/*

 * // Copyright (c) Radzivon Bartoshyk 7/2025. 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

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 */

use crate::common::dd_fmla;

use crate::double_double::DoubleDouble;

use crate::exponents::auxiliary::fast_ldexp;

use crate::exponents::exp::{EXP_REDUCE_T0, EXP_REDUCE_T1, to_denormal};

use crate::exponents::expf::{ExpfBackend, GenericExpfBackend};

use crate::rounding::CpuRoundTiesEven;

#[inline]

fn exp2_poly_dd(z: f64) -> DoubleDouble {

    const C: [(u64, u64); 6] = [

        (0x3bbabc9e3b39873e, 0x3f262e42fefa39ef),

        (0xbae5e43a53e44950, 0x3e4ebfbdff82c58f),

        (0xba0d3a15710d3d83, 0x3d6c6b08d704a0c0),

        (0x3914dd5d2a5e025a, 0x3c83b2ab6fba4e77),

        (0xb83dc47e47beb9dd, 0x3b95d87fe7a66459),

        (0xb744fcd51fcb7640, 0x3aa430912f9fb79d),

    ];

    let mut r = DoubleDouble::quick_mul_f64_add(

        DoubleDouble::from_bit_pair(C[5]),

        z,

        DoubleDouble::from_bit_pair(C[4]),

    );

    r = DoubleDouble::quick_mul_f64_add(r, z, DoubleDouble::from_bit_pair(C[3]));

    r = DoubleDouble::quick_mul_f64_add(r, z, DoubleDouble::from_bit_pair(C[2]));

    r = DoubleDouble::quick_mul_f64_add(r, z, DoubleDouble::from_bit_pair(C[1]));

    DoubleDouble::quick_mul_f64_add(r, z, DoubleDouble::from_bit_pair(C[0]))

}

#[cold]

fn exp2_accurate(x: f64) -> f64 {

    let mut ix = x.to_bits();

    let sx = 4096.0 * x;

    let fx = sx.cpu_round_ties_even();

    let z = sx - fx;

    let k: i64 = unsafe {

        fx.to_int_unchecked::<i64>() // this is already finite here

    };

    let i1 = k & 0x3f;

    let i0 = (k >> 6) & 0x3f;

    let ie = k >> 12;

    let t0 = DoubleDouble::from_bit_pair(EXP_REDUCE_T0[i0 as usize]);

    let t1 = DoubleDouble::from_bit_pair(EXP_REDUCE_T1[i1 as usize]);

    let dt = DoubleDouble::quick_mult(t0, t1);

    let mut f = exp2_poly_dd(z);

    f = DoubleDouble::quick_mult_f64(f, z);

    if ix <= 0xc08ff00000000000u64 {

        // x >= -1022

        // for -0x1.71547652b82fep-54 <= x <= 0x1.71547652b82fdp-53,

        // exp2(x) round to x to nearest

        if f64::from_bits(0xbc971547652b82fe) <= x && x <= f64::from_bits(0x3ca71547652b82fd) {

            return dd_fmla(x, 0.5, 1.0);

        } else if (k & 0xfff) == 0 {

            // 4096*x rounds to 4096*integer

            let zf = DoubleDouble::from_exact_add(dt.hi, f.hi);

            let zfl = DoubleDouble::from_exact_add(zf.lo, f.lo);

            f.hi = zf.hi;

            f.lo = zfl.hi;

            ix = zfl.hi.to_bits();

            if ix & 0x000fffffffffffff == 0 {

                // fl is a power of 2

                if ((ix >> 52) & 0x7ff) != 0 {

                    // |fl| is Inf

                    let v = zfl.lo.to_bits();

                    let d: i64 = ((((ix as i64) >> 63) ^ ((v as i64) >> 63)) as u64)

                        .wrapping_shl(1)

                        .wrapping_add(1) as i64;

                    ix = ix.wrapping_add(d as u64);

                    f.lo = f64::from_bits(ix);

                }

            }

        } else {

            f = DoubleDouble::quick_mult(f, dt);

            f = DoubleDouble::add(dt, f);

        }

        let hf = DoubleDouble::from_exact_add(f.hi, f.lo);

        fast_ldexp(hf.hi, ie as i32)

    } else {

        ix = 1u64.wrapping_sub(ie as u64).wrapping_shl(52);

        f = DoubleDouble::quick_mult(f, dt);

        f = DoubleDouble::add(dt, f);

        let zve = DoubleDouble::from_exact_add(f64::from_bits(ix), f.hi);

        f.hi = zve.hi;

        f.lo += zve.lo;

        to_denormal(f.to_f64())

    }

}

#[inline(always)]

fn exp2_gen<B: ExpfBackend>(x: f64, backend: B) -> f64 {

    let mut ix = x.to_bits();

    let ax = ix.wrapping_shl(1);

    if ax == 0 {

        return 1.0;

    }

    if ax >= 0x8120000000000000u64 {

        // |x| >= 1024

        if ax > 0xffe0000000000000u64 {

            return x + x; // nan

        }

        if ax == 0xffe0000000000000u64 {

            return if (ix >> 63) != 0 { 0.0 } else { x };

