/*

 * // 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.

 * //

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 * // 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

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

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

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 */

use crate::acospi::PI_OVER_TWO_F128;

use crate::asin::asin_eval;

use crate::asin_eval_dyadic::asin_eval_dyadic;

use crate::common::f_fmla;

use crate::double_double::DoubleDouble;

use crate::dyadic_float::{DyadicFloat128, DyadicSign};

use crate::rounding::CpuRound;

/// Computes acos(x)

///

/// Max found ULP 0.5

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

    let x_e = (x.to_bits() >> 52) & 0x7ff;

    const E_BIAS: u64 = (1u64 << (11 - 1u64)) - 1u64;

    const PI_OVER_TWO: DoubleDouble = DoubleDouble::new(

        f64::from_bits(0x3c91a62633145c07),

        f64::from_bits(0x3ff921fb54442d18),

    );

    let x_abs = f64::from_bits(x.to_bits() & 0x7fff_ffff_ffff_ffff);

    // |x| < 0.5.

    if x_e < E_BIAS - 1 {

        // |x| < 2^-55.

        if x_e < E_BIAS - 55 {

            // When |x| < 2^-55, acos(x) = pi/2

            return (x_abs + f64::from_bits(0x35f0000000000000)) + PI_OVER_TWO.hi;

        }

        let x_sq = DoubleDouble::from_exact_mult(x, x);

        let err = x_abs * f64::from_bits(0x3cc0000000000000);

        // Polynomial approximation:

        //   p ~ asin(x)/x

        let (p, err) = asin_eval(x_sq, err);

        // asin(x) ~ x * p

        let r0 = DoubleDouble::from_exact_mult(x, p.hi);

        // acos(x) = pi/2 - asin(x)

        //         ~ pi/2 - x * p

        //         = pi/2 - x * (p.hi + p.lo)

        let r_hi = f_fmla(-x, p.hi, PI_OVER_TWO.hi);

        // Use Dekker's 2SUM algorithm to compute the lower part.

        let mut r_lo = ((PI_OVER_TWO.hi - r_hi) - r0.hi) - r0.lo;

        r_lo = f_fmla(-x, p.lo, r_lo + PI_OVER_TWO.lo);

        let r_upper = r_hi + (r_lo + err);

        let r_lower = r_hi + (r_lo - err);

        if r_upper == r_lower {

            return r_upper;

        }

        return acos_less_0p5_hard(x, x_sq);

    }

    // |x| >= 0.5

    let x_sign = if x.is_sign_negative() { -1.0 } else { 1.0 };

    const PI: DoubleDouble = DoubleDouble::new(

        f64::from_bits(0x3ca1a62633145c07),

        f64::from_bits(0x400921fb54442d18),

    );

    // |x| >= 1

    if x_e >= E_BIAS {

        // x = +-1, asin(x) = +- pi/2

        if x_abs == 1.0 {

            // x = 1, acos(x) = 0,

            // x = -1, acos(x) = pi

            return if x == 1.0 {

                0.0

            } else {

                f_fmla(-x_sign, PI.hi, PI.lo)

            };

        }

        // |x| > 1, return NaN.

        return f64::NAN;

    }

    // When |x| >= 0.5, we perform range reduction as follow:

    //

    // When 0.5 <= x < 1, let:

    //   y = acos(x)

    // We will use the double angle formula:

    //   cos(2y) = 1 - 2 sin^2(y)

    // and the complement angle identity:

    //   x = cos(y) = 1 - 2 sin^2 (y/2)

    // So:

    //   sin(y/2) = sqrt( (1 - x)/2 )

    // And hence:

    //   y/2 = asin( sqrt( (1 - x)/2 ) )

    // Equivalently:

    //   acos(x) = y = 2 * asin( sqrt( (1 - x)/2 ) )

    // Let u = (1 - x)/2, then:

    //   acos(x) = 2 * asin( sqrt(u) )

    // Moreover, since 0.5 <= x < 1:

    //   0 < u <= 1/4, and 0 < sqrt(u) <= 0.5,

    // And hence we can reuse the same polynomial approximation of asin(x) when

    // |x| <= 0.5:

    //   acos(x) ~ 2 * sqrt(u) * P(u).

