closed_interval_set/

lib.rs

//! The `closed_interval_set` crate manipulates disjoint unions of
//! closed intervals that are represented as vectors ([`RangeVec`]) or
//! iterators ([`NormalizedRangeIter`]) of pairs of endpoints.  These
//! intervals are always closed (inclusive at both ends), so the
//! crate can naturally represent both the empty set (no interval),
//! and the universe (a closed interval from min to max).
//!
//! The crate is designed for usage patterns where sets are
//! constructed ahead of time (perhaps by combining different sets
//! together), then frozen (as vectors, internally) for read-only
//! access.  That said, its iterator implementations of set
//! [complementation](`NormalizedRangeIter::complement`),
//! [union](`NormalizedRangeIter::union`), and
//! [intersection](`NormalizedRangeIter::intersect`) are closed over
//! the [`NormalizedRangeIter`] trait, so it's reasonable to build up
//! complex expressions of type-erased [`NormalizedRangeIter`]s
//! before materializing the result to a [`RangeVec`].
//!
//! Using this crate usually starts by constructing [`Vec`]s of closed
//! ranges (of pairs of [`Endpoint`]s), and passing that to
//! [`RangeVec::from_vec`].  From that point, we have access to the
//! set operations on [`RangeVec`] and [`NormalizedRangeIter`].  The
//! toplevel functions (e.g., [`intersect_vec`] and [`normalize_vec`])
//! may be helpful to avoid excessive chaining or in subtle
//! situations, e.g., when the compiler knows whether the input is a
//! [`RangeVec`] or a [`Vec`] (or [`SmallVec`]) but it's annoying to
//! track by hand.
//!
//! Complementation is tricky when one handles only closed intervals.
//! We assume [`Endpoint`] types can enumerate values in total order
//! via [`Endpoint::decrease_toward`] and [`Endpoint::increase_toward`].
//! That's nonsense for [densely ordered sets](https://en.wikipedia.org/wiki/Dense_order)
//! like \\(\mathbb{Q},\\) but tends to work OK on computers: it's trivial
//! to enumerate bounded integers, and there is such a total order for
//! the finite set of floating point values.  Mathematically, this sorted
//! enumeration of floating point values makes no sense, nevertheless,
//! it can be useful in some domains, e.g., static program analysis.
//!
//! All operations take at most linear space and \\(\mathcal{O}(n \log
//! n)\\) time, where \\(n\\) is the total number of ranges in all the
//! inputs, before any normalization (simplification).  Set operations
//! on [`NormalizedRangeIter`] always use constant space, and many
//! operations on [`RangeVec`] reuse storage.
//!
//! The container type ([`SmallVec`]`<[_; 2]>`) is hardcoded, for
//! simplicity.  The [`Endpoint`] trait, however, is fully generic.
//! This crate comes with implementations of [`Endpoint`] for all
//! primitive fixed-width integer types ([`i8`], [`i16`], [`i32`], [`i64`],
//! [`i128`], [`u8`], [`u16`], [`u32`], [`u64`] and [`u128`]), for
//! [`isize`] and [`usize`], and for the standard floating point
//! types [`f32`] and [`f64`] (from \\(-\infty\\) to \\(+\infty\\),
//! with \\(-0\\) and \\(+0\\) as distinct values, and excluding NaNs,
//! in the same order as [`f32::total_cmp`] and [`f64::total_cmp`]).
//!
//! [`SmallVec`]: https://docs.rs/smallvec/latest/smallvec/struct.SmallVec.html
//! [`Vec`]: https://doc.rust-lang.org/std/vec/struct.Vec.html

