// Licensed to the Apache Software Foundation (ASF) under one

// or more contributor license agreements.  See the NOTICE file

// distributed with this work for additional information

// regarding copyright ownership.  The ASF licenses this file

// to you under the Apache License, Version 2.0 (the

// "License"); you may not use this file except in compliance

// with the License.  You may obtain a copy of the License at

//

//   http://www.apache.org/licenses/LICENSE-2.0

//

// Unless required by applicable law or agreed to in writing,

// software distributed under the License is distributed on an

// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY

// KIND, either express or implied.  See the License for the

// specific language governing permissions and limitations

// under the License.

use crate::arith::derive_arith;

use crate::bigint::div::div_rem;

use num_bigint::BigInt;

use num_traits::{FromPrimitive, ToPrimitive, cast::AsPrimitive};

use std::cmp::Ordering;

use std::num::ParseIntError;

use std::ops::{BitAnd, BitOr, BitXor, Neg, Shl, Shr};

use std::str::FromStr;

mod div;

/// An opaque error similar to [`std::num::ParseIntError`]

#[derive(Debug)]

pub struct ParseI256Error {}

impl From<ParseIntError> for ParseI256Error {

    fn from(_: ParseIntError) -> Self {

        Self {}

    }

}

impl std::fmt::Display for ParseI256Error {

    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {

        write!(f, "Failed to parse as i256")

    }

}

impl std::error::Error for ParseI256Error {}

/// Error returned by i256::DivRem

enum DivRemError {

    /// Division by zero

    DivideByZero,

    /// Division overflow

    DivideOverflow,

}

/// A signed 256-bit integer

#[allow(non_camel_case_types)]

#[derive(Copy, Clone, Default, Eq, PartialEq, Hash)]

#[repr(C)]

pub struct i256 {

    low: u128,

    high: i128,

}

impl std::fmt::Debug for i256 {

    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {

        write!(f, "{self}")

    }

}

impl std::fmt::Display for i256 {

    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {

        write!(f, "{}", BigInt::from_signed_bytes_le(&self.to_le_bytes()))

    }

}

impl FromStr for i256 {

    type Err = ParseI256Error;

    fn from_str(s: &str) -> Result<Self, Self::Err> {

        // i128 can store up to 38 decimal digits

        if s.len() <= 38 {

            return Ok(Self::from_i128(i128::from_str(s)?));

        }

        let (negative, s) = match s.as_bytes()[0] {

            b'-' => (true, &s[1..]),

            b'+' => (false, &s[1..]),

            _ => (false, s),

        };

        // Trim leading 0s

        let s = s.trim_start_matches('0');

        if s.is_empty() {

            return Ok(i256::ZERO);

        }

        if !s.as_bytes()[0].is_ascii_digit() {

            // Ensures no duplicate sign

            return Err(ParseI256Error {});

        }

        parse_impl(s, negative)

    }

}

impl From<i8> for i256 {

    fn from(value: i8) -> Self {

        Self::from_i128(value.into())

    }

}

impl From<i16> for i256 {

    fn from(value: i16) -> Self {

        Self::from_i128(value.into())

    }

}

impl From<i32> for i256 {

    fn from(value: i32) -> Self {

        Self::from_i128(value.into())

    }

}

impl From<i64> for i256 {

    fn from(value: i64) -> Self {

        Self::from_i128(value.into())

    }

}

/// Parse `s` with any sign and leading 0s removed

fn parse_impl(s: &str, negative: bool) -> Result<i256, ParseI256Error> {

    if s.len() <= 38 {

        let low = i128::from_str(s)?;

        return Ok(match negative {

            true => i256::from_parts(low.neg() as _, -1),

            false => i256::from_parts(low as _, 0),

        });

    }

    let split = s.len() - 38;

    if !s.as_bytes()[split].is_ascii_digit() {

        // Ensures not splitting codepoint and no sign

        return Err(ParseI256Error {});

    }

    let (hs, ls) = s.split_at(split);

    let mut low = i128::from_str(ls)?;

    let high = parse_impl(hs, negative)?;

    if negative {

        low = -low;

    }

    let low = i256::from_i128(low);

    high.checked_mul(i256::from_i128(10_i128.pow(38)))

        .and_then(|high| high.checked_add(low))

        .ok_or(ParseI256Error {})