    //

    // When -1 < x <= -0.5, we reduce to the previous case using the formula:

    //   acos(x) = pi - acos(-x)

    //           = pi - 2 * asin ( sqrt( (1 + x)/2 ) )

    //           ~ pi - 2 * sqrt(u) * P(u),

    // where u = (1 - |x|)/2.

    // u = (1 - |x|)/2

    let u = f_fmla(x_abs, -0.5, 0.5);

    // v_hi + v_lo ~ sqrt(u).

    // Let:

    //   h = u - v_hi^2 = (sqrt(u) - v_hi) * (sqrt(u) + v_hi)

    // Then:

    //   sqrt(u) = v_hi + h / (sqrt(u) + v_hi)

    //            ~ v_hi + h / (2 * v_hi)

    // So we can use:

    //   v_lo = h / (2 * v_hi).

    let v_hi = u.sqrt();

    let h;

    #[cfg(any(

        all(

            any(target_arch = "x86", target_arch = "x86_64"),

            target_feature = "fma"

        ),

        target_arch = "aarch64"

    ))]

    {

        h = f_fmla(v_hi, -v_hi, u);

    }

    #[cfg(not(any(

        all(

            any(target_arch = "x86", target_arch = "x86_64"),

            target_feature = "fma"

        ),

        target_arch = "aarch64"

    )))]

    {

        let v_hi_sq = DoubleDouble::from_exact_mult(v_hi, v_hi);

        h = (u - v_hi_sq.hi) - v_hi_sq.lo;

    }

    // Scale v_lo and v_hi by 2 from the formula:

    //   vh = v_hi * 2

    //   vl = 2*v_lo = h / v_hi.

    let vh = v_hi * 2.0;

    let vl = h / v_hi;

    // Polynomial approximation:

    //   p ~ asin(sqrt(u))/sqrt(u)

    let err = vh * f64::from_bits(0x3cc0000000000000);

    let (p, err) = asin_eval(DoubleDouble::new(0.0, u), err);

    // Perform computations in double-double arithmetic:

    //   asin(x) = pi/2 - (v_hi + v_lo) * (ASIN_COEFFS[idx][0] + p)

    let r0 = DoubleDouble::quick_mult(DoubleDouble::new(vl, vh), p);

    let r_hi;

    let r_lo;

    if x.is_sign_positive() {

        r_hi = r0.hi;

        r_lo = r0.lo;

    } else {

        let r = DoubleDouble::from_exact_add(PI.hi, -r0.hi);

        r_hi = r.hi;

        r_lo = (PI.lo - r0.lo) + r.lo;

    }

    let r_upper = r_hi + (r_lo + err);

    let r_lower = r_hi + (r_lo - err);

    if r_upper == r_lower {

        return r_upper;

    }

    acos_hard(x, u, v_hi, h, vh, vl)

}

#[cold]

#[inline(never)]

fn acos_hard(x: f64, u: f64, v_hi: f64, h: f64, vh: f64, vl: f64) -> f64 {

    // Ziv's accuracy test failed, we redo the computations in Float128.

    // Recalculate mod 1/64.

    let idx = (u * f64::from_bits(0x4050000000000000)).cpu_round() as usize;

    // After the first step of Newton-Raphson approximating v = sqrt(u), we have

    // that:

    //   sqrt(u) = v_hi + h / (sqrt(u) + v_hi)

    //      v_lo = h / (2 * v_hi)

    // With error:

    //   sqrt(u) - (v_hi + v_lo) = h * ( 1/(sqrt(u) + v_hi) - 1/(2*v_hi) )

    //                           = -h^2 / (2*v * (sqrt(u) + v)^2).

    // Since:

    //   (sqrt(u) + v_hi)^2 ~ (2sqrt(u))^2 = 4u,

    // we can add another correction term to (v_hi + v_lo) that is:

    //   v_ll = -h^2 / (2*v_hi * 4u)

    //        = -v_lo * (h / 4u)

    //        = -vl * (h / 8u),

    // making the errors:

    //   sqrt(u) - (v_hi + v_lo + v_ll) = O(h^3)

    // well beyond 128-bit precision needed.

