002500998624804212E-8,
    6.04699502254191894932E-8,
    -2.67079385394061173391E-7,
    1.11738753912010371815E-6,
    -4.41673835845875056359E-6,
    1.64484480707288970893E-5,
    -5.75419501008210370398E-5,
    1.88502885095841655729E-4,
    -5.76375574538582365885E-4,
    1.63947561694133579842E-3,
    -4.32430999505057594430E-3,
    1.05464603945949983183E-2,
    -2.37374148058994688156E-2,
    4.93052842396707084878E-2,
    -9.49010970480476444210E-2,
    1.71620901522208775349E-1,
    -3.04682672343198398683E-1,
    6.76795274409476084995E-1,
];

#[allow(clippy::unreadable_literal)]
#[allow(clippy::excessive_precision)]
const BESSI0_COEFFS_B: [f64; 25] = [
    -7.23318048787475395456E-18,
    -4.83050448594418207126E-18,
    4.46562142029675999901E-17,
    3.46122286769746109310E-17,
    -2.82762398051658348494E-16,
    -3.42548561967721913462E-16,
    1.77256013305652638360E-15,
    3.81168066935262242075E-15,
    -9.55484669882830764870E-15,
    -4.15056934728722208663E-14,
    1.54008621752140982691E-14,
    3.85277838274214270114E-13,
    7.18012445138366623367E-13,
    -1.79417853150680611778E-12,
    -1.32158118404477131188E-11,
    -3.14991652796324136454E-11,
    1.18891471078464383424E-11,
    4.94060238822496958910E-10,
    3.39623202570838634515E-9,
    2.26666899049817806459E-8,
    2.04891858946906374183E-7,
    2.89137052083475648297E-6,
    6.88975834691682398426E-5,
    3.36911647825569408990E-3,
    8.04490411014108831608E-1,
];

#[allow(clippy::unreadable_literal)]
#[allow(clippy::excessive_precision)]
const BESSI1_COEFFS_A: [f64; 29] = [
    2.77791411276104639959E-18,
    -2.11142121435816608115E-17,
    1.55363195773620046921E-16,
    -1.10559694773538630805E-15,
    7.60068429473540693410E-15,
    -5.04218550472791168711E-14,
    3.22379336594557470981E-13,
    -1.98397439776494371520E-12,
    1.17361862988909016308E-11,
    -6.66348972350202774223E-11,
    3.62559028155211703701E-10,
    -1.88724975172282928790E-9,
    9.38153738649577178388E-9,
    -4.44505912879632808065E-8,
    2.00329475355213526229E-7,
    -8.56872026469545474066E-7,
    3.47025130813767847674E-6,
    -1.32731636560394358279E-5,
    4.78156510755005422638E-5,
    -1.61760815825896745588E-4,
    5.12285956168575772895E-4,
    -1.51357245063125314899E-3,
    4.15642294431288815669E-3,
    -1.05640848946261981558E-2,
    2.47264490306265168283E-2,
    -5.29459812080949914269E-2,
    1.02643658689847095384E-1,
    -1.76416518357834055153E-1,
    2.52587186443633654823E-1,
];

#[allow(clippy::unreadable_literal)]
#[allow(clippy::excessive_precision)]
const BESSI1_COEFFS_B: [f64; 25] = [
    7.51729631084210481353E-18,
    4.41434832307170791151E-18,
    -4.65030536848935832153E-17,
    -3.20952592199342395980E-17,
    2.96262899764595013876E-16,
    3.30820231092092828324E-16,
    -1.88035477551078244854E-15,
    -3.81440307243700780478E-15,
    1.04202769841288027642E-14,
    4.27244001671195135429E-14,
    -2.10154184277266431302E-14,
    -4.08355111109219731823E-13,
    -7.19855177624590851209E-13,
    2.03562854414708950722E-12,
    1.41258074366137813316E-11,
    3.25260358301548823856E-11,
    -1.89749581235054123450E-11,
    -5.58974346219658380687E-10,
    -3.83538038596423702205E-9,
    -2.63146884688951950684E-8,
    -2.51223623787020892529E-7,
    -3.88256480887769039346E-6,
    -1.10588938762623716291E-4,
    -9.76109749136146840777E-3,
    7.78576235018280120474E-1,
];

fn chbevl(x: f64, coeffs: &[f64]) -> f64 {
    let mut b0 = coeffs[0];
    let mut b1 = 0.0;
    let mut b2 = 0.0;

    coeffs.iter().skip(1).for_each(|c| {
        b2 = b1;
        b1 = b0;
        b0 = x.mul_add(b1, *c) - b2;
    });

    0.5 * (b0 - b2)
}

/// Modified Bessel function, I<sub>0</sub>(x)
pub fn i0(x: f64) -> f64 {
    let ax = x.abs();

    if ax <= 8.0 {
        let y = ax.mul_add(0.5, -2.0);
        ax.exp() * chbevl(y, &BESSI0_COEFFS_A)
    } else {
        ax.exp() * chbevl(32.0_f64.mul_add(ax.recip(), -2.0), &BESSI0_COEFFS_B)
            / ax.sqrt()
    }
}

/// Modified Bessel function, I<sub>1</sub>(x)
pub fn i1(x: f64) -> f64 {
    let z = x.abs();
    let res = if z <= 8.0 {
        let y = z.mul_add(0.5, -2.0);
        chbevl(y, &BESSI1_COEFFS_A) * z * z.exp()
    } else {
        z.exp() * chbevl(32.0_f64.mul_add(x.recip(), -2.0), &BESSI1_COEFFS_B)
            / z.sqrt()
    };

    res * x.signum()
}

#[cfg(test)]
mod tests {
    use super::*;

    const TOL: f64 = 1E-12;

    #[test]
    fn chbevl_val() {
        assert::close(
            chbevl(2.23525, &BESSI0_COEFFS_A),
            0.1392518662822890,
            TOL,
        );
    }

    #[test]
    fn bessi0_small() {
        assert::close(i0(3.74), 9.0414968490127734, TOL);
        assert::close(i0(-3.74), 9.0414968490127734, TOL);
        assert::close(i0(8.0), 427.56411572180474, TOL);
    }

    #[test]
    fn bessi0_large() {
        assert::close(i0(8.1), 469.5006067101214, TOL);
        assert::close(i0(10.0), 2815.716628466254, TOL);
    }

    #[test]
    fn bessi1_small() {
        assert::close(i1(3.74), 7.709894215253694, TOL);
        assert::close(i1(-3.74), -7.709894215253694, TOL);
        assert::close(i1(0.0024), 0.0012000008640002072, TOL);
        assert::close(i1(8.0), 399.8731367825599, TOL);
    }

    #[test]
    fn bessi1_large() {
        assert::close(i1(8.1), 439.48430891035844, TOL);
        assert::close(i1(10.0), 2670.988303701255, TOL);
    }
}