}

impl PartialOrd for i256 {

    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {

        Some(self.cmp(other))

    }

}

impl Ord for i256 {

    fn cmp(&self, other: &Self) -> Ordering {

        // This is 25x faster than using a variable length encoding such

        // as BigInt as it avoids allocation and branching

        self.high.cmp(&other.high).then(self.low.cmp(&other.low))

    }

}

impl i256 {

    /// The additive identity for this integer type, i.e. `0`.

    pub const ZERO: Self = i256 { low: 0, high: 0 };

    /// The multiplicative identity for this integer type, i.e. `1`.

    pub const ONE: Self = i256 { low: 1, high: 0 };

    /// The multiplicative inverse for this integer type, i.e. `-1`.

    pub const MINUS_ONE: Self = i256 {

        low: u128::MAX,

        high: -1,

    };

    /// The maximum value that can be represented by this integer type

    pub const MAX: Self = i256 {

        low: u128::MAX,

        high: i128::MAX,

    };

    /// The minimum value that can be represented by this integer type

    pub const MIN: Self = i256 {

        low: u128::MIN,

        high: i128::MIN,

    };

    /// Create an integer value from its representation as a byte array in little-endian.

    #[inline]

    pub const fn from_le_bytes(b: [u8; 32]) -> Self {

        let (low, high) = split_array(b);

        Self {

            high: i128::from_le_bytes(high),

            low: u128::from_le_bytes(low),

        }

    }

    /// Create an integer value from its representation as a byte array in big-endian.

    #[inline]

    pub const fn from_be_bytes(b: [u8; 32]) -> Self {

        let (high, low) = split_array(b);

        Self {

            high: i128::from_be_bytes(high),

            low: u128::from_be_bytes(low),

        }

    }

    /// Create an `i256` value from a 128-bit value.

    pub const fn from_i128(v: i128) -> Self {

        Self::from_parts(v as u128, v >> 127)

    }

    /// Create an integer value from its representation as string.

    #[inline]

    pub fn from_string(value_str: &str) -> Option<Self> {

        value_str.parse().ok()

    }

    /// Create an optional i256 from the provided `f64`. Returning `None`

    /// if overflow occurred

    pub fn from_f64(v: f64) -> Option<Self> {

        BigInt::from_f64(v).and_then(|i| {

            let (integer, overflow) = i256::from_bigint_with_overflow(i);

            if overflow { None } else { Some(integer) }

        })

    }

    /// Create an i256 from the provided low u128 and high i128

    #[inline]

    pub const fn from_parts(low: u128, high: i128) -> Self {

        Self { low, high }

    }

    /// Returns this `i256` as a low u128 and high i128

    pub const fn to_parts(self) -> (u128, i128) {

        (self.low, self.high)

    }

    /// Converts this `i256` into an `i128` returning `None` if this would result

    /// in truncation/overflow

    pub fn to_i128(self) -> Option<i128> {

        let as_i128 = self.low as i128;

        let high_negative = self.high < 0;

        let low_negative = as_i128 < 0;

        let high_valid = self.high == -1 || self.high == 0;

        (high_negative == low_negative && high_valid).then_some(self.low as i128)

    }

    /// Wraps this `i256` into an `i128`

    pub fn as_i128(self) -> i128 {

        self.low as i128

    }

    /// Return the memory representation of this integer as a byte array in little-endian byte order.

    #[inline]

    pub const fn to_le_bytes(self) -> [u8; 32] {

        let low = self.low.to_le_bytes();

        let high = self.high.to_le_bytes();

        let mut t = [0; 32];

        let mut i = 0;

        while i != 16 {

            t[i] = low[i];

            t[i + 16] = high[i];

            i += 1;

        }

        t

    }

    /// Return the memory representation of this integer as a byte array in big-endian byte order.

