fix/

lib.rs

//! Fixed-point number types.
//!
//! # What?
//!
//! Fixed-point is a number representation with a fixed number of digits before and after the radix
//! point. This means that range is static rather than dynamic, as with floating-point. It also
//! means that they can be represented as integers, with their scale tracked by the type system.
//!
//! In this library, the scale of a `Fix` is represented as two type-level integers: the base and
//! the exponent. Any underlying integer primitive can be used to store the number. Arithmetic can
//! be performed on these numbers, and they can be converted to different scale exponents.
//!
//! # Why?
//!
//! A classic example: let's sum 10 cents and 20 cents using floating-point. We expect a result of
//! 30 cents.
//!
//! ```should_panic
//! assert_eq!(0.30, 0.10 + 0.20);
//! ```
//!
//! Wrong! We get an extra forty quintillionths of a dollar.
//!
//! ```text
//! assertion failed: `(left == right)` (left: `0.3`, right: `0.30000000000000004`)'
//! ```
//!
//! This is due to neither 0.1 nor 0.2 being exactly representable in base-2, just as a third can't
//! be represented exactly in base-10. With `Fix`, we can choose the precision we want in base-10,
//! at compile-time. In this case, hundredths of a dollar will do.
//!
//! ```
//! use fix::aliases::si::Centi; // Fix<_, U10, N2>
//! assert_eq!(Centi::new(0_30), Centi::new(0_10) + Centi::new(0_20));
//! ```
//!
//! But decimal is inefficient for binary computers, right? Multiplying and dividing by 10 is
//! slower than bit-shifting, but that's only needed when _moving_ the point. With `Fix`, this is
//! only done explicitly with the `convert` method.
//!
//! ```
//! use fix::aliases::si::{Centi, Milli};
//! assert_eq!(Milli::new(0_300), Centi::new(0_30).convert());
//! ```
//!
//! We can also choose a base-2 scale just as easily.
//!
//! ```
//! use fix::aliases::iec::{Kibi, Mebi};
//! assert_eq!(Kibi::new(1024), Mebi::new(1).convert());
//! ```
//!
//! It's also worth noting that the type-level scale changes when multiplying and dividing,
//! avoiding any implicit conversion.
//!
//! ```
//! use fix::aliases::iec::{Gibi, Kibi, Mebi};
//! assert_eq!(Mebi::new(3), Gibi::new(6) / Kibi::new(2));
//! ```
//!
//! # `no_std`
//!
//! This crate is `no_std`.

//#![no_std]

pub extern crate muldiv;
pub extern crate num_traits;
pub extern crate typenum;

pub mod aliases;
pub mod fix_value;
pub mod prelude;
pub mod util;

use core::cmp::{Eq, Ord, Ordering, PartialEq, PartialOrd};
use core::fmt::{Debug, Error, Formatter};
use core::hash::{Hash, Hasher};
use core::marker::PhantomData;
use core::ops::{Add, Div, Mul, Neg, Rem, Sub};
use core::ops::{AddAssign, DivAssign, MulAssign, RemAssign, SubAssign};

use muldiv::MulDiv;
use num_traits::{CheckedAdd, CheckedDiv, CheckedMul, CheckedSub, SaturatingAdd, SaturatingSub};
use paste::paste;
use typenum::consts::Z0;
use typenum::marker_traits::{Bit, Integer, Unsigned};
use typenum::operator_aliases::{AbsVal, Diff, Le, Sum};
use typenum::type_operators::{Abs, IsLess};

