/// Adapted from Rust's libm module, by the Rust core team,
/// licensed under the Apache License, version 2.0 - https://www.apache.org/licenses/LICENSE-2.0
/// https://github.com/rust-lang/libm/blob/master/LICENSE-APACHE
///
/// Thank you, Rust core team!

// From https://github.com/rust-lang/libm/blob/master/src/math/mod.rs
#[cfg(not(debug_assertions))]
macro_rules! i {
    ($array:expr, $index:expr) => {
        unsafe { *$array.get_unchecked($index) }
    };
    ($array:expr, $index:expr, = , $rhs:expr) => {
        unsafe {
            *$array.get_unchecked_mut($index) = $rhs;
        }
    };
    ($array:expr, $index:expr, += , $rhs:expr) => {
        unsafe {
            *$array.get_unchecked_mut($index) += $rhs;
        }
    };
    ($array:expr, $index:expr, -= , $rhs:expr) => {
        unsafe {
            *$array.get_unchecked_mut($index) -= $rhs;
        }
    };
    ($array:expr, $index:expr, &= , $rhs:expr) => {
        unsafe {
            *$array.get_unchecked_mut($index) &= $rhs;
        }
    };
    ($array:expr, $index:expr, == , $rhs:expr) => {
        unsafe { *$array.get_unchecked_mut($index) == $rhs }
    };
}

#[cfg(debug_assertions)]
macro_rules! i {
    ($array:expr, $index:expr) => {
        *$array.get($index).unwrap()
    };
    ($array:expr, $index:expr, = , $rhs:expr) => {
        *$array.get_mut($index).unwrap() = $rhs;
    };
    ($array:expr, $index:expr, -= , $rhs:expr) => {
        *$array.get_mut($index).unwrap() -= $rhs;
    };
    ($array:expr, $index:expr, += , $rhs:expr) => {
        *$array.get_mut($index).unwrap() += $rhs;
    };
    ($array:expr, $index:expr, &= , $rhs:expr) => {
        *$array.get_mut($index).unwrap() &= $rhs;
    };
    ($array:expr, $index:expr, == , $rhs:expr) => {
        *$array.get_mut($index).unwrap() == $rhs
    };
}

macro_rules! llvm_intrinsically_optimized {
    (#[cfg($($clause:tt)*)] $e:expr) => {
        #[cfg(all(feature = "unstable", $($clause)*))]
        {
            if true { // thwart the dead code lint
                $e
            }
        }
    };
}

macro_rules! force_eval {
    ($e:expr) => {
        unsafe {
            ::core::ptr::read_volatile(&$e);
        }
    };
}

// From https://github.com/rust-lang/libm/blob/master/src/math/atan.rs

// Clippy fails CI if we don't include overrides below. Since this is copied
// straight from the libm crate, I figure they had a reason to include
// the extra percision, so we just silence this warning.

#[allow(clippy::excessive_precision)]
const ATANHI: [f64; 4] = [
    4.63647609000806093515e-01, /* atan(0.5)hi 0x3FDDAC67, 0x0561BB4F */
    7.85398163397448278999e-01, /* atan(1.0)hi 0x3FE921FB, 0x54442D18 */
    9.82793723247329054082e-01, /* atan(1.5)hi 0x3FEF730B, 0xD281F69B */
    1.57079632679489655800e+00, /* atan(inf)hi 0x3FF921FB, 0x54442D18 */
];

#[allow(clippy::excessive_precision)]
const ATANLO: [f64; 4] = [
    2.26987774529616870924e-17, /* atan(0.5)lo 0x3C7A2B7F, 0x222F65E2 */
    3.06161699786838301793e-17, /* atan(1.0)lo 0x3C81A626, 0x33145C07 */
    1.39033110312309984516e-17, /* atan(1.5)lo 0x3C700788, 0x7AF0CBBD */
    6.12323399573676603587e-17, /* atan(inf)lo 0x3C91A626, 0x33145C07 */
];

#[allow(clippy::excessive_precision)]
const AT: [f64; 11] = [
    3.33333333333329318027e-01,  /* 0x3FD55555, 0x5555550D */
    -1.99999999998764832476e-01, /* 0xBFC99999, 0x9998EBC4 */
    1.42857142725034663711e-01,  /* 0x3FC24924, 0x920083FF */
    -1.11111104054623557880e-01, /* 0xBFBC71C6, 0xFE231671 */
    9.09088713343650656196e-02,  /* 0x3FB745CD, 0xC54C206E */
    -7.69187620504482999495e-02, /* 0xBFB3B0F2, 0xAF749A6D */
    6.66107313738753120669e-02,  /* 0x3FB10D66, 0xA0D03D51 */
    -5.83357013379057348645e-02, /* 0xBFADDE2D, 0x52DEFD9A */
    4.97687799461593236017e-02,  /* 0x3FA97B4B, 0x24760DEB */
    -3.65315727442169155270e-02, /* 0xBFA2B444, 0x2C6A6C2F */
    1.62858201153657823623e-02,  /* 0x3F90AD3A, 0xE322DA11 */
];

#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
pub fn fabs(x: f64) -> f64 {
    // On wasm32 we know that LLVM's intrinsic will compile to an optimized
    // `f64.abs` native instruction, so we can leverage this for both code size
    // and speed.
    llvm_intrinsically_optimized! {
        #[cfg(target_arch = "wasm32")] {
            return unsafe { ::core::intrinsics::fabsf64(x) }
        }
    }
    f64::from_bits(x.to_bits() & (u64::MAX / 2))
}

#[inline(always)]
#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]
pub fn atan(x: f64) -> f64 {
    let mut x = x;
    let mut ix = (x.to_bits() >> 32) as u32;
    let sign = ix >> 31;
    ix &= 0x7fff_ffff;
    if ix >= 0x4410_0000 {
        if x.is_nan() {
            return x;
        }

        let z = ATANHI[3] + f64::from_bits(0x0380_0000); // 0x1p-120f
        return if sign != 0 { -z } else { z };
    }

    let id = if ix < 0x3fdc_0000 {
        /* |x| < 0.4375 */
        if ix < 0x3e40_0000 {
            /* |x| < 2^-27 */
            if ix < 0x0010_0000 {
                /* raise underflow for subnormal x */
                force_eval!(x as f32);
            }

            return x;
        }

        -1
    } else {
        x = fabs(x);
        if ix < 0x3ff30000 {
            /* |x| < 1.1875 */
            if ix < 0x3fe60000 {
                /* 7/16 <= |x| < 11/16 */
                x = (2. * x - 1.) / (2. + x);
                0
            } else {
                /* 11/16 <= |x| < 19/16 */
                x = (x - 1.) / (x + 1.);
                1
            }
        } else if ix < 0x40038000 {
            /* |x| < 2.4375 */
            x = (x - 1.5) / (1. + 1.5 * x);
            2
        } else {
            /* 2.4375 <= |x| < 2^66 */
            x = -1. / x;
            3
        }
    };

    let z = x * x;
    let w = z * z;
    /* break sum from i=0 to 10 AT[i]z**(i+1) into odd and even poly */
    let s1 = z * (AT[0] + w * (AT[2] + w * (AT[4] + w * (AT[6] + w * (AT[8] + w * AT[10])))));
    let s2 = w * (AT[1] + w * (AT[3] + w * (AT[5] + w * (AT[7] + w * AT[9]))));

    if id < 0 {
        return x - x * (s1 + s2);
    }

    let z = i!(ATANHI, id as usize) - (x * (s1 + s2) - i!(ATANLO, id as usize) - x);

    if sign != 0 {
        -z
    } else {
        z
    }
}