    #[inline]

    pub const fn to_be_bytes(self) -> [u8; 32] {

        let low = self.low.to_be_bytes();

        let high = self.high.to_be_bytes();

        let mut t = [0; 32];

        let mut i = 0;

        while i != 16 {

            t[i] = high[i];

            t[i + 16] = low[i];

            i += 1;

        }

        t

    }

    /// Create an i256 from the provided [`BigInt`] returning a bool indicating

    /// if overflow occurred

    fn from_bigint_with_overflow(v: BigInt) -> (Self, bool) {

        let v_bytes = v.to_signed_bytes_le();

        match v_bytes.len().cmp(&32) {

            Ordering::Less => {

                let mut bytes = if num_traits::Signed::is_negative(&v) {

                    [255_u8; 32]

                } else {

                    [0; 32]

                };

                bytes[0..v_bytes.len()].copy_from_slice(&v_bytes[..v_bytes.len()]);

                (Self::from_le_bytes(bytes), false)

            }

            Ordering::Equal => (Self::from_le_bytes(v_bytes.try_into().unwrap()), false),

            Ordering::Greater => (Self::from_le_bytes(v_bytes[..32].try_into().unwrap()), true),

        }

    }

    /// Computes the absolute value of this i256

    #[inline]

    pub fn wrapping_abs(self) -> Self {

        // -1 if negative, otherwise 0

        let sa = self.high >> 127;

        let sa = Self::from_parts(sa as u128, sa);

        // Inverted if negative

        Self::from_parts(self.low ^ sa.low, self.high ^ sa.high).wrapping_sub(sa)

    }

    /// Computes the absolute value of this i256 returning `None` if `Self == Self::MIN`

    #[inline]

    pub fn checked_abs(self) -> Option<Self> {

        (self != Self::MIN).then(|| self.wrapping_abs())

    }

    /// Negates this i256

    #[inline]

    pub fn wrapping_neg(self) -> Self {

        Self::from_parts(!self.low, !self.high).wrapping_add(i256::ONE)

    }

    /// Negates this i256 returning `None` if `Self == Self::MIN`

    #[inline]

    pub fn checked_neg(self) -> Option<Self> {

        (self != Self::MIN).then(|| self.wrapping_neg())

    }

    /// Performs wrapping addition

    #[inline]

    pub fn wrapping_add(self, other: Self) -> Self {

        let (low, carry) = self.low.overflowing_add(other.low);

        let high = self.high.wrapping_add(other.high).wrapping_add(carry as _);

        Self { low, high }

    }

    /// Performs checked addition

    #[inline]

    pub fn checked_add(self, other: Self) -> Option<Self> {

        let r = self.wrapping_add(other);

        ((other.is_negative() && r < self) || (!other.is_negative() && r >= self)).then_some(r)

    }

    /// Performs wrapping subtraction

    #[inline]

    pub fn wrapping_sub(self, other: Self) -> Self {

        let (low, carry) = self.low.overflowing_sub(other.low);

        let high = self.high.wrapping_sub(other.high).wrapping_sub(carry as _);

        Self { low, high }

    }

    /// Performs checked subtraction

    #[inline]

    pub fn checked_sub(self, other: Self) -> Option<Self> {

        let r = self.wrapping_sub(other);

        ((other.is_negative() && r > self) || (!other.is_negative() && r <= self)).then_some(r)

    }

    /// Performs wrapping multiplication

    #[inline]

    pub fn wrapping_mul(self, other: Self) -> Self {

        let (low, high) = mulx(self.low, other.low);

        // Compute the high multiples, only impacting the high 128-bits

        let hl = self.high.wrapping_mul(other.low as i128);

        let lh = (self.low as i128).wrapping_mul(other.high);

        Self {

            low,

            high: (high as i128).wrapping_add(hl).wrapping_add(lh),

        }

    }

    /// Performs checked multiplication

    #[inline]

    pub fn checked_mul(self, other: Self) -> Option<Self> {

        if self == i256::ZERO || other == i256::ZERO {

            return Some(i256::ZERO);

        }

        // Shift sign bit down to construct mask of all set bits if negative

        let l_sa = self.high >> 127;

        let r_sa = other.high >> 127;

        let out_sa = (l_sa ^ r_sa) as u128;

        // Compute absolute values

        let l_abs = self.wrapping_abs();

        let r_abs = other.wrapping_abs();

        // Overflow if both high parts are non-zero

        if l_abs.high != 0 && r_abs.high != 0 {

            return None;

        }

        // Perform checked multiplication on absolute values

        let (low, high) = mulx(l_abs.low, r_abs.low);

        // Compute the high multiples, only impacting the high 128-bits

        let hl = (l_abs.high as u128).checked_mul(r_abs.low)?;

        let lh = l_abs.low.checked_mul(r_abs.high as u128)?;

        let high = high.checked_add(hl)?.checked_add(lh)?;

        // Reverse absolute value, if necessary

        let (low, c) = (low ^ out_sa).overflowing_sub(out_sa);

        let high = (high ^ out_sa).wrapping_sub(out_sa).wrapping_sub(c as u128) as i128;

        // Check for overflow in final conversion

        (high.is_negative() == (self.is_negative() ^ other.is_negative()))

            .then_some(Self { low, high })

    }

    /// Division operation, returns (quotient, remainder).