/// Fixed-point number representing _Bits × Base <sup>Exp</sup>_.
///
/// - `Bits` is an integer primitive type, or any type which can be created from a type-level
///   integer and exponentiated.
/// - `Base` is an [`Unsigned`] type-level integer.
/// - `Exp` is a signed type-level [`Integer`].
///
/// [`Unsigned`]: ../typenum/marker_traits/trait.Unsigned.html
/// [`Integer`]: ../typenum/marker_traits/trait.Integer.html
///
/// # Summary of operations
///
/// Lower case variables represent values of _Bits_. Upper case _B_ and _E_ represent type-level
/// integers _Base_ and _Exp_, respectively.
///
/// - _−(x B<sup>E</sup>) = (−x) B<sup>E</sup>_
/// - _(x B<sup>E</sup>) + (y B<sup>E</sup>) = (x + y) B<sup>E</sup>_
/// - _(x B<sup>E</sup>) − (y B<sup>E</sup>) = (x − y) B<sup>E</sup>_
/// - _(x B<sup>E<sub>x</sub></sup>) × (y B<sup>E<sub>y</sub></sup>) =
///   (x × y) B<sup>E<sub>x</sub> + E<sub>y</sub></sup>_
/// - _(x B<sup>E<sub>x</sub></sup>) ÷ (y B<sup>E<sub>y</sub></sup>) =
///   (x ÷ y) B<sup>E<sub>x</sub> − E<sub>y</sub></sup>_
/// - _(x B<sup>E<sub>x</sub></sup>) % (y B<sup>E<sub>y</sub></sup>) =
///   (x % y) B<sup>E<sub>x</sub></sup>_
/// - _(x B<sup>E</sup>) × y = (x × y) B<sup>E</sup>_
/// - _(x B<sup>E</sup>) ÷ y = (x ÷ y) B<sup>E</sup>_
/// - _(x B<sup>E</sup>) % y = (x % y) B<sup>E</sup>_
pub struct Fix<Bits, Base, Exp> {
    /// The underlying integer.
    pub bits: Bits,

    marker: PhantomData<(Base, Exp)>,
}

impl<Bits, Base, Exp> Fix<Bits, Base, Exp> {
    /// Creates a number.
    ///
    /// # Examples
    ///
    /// ```
    /// use fix::aliases::si::{Kilo, Milli};
    /// Milli::new(25); // 0.025
    /// Kilo::new(25); // 25 000
    /// ```
    pub fn new(bits: Bits) -> Self {
        Fix {
            bits,
            marker: PhantomData,
        }
    }

    /// Like `Self::new`, but creates numbers in the constant context.
    pub const fn constant(bits: Bits) -> Self {
        Fix {
            bits,
            marker: PhantomData,
        }
    }

    /// Converts to another _Exp_.
    ///
    /// # Examples
    ///
    /// ```
    /// use fix::aliases::si::{Kilo, Milli};
    /// let kilo = Kilo::new(5);
    /// let milli = Milli::new(5_000_000);
    /// assert_eq!(kilo, milli.convert());
    /// assert_eq!(milli, kilo.convert());
    /// ```
    pub fn convert<ToExp>(self) -> Fix<Bits, Base, ToExp>
    where
        Bits: FromUnsigned + Pow + Mul<Output = Bits> + Div<Output = Bits>,
        Base: Unsigned,
        Exp: Sub<ToExp>,
        Diff<Exp, ToExp>: Abs + IsLess<Z0>,
        AbsVal<Diff<Exp, ToExp>>: Integer,
    {
        let base = Bits::from_unsigned::<Base>();
        let diff = AbsVal::<Diff<Exp, ToExp>>::to_i32();
        let inverse = Le::<Diff<Exp, ToExp>, Z0>::to_bool();

        // FIXME: Would like to do this with typenum::Pow, but that
        // seems to result in overflow evaluating requirements.
        let ratio = base.pow(diff.unsigned_abs());

        if inverse {
            Fix::new(self.bits / ratio)
        } else {
            Fix::new(self.bits * ratio)
        }
    }

    /// Converts the underlying bits to a wider type.
    ///
    /// # Examples
    ///
    /// ```
    /// use fix::aliases::si::Milli;
    /// let one = Milli::new(16899u64);
    /// let mapped = one.widen::<u128>();
    /// assert_eq!(mapped, Milli::new(16899u128));
    /// ```
    ///
    pub fn widen<ToBits>(self) -> Fix<ToBits, Base, Exp>
    where
        ToBits: From<Bits>,
    {
        Fix::<ToBits, Base, Exp>::new(self.bits.into())
    }