#![deny(missing_docs)]
// https://github.com/taiki-e/cargo-llvm-cov?tab=readme-ov-file#exclude-code-from-coverage
#![cfg_attr(coverage_nightly, feature(coverage_attribute))]
// `cargo build --target thumbv6m-none-eabi` is (maybe?) a decent way to check we don't
// indirectly use the full stdlib.
#![cfg_attr(not(test), no_std)]
extern crate alloc; // for `alloc::Vec`

use smallvec::SmallVec;

mod complement;
mod intersection;
mod intersection_iterator;
pub mod iterator_wrapper;
mod normalize;
mod primitive_endpoint;
mod range_case;
mod range_vec;
mod slice_sequence;
mod union;
mod union_iterator;

pub use range_case::RangeCase;
pub use range_vec::RangeVec;

pub use normalize::is_normalized;
pub use normalize::normalize_vec;

pub use complement::complement_vec;
pub use intersection::intersect_vec;
pub use union::union_vec;

/// Inline storage (in ranges) reserved in a [`RangeVec`].
///
/// Controlled by the `inline_storage` feature, which is enabled by
/// default.  When the `inline_storage` feature is *not* enabled,
/// this constant is set to 0.
pub const INLINE_SIZE: usize = if cfg!(feature = "inline_storage") {
    2
} else {
    0
};

/// Our internal storage type for [`RangeVec`].
type Backing<T> = SmallVec<[(T, T); INLINE_SIZE]>;

/// An [`Endpoint`] is the left or right limit of a closed interval
/// `[left, right]`.
///
/// [`Endpoint`] types must have maximum and minimum values.  For
/// bounded integer types, that's simply `T::MIN` or `T::MAX`;
/// in general, types may have to be extended, just like floating
/// point values have +/- infinity.
///
/// [`Endpoint`] types must also be enumerable in both ascending and
/// descending order.
///
/// There is an implementation for all 10 primitive fixed-width
/// integer types (signed/unsigned 8, 16, 32, 64, and 128 bits), for
/// [`isize`] and [`usize`], and for the IEEE floating point types
/// [`f32`] and [`f64`].
pub trait Endpoint: Copy {
    /// The minimum value for values of type [`Endpoint`]:
    ///
    /// \\[ \forall x : \mathtt{Self}, x \geq \mathtt{Self::min\\_value}() \\]
    fn min_value() -> Self;

    /// The maximum value for values of type [`Endpoint`]:
    ///
    /// \\[ \forall x : \mathtt{Self}, x \leq \mathtt{Self::max\\_value}() \\]
    fn max_value() -> Self;

    /// Returns whether `self` is comparable.
    fn is_valid(self) -> bool;

    /// Compares `self <=> other`.  Both `self` and `other` are
    /// guaranteed to satisfy [`Endpoint::is_valid()`].
    /// Implementations may return an arbitrary ordering if that's not
    /// the case.
    ///
    /// See [`core::cmp::Ord`]
    fn cmp_end(self, other: Self) -> core::cmp::Ordering;

    /// Returns the minimum [`Endpoint`] value strictly
    /// greater than `self`, or `None` if there is no
    /// such value (iff `self == Self::max_value()`).
    ///
    /// \\[ \forall \mathtt{self}, x: \mathtt{Self}, x > \mathtt{self} \Rightarrow x \geq \mathtt{self.next\\_after}() \\]
    #[inline(always)]
    fn next_after(self) -> Option<Self> {
        self.increase_toward(Self::max_value())
    }

    /// Returns the maximum [`Endpoint`] value strictly
    /// less than `self`, or `None` if there is no
    /// such value (iff `self == Self::min_value()`).
    ///
    /// \\[ \forall \mathtt{self}, x: \mathtt{Self}, x < \mathtt{self} \Rightarrow x \leq \mathtt{self.prev\\_before}() \\]
    #[inline(always)]
    fn prev_before(self) -> Option<Self> {
        self.decrease_toward(Self::min_value())
    }

    /// Returns [`prev_before()`] iff `other < self`, and [`None`]
    /// otherwise.
    ///
    /// In practice, it's usually easier to directly implement this
    /// method instead of [`prev_before()`] (`other < self` guarantees
    /// there is a previous value for `self`), so [`prev_before()`] is
    /// implemented in terms of [`Self::decrease_toward()`].
    ///
    /// [`prev_before()`]: `Self::prev_before`
    fn decrease_toward(self, other: Self) -> Option<Self>;