    /// This basically implements [Long division]: `<https://en.wikipedia.org/wiki/Division_algorithm>`

    #[inline]

    fn div_rem(self, other: Self) -> Result<(Self, Self), DivRemError> {

        if other == Self::ZERO {

            return Err(DivRemError::DivideByZero);

        }

        if other == Self::MINUS_ONE && self == Self::MIN {

            return Err(DivRemError::DivideOverflow);

        }

        let a = self.wrapping_abs();

        let b = other.wrapping_abs();

        let (div, rem) = div_rem(&a.as_digits(), &b.as_digits());

        let div = Self::from_digits(div);

        let rem = Self::from_digits(rem);

        Ok((

            if self.is_negative() == other.is_negative() {

                div

            } else {

                div.wrapping_neg()

            },

            if self.is_negative() {

                rem.wrapping_neg()

            } else {

                rem

            },

        ))

    }

    /// Interpret this [`i256`] as 4 `u64` digits, least significant first

    fn as_digits(self) -> [u64; 4] {

        [

            self.low as u64,

            (self.low >> 64) as u64,

            self.high as u64,

            (self.high as u128 >> 64) as u64,

        ]

    }

    /// Interpret 4 `u64` digits, least significant first, as a [`i256`]

    fn from_digits(digits: [u64; 4]) -> Self {

        Self::from_parts(

            digits[0] as u128 | ((digits[1] as u128) << 64),

            digits[2] as i128 | ((digits[3] as i128) << 64),

        )

    }

    /// Performs wrapping division

    #[inline]

    pub fn wrapping_div(self, other: Self) -> Self {

        match self.div_rem(other) {

            Ok((v, _)) => v,

            Err(DivRemError::DivideByZero) => panic!("attempt to divide by zero"),

            Err(_) => Self::MIN,

        }

    }

    /// Performs checked division

    #[inline]

    pub fn checked_div(self, other: Self) -> Option<Self> {

        self.div_rem(other).map(|(v, _)| v).ok()

    }

    /// Performs wrapping remainder

    #[inline]

    pub fn wrapping_rem(self, other: Self) -> Self {

        match self.div_rem(other) {

            Ok((_, v)) => v,

            Err(DivRemError::DivideByZero) => panic!("attempt to divide by zero"),

            Err(_) => Self::ZERO,

        }

    }

    /// Performs checked remainder

    #[inline]

    pub fn checked_rem(self, other: Self) -> Option<Self> {

        self.div_rem(other).map(|(_, v)| v).ok()

    }

    /// Performs checked exponentiation

    #[inline]

    pub fn checked_pow(self, mut exp: u32) -> Option<Self> {

        if exp == 0 {

            return Some(i256::from_i128(1));

        }

        let mut base = self;

        let mut acc: Self = i256::from_i128(1);

        while exp > 1 {

            if (exp & 1) == 1 {

                acc = acc.checked_mul(base)?;

            }

            exp /= 2;

            base = base.checked_mul(base)?;

        }

        // since exp!=0, finally the exp must be 1.

        // Deal with the final bit of the exponent separately, since

        // squaring the base afterwards is not necessary and may cause a

        // needless overflow.

        acc.checked_mul(base)

    }

    /// Performs wrapping exponentiation

    #[inline]

    pub fn wrapping_pow(self, mut exp: u32) -> Self {

        if exp == 0 {

            return i256::from_i128(1);

        }

        let mut base = self;

        let mut acc: Self = i256::from_i128(1);

        while exp > 1 {

            if (exp & 1) == 1 {

                acc = acc.wrapping_mul(base);

            }

            exp /= 2;

            base = base.wrapping_mul(base);

        }

        // since exp!=0, finally the exp must be 1.

        // Deal with the final bit of the exponent separately, since

        // squaring the base afterwards is not necessary and may cause a

        // needless overflow.

        acc.wrapping_mul(base)

    }

    /// Returns a number [`i256`] representing sign of this [`i256`].