    /// Attempts to converts underlying bits to a narrower type.
    /// Returns `None` if conversion fails.
    ///
    /// # Examples
    ///
    /// ```
    /// use fix::aliases::si::Milli;
    /// let one = Milli::new(16899u128);
    /// let mapped = one.narrow::<u64>();
    /// assert_eq!(mapped, Some(Milli::new(16899u64)));
    /// ```
    ///
    pub fn narrow<ToBits>(self) -> Option<Fix<ToBits, Base, Exp>>
    where
        ToBits: TryFrom<Bits>,
    {
        self.bits.try_into().ok().map(Fix::<ToBits, Base, Exp>::new)
    }
}

/// Conversion from type-level [`Unsigned`] integers.
///
/// Enables being generic over types which can be created from type-level integers. It should
/// probably be in `typenum` itself...
///
/// [`Unsigned`]: ../typenum/marker_traits/trait.Unsigned.html
pub trait FromUnsigned {
    /// Creates a value from a type.
    fn from_unsigned<U>() -> Self
    where
        U: Unsigned;
}

macro_rules! impl_from_unsigned {
    ($ty:ident) => {
        impl FromUnsigned for $ty {
            fn from_unsigned<U: Unsigned>() -> Self {
                paste! { U::[<to_$ty>]() }
            }
        }
    };
}

impl_from_unsigned!(u8);
impl_from_unsigned!(u16);
impl_from_unsigned!(u32);
impl_from_unsigned!(u64);
impl_from_unsigned!(u128);
impl_from_unsigned!(usize);
impl_from_unsigned!(i8);
impl_from_unsigned!(i16);
impl_from_unsigned!(i32);
impl_from_unsigned!(i64);
impl_from_unsigned!(i128);
impl_from_unsigned!(isize);

/// Exponentiation.
///
/// Enables being generic over integers which can be exponentiated. Why must we do this, standard
/// library?
pub trait Pow {
    /// Raises `self` to the power of `exp`.
    #[must_use]
    fn pow(self, exp: u32) -> Self;
}

macro_rules! impl_pow {
    ($ty:ident) => {
        impl Pow for $ty {
            #[inline]
            fn pow(self, exp: u32) -> Self {
                self.pow(exp)
            }
        }
    };
}

impl_pow!(u8);
impl_pow!(u16);
impl_pow!(u32);
impl_pow!(u64);
impl_pow!(u128);
impl_pow!(usize);
impl_pow!(i8);
impl_pow!(i16);
impl_pow!(i32);
impl_pow!(i64);
impl_pow!(i128);
impl_pow!(isize);

// The usual traits.

impl<Bits, Base, Exp> Copy for Fix<Bits, Base, Exp> where Bits: Copy {}

impl<Bits, Base, Exp> Clone for Fix<Bits, Base, Exp>
where
    Bits: Clone,
{
    fn clone(&self) -> Self {
        Self::new(self.bits.clone())
    }
}

impl<Bits, Base, Exp> Default for Fix<Bits, Base, Exp>
where
    Bits: Default,
{
    fn default() -> Self {
        Self::new(Bits::default())
    }
}

impl<Bits, Base, Exp> Hash for Fix<Bits, Base, Exp>
where
    Bits: Hash,
{
    fn hash<H>(&self, state: &mut H)
    where
        H: Hasher,
    {
        self.bits.hash(state);
    }
}

impl<Bits, Base, Exp> Debug for Fix<Bits, Base, Exp>
where
    Bits: Debug,
    Base: Unsigned,
    Exp: Integer,
{
    fn fmt(&self, f: &mut Formatter) -> Result<(), Error> {
        write!(f, "{:?}x{}^{}", self.bits, Base::to_u64(), Exp::to_i64())
    }
}

// Comparison.

impl<Bits, Base, Exp> Eq for Fix<Bits, Base, Exp> where Bits: Eq {}
impl<Bits, Base, Exp> PartialEq for Fix<Bits, Base, Exp>
where
    Bits: PartialEq,
{
    fn eq(&self, rhs: &Self) -> bool {
        self.bits == rhs.bits
    }
}

impl<Bits, Base, Exp> PartialOrd for Fix<Bits, Base, Exp>
where
    Bits: PartialOrd,
{
    fn partial_cmp(&self, rhs: &Self) -> Option<Ordering> {
        self.bits.partial_cmp(&rhs.bits)
    }
}

impl<Bits, Base, Exp> Ord for Fix<Bits, Base, Exp>
where
    Bits: Ord,
{
    fn cmp(&self, rhs: &Self) -> Ordering {
        self.bits.cmp(&rhs.bits)
    }
}