    // Get the rounding error of vl = 2 * v_lo ~ h / vh

    // Get full product of vh * vl

    let vl_lo;

    #[cfg(any(

        all(

            any(target_arch = "x86", target_arch = "x86_64"),

            target_feature = "fma"

        ),

        target_arch = "aarch64"

    ))]

    {

        vl_lo = f_fmla(-v_hi, vl, h) / v_hi;

    }

    #[cfg(not(any(

        all(

            any(target_arch = "x86", target_arch = "x86_64"),

            target_feature = "fma"

        ),

        target_arch = "aarch64"

    )))]

    {

        let vh_vl = DoubleDouble::from_exact_mult(v_hi, vl);

        vl_lo = ((h - vh_vl.hi) - vh_vl.lo) / v_hi;

    }

    let t = h * (-0.25) / u;

    let vll = f_fmla(vl, t, vl_lo);

    let m_v_p = DyadicFloat128::new_from_f64(vl) + DyadicFloat128::new_from_f64(vll);

    let mut m_v = DyadicFloat128::new_from_f64(vh) + m_v_p;

    m_v.sign = if x.is_sign_negative() {

        DyadicSign::Neg

    } else {

        DyadicSign::Pos

    };

    // Perform computations in Float128:

    //   acos(x) = (v_hi + v_lo + vll) * P(u)         , when 0.5 <= x < 1,

    //           = pi - (v_hi + v_lo + vll) * P(u)    , when -1 < x <= -0.5.

    let y_f128 =

        DyadicFloat128::new_from_f64(f_fmla(idx as f64, f64::from_bits(0xbf90000000000000), u));

    let p_f128 = asin_eval_dyadic(y_f128, idx);

    let mut r_f128 = m_v * p_f128;

    if x.is_sign_negative() {

        const PI_F128: DyadicFloat128 = DyadicFloat128 {

            sign: DyadicSign::Pos,

            exponent: -126,

            mantissa: 0xc90fdaa2_2168c234_c4c6628b_80dc1cd1_u128,

        };

        r_f128 = PI_F128 + r_f128;

    }

    r_f128.fast_as_f64()

}

#[cold]

#[inline(never)]

fn acos_less_0p5_hard(x: f64, x_sq: DoubleDouble) -> f64 {

    // Ziv's accuracy test failed, perform 128-bit calculation.

    // Recalculate mod 1/64.

    let idx = (x_sq.hi * f64::from_bits(0x4050000000000000)).cpu_round() as usize;

    // Get x^2 - idx/64 exactly.  When FMA is available, double-double

    // multiplication will be correct for all rounding modes. Otherwise, we use

    // Float128 directly.

    let mut x_f128 = DyadicFloat128::new_from_f64(x);

    let u: DyadicFloat128;

    #[cfg(any(

        all(

            any(target_arch = "x86", target_arch = "x86_64"),

            target_feature = "fma"

        ),

        target_arch = "aarch64"

    ))]

    {

        // u = x^2 - idx/64

        let u_hi = DyadicFloat128::new_from_f64(f_fmla(

            idx as f64,

            f64::from_bits(0xbf90000000000000),

            x_sq.hi,

        ));

        u = u_hi.quick_add(&DyadicFloat128::new_from_f64(x_sq.lo));

    }

    #[cfg(not(any(

        all(

            any(target_arch = "x86", target_arch = "x86_64"),

            target_feature = "fma"

        ),

        target_arch = "aarch64"

    )))]

    {

        let x_sq_f128 = x_f128.quick_mul(&x_f128);

        u = x_sq_f128.quick_add(&DyadicFloat128::new_from_f64(

            idx as f64 * f64::from_bits(0xbf90000000000000),

        ));

    }

    let p_f128 = asin_eval_dyadic(u, idx);

    // Flip the sign of x_f128 to perform subtraction.

    x_f128.sign = x_f128.sign.negate();

    let r = PI_OVER_TWO_F128.quick_add(&x_f128.quick_mul(&p_f128));

    r.fast_as_f64()

}

#[cfg(test)]

mod tests {

    use super::*;

    #[test]

    fn f_acos_test() {

        assert_eq!(f_acos(0.7), 0.7953988301841436);

        assert_eq!(f_acos(-0.1), 1.6709637479564565);

        assert_eq!(f_acos(-0.4), 1.9823131728623846);

    }

}