    ///

    /// 0 if the number is zero

    /// 1 if the number is positive

    /// -1 if the number is negative

    pub const fn signum(self) -> Self {

        if self.is_positive() {

            i256::ONE

        } else if self.is_negative() {

            i256::MINUS_ONE

        } else {

            i256::ZERO

        }

    }

    /// Returns `true` if this [`i256`] is negative

    #[inline]

    pub const fn is_negative(self) -> bool {

        self.high.is_negative()

    }

    /// Returns `true` if this [`i256`] is positive

    pub const fn is_positive(self) -> bool {

        self.high.is_positive() || self.high == 0 && self.low != 0

    }

    fn leading_zeros(&self) -> u32 {

        match self.high {

            0 => u128::BITS + self.low.leading_zeros(),

            _ => self.high.leading_zeros(),

        }

    }

    fn redundant_leading_sign_bits_i256(n: i256) -> u8 {

        let mask = n >> 255; // all ones or all zeros

        ((n ^ mask).leading_zeros() - 1) as u8 // we only need one sign bit

    }

    fn i256_to_f64(input: i256) -> f64 {

        let k = i256::redundant_leading_sign_bits_i256(input);

        let n = input << k; // left-justify (no redundant sign bits)

        let n = (n.high >> 64) as i64; // throw away the lower 192 bits

        (n as f64) * f64::powi(2.0, 192 - (k as i32)) // convert to f64 and scale it, as we left-shift k bit previous, so we need to scale it by 2^(192-k)

    }

}

/// Temporary workaround due to lack of stable const array slicing

/// See <https://github.com/rust-lang/rust/issues/90091>

const fn split_array<const N: usize, const M: usize>(vals: [u8; N]) -> ([u8; M], [u8; M]) {

    let mut a = [0; M];

    let mut b = [0; M];

    let mut i = 0;

    while i != M {

        a[i] = vals[i];

        b[i] = vals[i + M];

        i += 1;

    }

    (a, b)

}

/// Performs an unsigned multiplication of `a * b` returning a tuple of

/// `(low, high)` where `low` contains the lower 128-bits of the result

/// and `high` the higher 128-bits

///

/// This mirrors the x86 mulx instruction but for 128-bit types

#[inline]

fn mulx(a: u128, b: u128) -> (u128, u128) {

    let split = |a: u128| (a & (u64::MAX as u128), a >> 64);

    const MASK: u128 = u64::MAX as _;

    let (a_low, a_high) = split(a);

    let (b_low, b_high) = split(b);

    // Carry stores the upper 64-bits of low and lower 64-bits of high

    let (mut low, mut carry) = split(a_low * b_low);

    carry += a_high * b_low;

    // Update low and high with corresponding parts of carry

    low += carry << 64;

    let mut high = carry >> 64;

    // Update carry with overflow from low

    carry = low >> 64;

    low &= MASK;

    // Perform multiply including overflow from low

    carry += b_high * a_low;

    // Update low and high with values from carry

    low += carry << 64;

    high += carry >> 64;

    // Perform 4th multiplication

    high += a_high * b_high;

    (low, high)

}

derive_arith!(

    i256,

    Add,

    AddAssign,

    add,

    add_assign,

    wrapping_add,

    checked_add

);

derive_arith!(

    i256,

    Sub,

    SubAssign,

    sub,

    sub_assign,

    wrapping_sub,

    checked_sub

);

derive_arith!(

    i256,

    Mul,

    MulAssign,

    mul,

    mul_assign,

    wrapping_mul,

    checked_mul

);

derive_arith!(

    i256,

    Div,

    DivAssign,

    div,

    div_assign,

    wrapping_div,

    checked_div

);

derive_arith!(

    i256,

    Rem,

    RemAssign,

    rem,

    rem_assign,

    wrapping_rem,

    checked_rem

);

impl Neg for i256 {

    type Output = i256;

    #[cfg(debug_assertions)]

    fn neg(self) -> Self::Output {

        self.checked_neg().expect("i256 overflow")

    }

    #[cfg(not(debug_assertions))]

    fn neg(self) -> Self::Output {

        self.wrapping_neg()

    }

}

impl BitAnd for i256 {

    type Output = i256;