// Arithmetic.

impl<Bits, Base, Exp> Neg for Fix<Bits, Base, Exp>
where
    Bits: Neg<Output = Bits>,
{
    type Output = Self;
    fn neg(self) -> Self {
        Self::new(-self.bits)
    }
}

impl<Bits, Base, Exp> Add for Fix<Bits, Base, Exp>
where
    Bits: Add<Output = Bits>,
{
    type Output = Self;
    fn add(self, rhs: Self) -> Self {
        Self::new(self.bits + rhs.bits)
    }
}

impl<Bits, Base, Exp> Sub for Fix<Bits, Base, Exp>
where
    Bits: Sub<Output = Bits>,
{
    type Output = Self;
    fn sub(self, rhs: Self) -> Self {
        Self::new(self.bits - rhs.bits)
    }
}

impl<Bits, Base, LExp, RExp> Mul<Fix<Bits, Base, RExp>> for Fix<Bits, Base, LExp>
where
    Bits: Mul<Output = Bits>,
    LExp: Add<RExp>,
{
    type Output = Fix<Bits, Base, Sum<LExp, RExp>>;
    fn mul(self, rhs: Fix<Bits, Base, RExp>) -> Self::Output {
        Self::Output::new(self.bits * rhs.bits)
    }
}

impl<Bits, Base, LExp, RExp> Div<Fix<Bits, Base, RExp>> for Fix<Bits, Base, LExp>
where
    Bits: Div<Output = Bits>,
    LExp: Sub<RExp>,
{
    type Output = Fix<Bits, Base, Diff<LExp, RExp>>;
    fn div(self, rhs: Fix<Bits, Base, RExp>) -> Self::Output {
        Self::Output::new(self.bits / rhs.bits)
    }
}

impl<Bits, Base, Exp> Rem for Fix<Bits, Base, Exp>
where
    Bits: Rem<Output = Bits>,
{
    type Output = Self;
    fn rem(self, rhs: Self) -> Self {
        Self::new(self.bits % rhs.bits)
    }
}

impl<Bits, Base, Exp> Mul<Bits> for Fix<Bits, Base, Exp>
where
    Bits: Mul<Output = Bits>,
{
    type Output = Self;
    fn mul(self, rhs: Bits) -> Self {
        Self::new(self.bits * rhs)
    }
}

impl<Bits, Base, Exp> Div<Bits> for Fix<Bits, Base, Exp>
where
    Bits: Div<Output = Bits>,
{
    type Output = Self;
    fn div(self, rhs: Bits) -> Self {
        Self::new(self.bits / rhs)
    }
}

impl<Bits, Base, Exp> Rem<Bits> for Fix<Bits, Base, Exp>
where
    Bits: Rem<Output = Bits>,
{
    type Output = Self;
    fn rem(self, rhs: Bits) -> Self {
        Self::new(self.bits % rhs)
    }
}

impl<Bits, Base, Exp> AddAssign for Fix<Bits, Base, Exp>
where
    Bits: AddAssign,
{
    fn add_assign(&mut self, rhs: Self) {
        self.bits += rhs.bits;
    }
}

impl<Bits, Base, Exp> SubAssign for Fix<Bits, Base, Exp>
where
    Bits: SubAssign,
{
    fn sub_assign(&mut self, rhs: Self) {
        self.bits -= rhs.bits;
    }
}

impl<Bits, Base, Exp> MulAssign<Bits> for Fix<Bits, Base, Exp>
where
    Bits: MulAssign,
{
    fn mul_assign(&mut self, rhs: Bits) {
        self.bits *= rhs;
    }
}

impl<Bits, Base, Exp> DivAssign<Bits> for Fix<Bits, Base, Exp>
where
    Bits: DivAssign,
{
    fn div_assign(&mut self, rhs: Bits) {
        self.bits /= rhs;
    }
}