    /// Returns [`next_after()`] iff `other > self`, and [`None`]
    /// otherwise.
    ///
    /// In practice, it's usually easier to directly implement this
    /// method instead of [`next_after()`] (`other > self` guarantees
    /// there is a next value for `self`), so [`next_after()`] is
    /// implemented in terms of [`Self::increase_toward()`].
    ///
    /// [`next_after()`]: `Self::next_after`
    fn increase_toward(self, other: Self) -> Option<Self>;

    /// Compares two ranges of endpoints.
    #[doc(hidden)]
    #[inline(always)]
    fn cmp_range(left: (Self, Self), right: (Self, Self)) -> core::cmp::Ordering {
        match left.0.cmp_end(right.0) {
            core::cmp::Ordering::Equal => left.1.cmp_end(right.1),
            any => any,
        }
    }

    /// Returns the max of two endpoints.
    #[doc(hidden)]
    #[inline(always)]
    fn bot_end(self, other: Self) -> Self {
        core::cmp::min_by(self, other, |x, y| Self::cmp_end(*x, *y))
    }

    /// Returns the min of two endpoints.
    #[doc(hidden)]
    #[inline(always)]
    fn top_end(self, other: Self) -> Self {
        core::cmp::max_by(self, other, |x, y| Self::cmp_end(*x, *y))
    }
}

/// We represent closed ranges as pairs of [`Endpoint`]s.
type Pair<T> = (T, T);

mod private {
    pub trait Sealed {}
}

/// A [`ClosedRange`] represents a closed range of values with pairs
/// of [`Endpoint`]s.
///
/// This trait stands for `(T, T)` `&(T, T)`, where `T` implements
/// [`Endpoint`].
///
/// The [`ClosedRange`] trait is sealed and cannot be implemented for
/// types outside this crate.  External code *may* have to write down
/// the trait's name, but most likely shouldn't try to actually invoke
/// any method on that trait.
pub trait ClosedRange: Copy + private::Sealed {
    /// The type of the endpoints for this range.
    #[doc(hidden)]
    type EndT: Endpoint;

    /// Returns a copy of the range represented by this
    /// [`ClosedRange`] instance.
    #[doc(hidden)]
    fn get(self) -> Pair<Self::EndT>;
}

/// A [`NormalizedRangeIter`] yields a sorted sequence of
/// non-overlapping, non-adjacent, non-empty closed ranges.
///
/// It's hard to check for this property at runtime, so this
/// trait is sealed.
pub trait NormalizedRangeIter: private::Sealed + Iterator<Item: ClosedRange> {
    /// Determines whether this range iterator is equivalent to
    /// (represents the same set of values as) another.
    ///
    /// This operation takes constant space and time linear in
    /// the shorter length of the two input iterators.
    #[must_use]
    fn eqv(
        mut self,
        other: impl IntoNormalizedRangeIter<Item: ClosedRange<EndT = ClosedRangeEnd<Self::Item>>>,
    ) -> bool
    where
        Self: Sized,
    {
        use core::cmp::Ordering;

        let mut other = other.into_iter();
        loop {
            // No need to fuse, we bail as soon as one iterator returns `None`.
            match (self.next(), other.next()) {
                (Some(a), Some(b)) => {
                    if Endpoint::cmp_range(a.get(), b.get()) != Ordering::Equal {
                        return false;
                    }
                }
                (None, None) => return true,
                _ => return false,
            }
        }
    }

    /// Returns an iterator for the complement of this normalized range iterator.
    ///
    /// Running the resulting iterator to exhaustion takes constant space and time
    /// linear in the length of the input iterator.
    ///
    /// The result is also a [`NormalizedRangeIter`].
    #[inline(always)]
    #[must_use]
    fn complement(self) -> complement::ComplementIterator<Self>
    where
        Self: Sized,
    {
        complement::ComplementIterator::new(self)
    }