    #[inline]

    fn bitand(self, rhs: Self) -> Self::Output {

        Self {

            low: self.low & rhs.low,

            high: self.high & rhs.high,

        }

    }

}

impl BitOr for i256 {

    type Output = i256;

    #[inline]

    fn bitor(self, rhs: Self) -> Self::Output {

        Self {

            low: self.low | rhs.low,

            high: self.high | rhs.high,

        }

    }

}

impl BitXor for i256 {

    type Output = i256;

    #[inline]

    fn bitxor(self, rhs: Self) -> Self::Output {

        Self {

            low: self.low ^ rhs.low,

            high: self.high ^ rhs.high,

        }

    }

}

impl Shl<u8> for i256 {

    type Output = i256;

    #[inline]

    fn shl(self, rhs: u8) -> Self::Output {

        if rhs == 0 {

            self

        } else if rhs < 128 {

            Self {

                high: (self.high << rhs) | (self.low >> (128 - rhs)) as i128,

                low: self.low << rhs,

            }

        } else {

            Self {

                high: (self.low << (rhs - 128)) as i128,

                low: 0,

            }

        }

    }

}

impl Shr<u8> for i256 {

    type Output = i256;

    #[inline]

    fn shr(self, rhs: u8) -> Self::Output {

        if rhs == 0 {

            self

        } else if rhs < 128 {

            Self {

                high: self.high >> rhs,

                low: (self.low >> rhs) | ((self.high as u128) << (128 - rhs)),

            }

        } else {

            Self {

                high: self.high >> 127,

                low: (self.high >> (rhs - 128)) as u128,

            }

        }

    }

}

macro_rules! define_as_primitive {

    ($native_ty:ty) => {

        impl AsPrimitive<i256> for $native_ty {

            fn as_(self) -> i256 {

                i256::from_i128(self as i128)

            }

        }

    };

}

define_as_primitive!(i8);

define_as_primitive!(i16);

define_as_primitive!(i32);

define_as_primitive!(i64);

define_as_primitive!(u8);

define_as_primitive!(u16);

define_as_primitive!(u32);

define_as_primitive!(u64);

impl ToPrimitive for i256 {

    fn to_i64(&self) -> Option<i64> {

        let as_i128 = self.low as i128;

        let high_negative = self.high < 0;

        let low_negative = as_i128 < 0;

        let high_valid = self.high == -1 || self.high == 0;

        if high_negative == low_negative && high_valid {

            let (low_bytes, high_bytes) = split_array(u128::to_le_bytes(self.low));

            let high = i64::from_le_bytes(high_bytes);

            let low = i64::from_le_bytes(low_bytes);

            let high_negative = high < 0;

            let low_negative = low < 0;

            let high_valid = self.high == -1 || self.high == 0;

            (high_negative == low_negative && high_valid).then_some(low)

        } else {

            None

        }

    }

    fn to_f64(&self) -> Option<f64> {

        match *self {

            Self::MIN => Some(-2_f64.powi(255)),

            Self::ZERO => Some(0f64),

            Self::ONE => Some(1f64),

            n => Some(Self::i256_to_f64(n)),

        }

    }

    fn to_u64(&self) -> Option<u64> {

        let as_i128 = self.low as i128;

        let high_negative = self.high < 0;

        let low_negative = as_i128 < 0;

        let high_valid = self.high == -1 || self.high == 0;

        if high_negative == low_negative && high_valid {

            self.low.to_u64()

        } else {

            None

        }

    }

}

#[cfg(all(test, not(miri)))] // llvm.x86.subborrow.64 not supported by MIRI

mod tests {

    use super::*;

    use num_traits::Signed;

    use rand::{Rng, rng};

    #[test]

    fn test_signed_cmp() {

        let a = i256::from_parts(i128::MAX as u128, 12);

        let b = i256::from_parts(i128::MIN as u128, 12);

        assert!(a < b);

        let a = i256::from_parts(i128::MAX as u128, 12);

        let b = i256::from_parts(i128::MIN as u128, -12);

        assert!(a > b);

    }

    #[test]

    fn test_to_i128() {

        let vals = [

            BigInt::from_i128(-1).unwrap(),

            BigInt::from_i128(i128::MAX).unwrap(),

            BigInt::from_i128(i128::MIN).unwrap(),

            BigInt::from_u128(u128::MIN).unwrap(),

            BigInt::from_u128(u128::MAX).unwrap(),

        ];

        for v in vals {

            let (t, overflow) = i256::from_bigint_with_overflow(v.clone());

            assert!(!overflow);

            assert_eq!(t.to_i128(), v.to_i128(), "{v} vs {t}");