impl<Bits, Base, LExp, RExp> RemAssign<Fix<Bits, Base, RExp>> for Fix<Bits, Base, LExp>
where
    Bits: RemAssign,
{
    fn rem_assign(&mut self, rhs: Fix<Bits, Base, RExp>) {
        self.bits %= rhs.bits;
    }
}

impl<Bits, Base, Exp> RemAssign<Bits> for Fix<Bits, Base, Exp>
where
    Bits: RemAssign,
{
    fn rem_assign(&mut self, rhs: Bits) {
        self.bits %= rhs;
    }
}

// Checked arithmetic.

impl<Bits, Base, Exp> CheckedAdd for Fix<Bits, Base, Exp>
where
    Bits: CheckedAdd,
{
    fn checked_add(&self, v: &Self) -> Option<Self> {
        self.bits.checked_add(&v.bits).map(Self::new)
    }
}

impl<Bits, Base, Exp> CheckedSub for Fix<Bits, Base, Exp>
where
    Bits: CheckedSub,
{
    fn checked_sub(&self, v: &Self) -> Option<Self> {
        self.bits.checked_sub(&v.bits).map(Self::new)
    }
}

impl<Bits, Base, Exp> Fix<Bits, Base, Exp>
where
    Self: CheckedSub,
    Bits: Copy,
{
    #[must_use]
    pub fn abs_diff(&self, v: &Self) -> Fix<Bits, Base, Exp> {
        self.checked_sub(v).unwrap_or_else(|| *v - *self)
    }
}

/// Adapts `CheckedMul` concept to this library with computed `Output` type.
pub trait CheckedMulFix<Rhs> {
    type Output;
    fn checked_mul(&self, v: &Rhs) -> Option<Self::Output>;
}

impl<Bits, Base, LExp, RExp> CheckedMulFix<Fix<Bits, Base, RExp>> for Fix<Bits, Base, LExp>
where
    Bits: CheckedMul,
    LExp: Add<RExp>,
{
    type Output = Fix<Bits, Base, Sum<LExp, RExp>>;
    fn checked_mul(&self, v: &Fix<Bits, Base, RExp>) -> Option<Self::Output> {
        self.bits.checked_mul(&v.bits).map(Self::Output::new)
    }
}

/// Adapts `CheckedDiv` to this library with computed `Output` type.
pub trait CheckedDivFix<Rhs> {
    type Output;
    fn checked_div(&self, v: &Rhs) -> Option<Self::Output>;
}

impl<Bits, Base, LExp, RExp> CheckedDivFix<Fix<Bits, Base, RExp>> for Fix<Bits, Base, LExp>
where
    Bits: CheckedDiv,
    LExp: Sub<RExp>,
{
    type Output = Fix<Bits, Base, Diff<LExp, RExp>>;
    fn checked_div(&self, v: &Fix<Bits, Base, RExp>) -> Option<Self::Output> {
        self.bits.checked_div(&v.bits).map(Self::Output::new)
    }
}

impl<Bits, Base, LExp, RExp> MulDiv<Fix<Bits, Base, RExp>> for Fix<Bits, Base, LExp>
where
    Bits: MulDiv,
{
    type Output = Fix<<Bits as MulDiv>::Output, Base, LExp>;
    fn mul_div_ceil(
        self,
        num: Fix<Bits, Base, RExp>,
        denom: Fix<Bits, Base, RExp>,
    ) -> Option<Self::Output> {
        self.bits
            .mul_div_ceil(num.bits, denom.bits)
            .map(Self::Output::new)
    }
    fn mul_div_floor(
        self,
        num: Fix<Bits, Base, RExp>,
        denom: Fix<Bits, Base, RExp>,
    ) -> Option<Self::Output> {
        self.bits
            .mul_div_floor(num.bits, denom.bits)
            .map(Self::Output::new)
    }
    fn mul_div_round(
        self,
        num: Fix<Bits, Base, RExp>,
        denom: Fix<Bits, Base, RExp>,
    ) -> Option<Self::Output> {
        self.bits
            .mul_div_round(num.bits, denom.bits)
            .map(Self::Output::new)
    }
}