        }

        // +/-inf

        if (ix >> 63) != 0 {

            // x <= -1024

            if ix >= 0xc090cc0000000000u64 {

                // x <= -1075

                const Z: f64 = f64::from_bits(0x0010000000000000);

                return Z * Z;

            }

        } else {

            // x >= 1024

            return f64::from_bits(0x7fe0000000000000) * x;

        }

    }

    // for |x| <= 0x1.71547652b82fep-54, 2^x rounds to 1 to nearest

    // this avoids a spurious underflow in z*z below

    if ax <= 0x792e2a8eca5705fcu64 {

        return 1.0 + f64::copysign(f64::from_bits(0x3c90000000000000), x);

    }

    let m = ix.wrapping_shl(12);

    let ex = (ax >> 53).wrapping_sub(0x3ff);

    let frac = ex >> 63 | m << (ex & 63);

    let sx = 4096.0 * x;

    let fx = backend.round_ties_even(sx);

    let z = sx - fx;

    let z2 = z * z;

    let k = unsafe {

        fx.to_int_unchecked::<i64>() // this already finite here

    };

    let i1 = k & 0x3f;

    let i0 = (k >> 6) & 0x3f;

    let ie = k >> 12;

    let t0 = DoubleDouble::from_bit_pair(EXP_REDUCE_T0[i0 as usize]);

    let t1 = DoubleDouble::from_bit_pair(EXP_REDUCE_T1[i1 as usize]);

    let ti0 = backend.quick_mult(t0, t1);

    const C: [u64; 4] = [

        0x3f262e42fefa39ef,

        0x3e4ebfbdff82c58f,

        0x3d6c6b08d73b3e01,

        0x3c83b2ab6fdda001,

    ];

    let tz = ti0.hi * z;

    let mut fh = ti0.hi;

    let p0 = backend.fma(z, f64::from_bits(C[1]), f64::from_bits(C[0]));

    let p1 = backend.fma(z, f64::from_bits(C[3]), f64::from_bits(C[2]));

    let p2 = backend.fma(z2, p1, p0);

    let mut fl = backend.fma(tz, p2, ti0.lo);

    const EPS: f64 = f64::from_bits(0x3c0833beace2b6fe);

    if ix <= 0xc08ff00000000000u64 {

        // x >= -1022

        if frac != 0 {

            let ub = fh + (fl + EPS);

            fh += fl - EPS;

            if ub != fh {

                return exp2_accurate(x);

            }

        }

        fh = fast_ldexp(fh, ie as i32);

    } else {

        // subnormal case

        ix = 1u64.wrapping_sub(ie as u64).wrapping_shl(52);

        let rs = DoubleDouble::from_exact_add(f64::from_bits(ix), fh);

        fl += rs.lo;

        fh = rs.hi;

        if frac != 0 {

            let ub = fh + (fl + EPS);

            fh += fl - EPS;

            if ub != fh {

                return exp2_accurate(x);

            }

        }

        // when 2^x is exact, no underflow should be raised

        fh = to_denormal(fh);

    }

    fh

}

Unexecuted instantiation: pxfm::exponents::exp2::exp2_gen::<pxfm::exponents::expf::FmaBackend>

Unexecuted instantiation: pxfm::exponents::exp2::exp2_gen::<pxfm::exponents::expf::GenericExpfBackend>

#[cfg(any(target_arch = "x86", target_arch = "x86_64"))]

#[target_feature(enable = "avx", enable = "fma")]

unsafe fn exp2_fma_impl(x: f64) -> f64 {

    use crate::exponents::expf::FmaBackend;

    exp2_gen(x, FmaBackend {})

}

/// Computes exp2

///

/// Max found ULP 0.5

pub fn f_exp2(x: f64) -> f64 {

    #[cfg(not(any(target_arch = "x86", target_arch = "x86_64")))]

    {

        exp2_gen(x, GenericExpfBackend {})

    }

    #[cfg(any(target_arch = "x86", target_arch = "x86_64"))]

    {

        use std::sync::OnceLock;

        static EXECUTOR: OnceLock<unsafe fn(f64) -> f64> = OnceLock::new();

        let q = EXECUTOR.get_or_init(|| {

            if std::arch::is_x86_feature_detected!("avx")

                && std::arch::is_x86_feature_detected!("fma")

            {

                exp2_fma_impl

            } else {

                fn def_exp2(x: f64) -> f64 {

                    exp2_gen(x, GenericExpfBackend {})

                }

                def_exp2

            }

        });

        unsafe { q(x) }

    }

}

#[cfg(test)]

mod tests {

    use super::*;

    #[test]

    fn test_exp2d() {

        assert_eq!(f_exp2(2.0), 4.0);

        assert_eq!(f_exp2(3.0), 8.0);

        assert_eq!(f_exp2(4.0), 16.0);

        assert_eq!(f_exp2(0.35f64), 1.2745606273192622);

        assert_eq!(f_exp2(-0.6f64), 0.6597539553864471);

        assert_eq!(f_exp2(f64::INFINITY), f64::INFINITY);

        assert_eq!(f_exp2(f64::NEG_INFINITY), 0.);

        assert!(f_exp2(f64::NAN).is_nan());

    }

}