        }

    }

    /// Tests operations against the two provided [`i256`]

    fn test_ops(il: i256, ir: i256) {

        let bl = BigInt::from_signed_bytes_le(&il.to_le_bytes());

        let br = BigInt::from_signed_bytes_le(&ir.to_le_bytes());

        // Comparison

        assert_eq!(il.cmp(&ir), bl.cmp(&br), "{bl} cmp {br}");

        // Conversions

        assert_eq!(i256::from_le_bytes(il.to_le_bytes()), il);

        assert_eq!(i256::from_be_bytes(il.to_be_bytes()), il);

        assert_eq!(i256::from_le_bytes(ir.to_le_bytes()), ir);

        assert_eq!(i256::from_be_bytes(ir.to_be_bytes()), ir);

        // To i128

        assert_eq!(il.to_i128(), bl.to_i128(), "{bl}");

        assert_eq!(ir.to_i128(), br.to_i128(), "{br}");

        // Absolute value

        let (abs, overflow) = i256::from_bigint_with_overflow(bl.abs());

        assert_eq!(il.wrapping_abs(), abs);

        assert_eq!(il.checked_abs().is_none(), overflow);

        let (abs, overflow) = i256::from_bigint_with_overflow(br.abs());

        assert_eq!(ir.wrapping_abs(), abs);

        assert_eq!(ir.checked_abs().is_none(), overflow);

        // Negation

        let (neg, overflow) = i256::from_bigint_with_overflow(bl.clone().neg());

        assert_eq!(il.wrapping_neg(), neg);

        assert_eq!(il.checked_neg().is_none(), overflow);

        // Negation

        let (neg, overflow) = i256::from_bigint_with_overflow(br.clone().neg());

        assert_eq!(ir.wrapping_neg(), neg);

        assert_eq!(ir.checked_neg().is_none(), overflow);

        // Addition

        let actual = il.wrapping_add(ir);

        let (expected, overflow) = i256::from_bigint_with_overflow(bl.clone() + br.clone());

        assert_eq!(actual, expected);

        let checked = il.checked_add(ir);

        match overflow {

            true => assert!(checked.is_none()),

            false => assert_eq!(checked, Some(actual)),

        }

        // Subtraction

        let actual = il.wrapping_sub(ir);

        let (expected, overflow) = i256::from_bigint_with_overflow(bl.clone() - br.clone());

        assert_eq!(actual.to_string(), expected.to_string());

        let checked = il.checked_sub(ir);

        match overflow {

            true => assert!(checked.is_none()),

            false => assert_eq!(checked, Some(actual), "{bl} - {br} = {expected}"),

        }

        // Multiplication

        let actual = il.wrapping_mul(ir);

        let (expected, overflow) = i256::from_bigint_with_overflow(bl.clone() * br.clone());

        assert_eq!(actual.to_string(), expected.to_string());

        let checked = il.checked_mul(ir);

        match overflow {

            true => assert!(

                checked.is_none(),

                "{il} * {ir} = {actual} vs {bl} * {br} = {expected}"

            ),

            false => assert_eq!(

                checked,

                Some(actual),

                "{il} * {ir} = {actual} vs {bl} * {br} = {expected}"

            ),

        }

        // Division

        if ir != i256::ZERO {

            let actual = il.wrapping_div(ir);

            let expected = bl.clone() / br.clone();

            let checked = il.checked_div(ir);

            if ir == i256::MINUS_ONE && il == i256::MIN {

                // BigInt produces an integer over i256::MAX

                assert_eq!(actual, i256::MIN);

                assert!(checked.is_none());

            } else {

                assert_eq!(actual.to_string(), expected.to_string());

                assert_eq!(checked.unwrap().to_string(), expected.to_string());

            }

        } else {

            // `wrapping_div` panics on division by zero

            assert!(il.checked_div(ir).is_none());

        }

        // Remainder

        if ir != i256::ZERO {

            let actual = il.wrapping_rem(ir);

            let expected = bl.clone() % br.clone();

            let checked = il.checked_rem(ir);

            assert_eq!(actual.to_string(), expected.to_string(), "{il} % {ir}");