// Saturating arithmetic.

impl<Bits, Base, Exp> SaturatingAdd for Fix<Bits, Base, Exp>
where
    Bits: SaturatingAdd,
{
    fn saturating_add(&self, v: &Self) -> Self {
        Self::new(self.bits.saturating_add(&v.bits))
    }
}

impl<Bits, Base, Exp> SaturatingSub for Fix<Bits, Base, Exp>
where
    Bits: SaturatingSub,
{
    fn saturating_sub(&self, v: &Self) -> Self {
        Self::new(self.bits.saturating_sub(&v.bits))
    }
}

#[cfg(test)]
mod tests {
    use num_traits::{SaturatingAdd, SaturatingSub};

    use crate::aliases::si::{Kilo, Milli, Unit};
    use crate::{CheckedAdd, CheckedDivFix, CheckedMulFix, CheckedSub, MulDiv};

    #[test]
    fn convert_milli_to_kilo() {
        assert_eq!(Kilo::new(15), Milli::new(15_000_000).convert());
    }

    #[test]
    fn convert_kilo_to_milli() {
        assert_eq!(Milli::new(15_000_000), Kilo::new(15).convert());
    }

    #[test]
    fn cmp() {
        assert!(Kilo::new(1) < Kilo::new(2));
    }

    #[test]
    fn neg() {
        assert_eq!(Kilo::new(-1), -Kilo::new(1i32));
    }

    #[test]
    fn add() {
        assert_eq!(Kilo::new(3), Kilo::new(1) + Kilo::new(2));
    }

    #[test]
    fn sub() {
        assert_eq!(Kilo::new(1), Kilo::new(3) - Kilo::new(2));
    }

    #[test]
    fn mul() {
        assert_eq!(Unit::new(6), Kilo::new(2) * Milli::new(3));
    }

    #[test]
    fn div() {
        assert_eq!(Unit::new(3), Kilo::new(6) / Kilo::new(2));
    }

    #[test]
    fn rem() {
        assert_eq!(Kilo::new(1), Kilo::new(6) % Kilo::new(5));
    }

    #[test]
    fn mul_bits() {
        assert_eq!(Kilo::new(6), Kilo::new(2) * 3);
    }

    #[test]
    fn div_bits() {
        assert_eq!(Kilo::new(3), Kilo::new(6) / 2);
    }

    #[test]
    fn rem_bits() {
        assert_eq!(Kilo::new(1), Kilo::new(6) % 5);
    }

    #[test]
    fn add_assign() {
        let mut a = Kilo::new(1);
        a += Kilo::new(2);
        assert_eq!(Kilo::new(3), a);
    }

    #[test]
    fn sub_assign() {
        let mut a = Kilo::new(3);
        a -= Kilo::new(2);
        assert_eq!(Kilo::new(1), a);
    }

    #[test]
    fn mul_assign_bits() {
        let mut a = Kilo::new(2);
        a *= 3;
        assert_eq!(Kilo::new(6), a);
    }

    #[test]
    fn div_assign_bits() {
        let mut a = Kilo::new(6);
        a /= 2;
        assert_eq!(Kilo::new(3), a);
    }

    #[test]
    fn rem_assign() {
        let mut a = Kilo::new(6);
        a %= Milli::new(5);
        assert_eq!(Kilo::new(1), a);
    }

    #[test]
    fn rem_assign_bits() {
        let mut a = Kilo::new(6);
        a %= 5;
        assert_eq!(Kilo::new(1), a);
    }

    #[test]
    fn checked_add_neg() {
        let max = Kilo::new(u8::MAX);
        let one = Kilo::new(1);
        assert!(max.checked_add(&one).is_none())
    }

    #[test]
    fn checked_add_pos() {
        let forty = Kilo::new(40);
        let two = Kilo::new(2);
        assert_eq!(forty.checked_add(&two), Some(Kilo::new(42)))
    }

    #[test]
    fn checked_sub_neg() {
        let one = Kilo::new(1);
        let max = Kilo::new(u8::MAX);
        assert!(one.checked_sub(&max).is_none())
    }

    #[test]
    fn checked_sub_pos() {
        let fifty = Kilo::new(50);
        let eight = Kilo::new(8);
        assert_eq!(fifty.checked_sub(&eight), Some(Kilo::new(42)))
    }

    #[test]
    fn checked_mul_neg() {
        let fifty = Kilo::new(50);
        let max = Kilo::new(u8::MAX);
        assert!(fifty.checked_mul(&max).is_none())
    }

    #[test]
    fn checked_mul_pos() {
        let fifty = Kilo::new(50_u64);
        assert_eq!(
            fifty.checked_mul(&fifty).map(|out| out.convert()),
            Some(Kilo::new(2_500_000_u64))
        )
    }

    #[test]
    fn checked_div_neg() {
        let one = Unit::new(0);
        assert!(one.checked_div(&one).is_none())
    }

    #[test]
    fn checked_div_pos() {
        let hundred = Kilo::new(100);
        let five = Kilo::new(5);
        assert_eq!(hundred.checked_div(&five), Some(Unit::new(20)))
    }

    #[test]
    fn narrow_succeeds() {
        let one = Milli::new(1000u128);
        let mapped = one.narrow::<u64>();
        assert_eq!(mapped, Some(Milli::new(1000u64)));
    }

    #[test]
    fn narrow_fails() {
        let one = Milli::new(1699u64);
        let mapped = one.narrow::<u8>();
        assert_eq!(mapped, None);
    }

    #[test]
    fn widen_succeeds() {
        let one = Milli::new(1340191u64);
        let mapped = one.widen::<u128>();
        assert_eq!(mapped, Milli::new(1340191u128));
    }

    #[test]
    fn mul_div_ceil() {
        let start = Milli::new(313459u64);
        let mul = Milli::new(1200u64);
        let div = Milli::new(2450u64);
        assert_eq!(start.mul_div_ceil(mul, div), Some(Milli::new(153531)));
    }

    #[test]
    fn mul_div_ceil_unit() {
        let start = Milli::new(31345934u64);
        let mul = Milli::new(1000u64);
        let div = Milli::new(2000u64);
        assert_eq!(start.mul_div_ceil(mul, div), Some(Milli::new(15672967u64)));
    }

    #[test]
    fn mul_div_floor() {
        let start = Milli::new(69_693u64);
        let mul = Milli::new(5_192u64);
        let div = Milli::new(190u64);
        assert_eq!(start.mul_div_floor(mul, div), Some(Milli::new(1904452u64)));
    }

    #[test]
    fn mul_div_floor_unit() {
        let start = Milli::new(69_693u64);
        let mul = Milli::new(1000u64);
        let div = Milli::new(9u64);
        assert_eq!(start.mul_div_floor(mul, div), Some(Milli::new(7743666u64)));
    }

    #[test]
    fn mul_div_round() {
        let start = Milli::new(1892u64);
        let mul = Milli::new(3222u64);
        let div = Milli::new(9999u64);
        assert_eq!(start.mul_div_round(mul, div), Some(Milli::new(610u64)));
    }

    #[test]
    fn mul_div_round_unit() {
        let start = Milli::new(1892u64);
        let mul = Milli::new(1000u64);
        let div = Milli::new(322u64);
        assert_eq!(start.mul_div_round(mul, div), Some(Milli::new(5876u64)));
    }

    #[test]
    fn abs_diff() {
        let start = Milli::new(u128::MIN);
        let end = Milli::new(u128::MAX);
        assert_eq!(start.abs_diff(&end), end);
    }

    #[test]
    fn constant() {
        assert_eq!(Kilo::constant(69u64), Kilo::new(69u64));
    }

    #[test]
    fn saturating_sub() {
        let zero = Kilo::constant(0);
        let result = zero.saturating_sub(&Kilo::new(69u64));
        assert_eq!(zero, result);
    }

    #[test]
    fn saturating_add() {
        let max = Kilo::new(u64::MAX);
        let result = max.saturating_add(&Kilo::new(69u64));
        assert_eq!(max, result);
    }
}