    /// Returns an iterator for the intersection of this normalized range iterator
    /// and another [`RangeVec`] of normalized ranges.
    ///
    /// Running the resulting iterator to exhaustion takes constant space and
    /// \\(\mathcal{O}(\min(m + n, m \log n))\\) time, where \\(m\\) is the
    /// size of `self`, and \\(n\\) that of `other`.
    ///
    /// The result is also a [`NormalizedRangeIter`].
    #[inline(always)]
    #[must_use]
    fn intersect_vec<'a>(
        self,
        other: &'a RangeVec<ClosedRangeEnd<Self::Item>>,
    ) -> intersection::IntersectionIterator<'a, Self>
    where
        Self: 'a + Sized,
    {
        // Unsafe because the interface assumes both arguments are normalized.
        unsafe { crate::intersection::intersect(self, other) }
    }

    /// Returns an iterator for the intersection of this normalized range iterator
    /// and another iterator of normalized ranges.
    ///
    /// Running the resulting iterator to exhaustion takes constant space and
    /// time linear in the total length of the two input iterators.
    ///
    /// The result is also a [`NormalizedRangeIter`].
    #[inline(always)]
    #[must_use]
    fn intersect<Other>(
        self,
        other: Other,
    ) -> intersection_iterator::LinearIntersectionIterator<
        ClosedRangeEnd<Self::Item>,
        Self,
        <Other as IntoIterator>::IntoIter,
    >
    where
        Self: Sized,
        Other: IntoNormalizedRangeIter<Item: ClosedRange<EndT = ClosedRangeEnd<Self::Item>>>,
    {
        intersection_iterator::LinearIntersectionIterator::new(self, other.into_iter())
    }

    /// Returns an interator for the union of this normalized range
    /// iterator and another normalized range iterator.
    ///
    /// Running the resulting iterator to exhaustion takes constant space and
    /// time linear in the total length of the two input iterators.
    ///
    /// The result is also a [`NormalizedRangeIter`].
    #[inline(always)]
    #[must_use]
    fn union<Other>(
        self,
        other: Other,
    ) -> union_iterator::UnionIterator<
        ClosedRangeEnd<Self::Item>,
        Self,
        <Other as IntoIterator>::IntoIter,
    >
    where
        Self: Sized,
        Other: IntoNormalizedRangeIter<Item: ClosedRange<EndT = ClosedRangeEnd<Self::Item>>>,
    {
        union_iterator::UnionIterator::new(self, other.into_iter())
    }

    /// Collects the contents of a [`NormalizedRangeIter`] into a [`RangeVec`].
    ///
    /// This takes time linear in the length of the input iterator (in addition
    /// to the resources used by the iterator itself).
    #[must_use]
    fn collect_range_vec(self) -> RangeVec<ClosedRangeEnd<Self::Item>>
    where
        Self: Sized,
    {
        #[cfg(feature = "internal_checks")]
        let hint = self.size_hint();

        let inner: SmallVec<[_; INLINE_SIZE]> = self.map(|range| range.get()).collect();

        #[cfg(feature = "internal_checks")]
        {
            assert!(hint.0 <= inner.len());
            assert!(inner.len() <= hint.1.unwrap_or(usize::MAX));
            assert!(is_normalized(&inner));
        }

        unsafe { RangeVec::new_unchecked(inner) }
    }
}

/// Boxes of iterators are iterators.
impl<T: NormalizedRangeIter + ?Sized> private::Sealed for alloc::boxed::Box<T> {}
impl<T: NormalizedRangeIter + ?Sized> NormalizedRangeIter for alloc::boxed::Box<T> {}

/// A [`IntoNormalizedRangeIter`] is an [`IntoIterator`] that turns
/// into an [`NormalizedRangeIter`].
pub trait IntoNormalizedRangeIter: IntoIterator<IntoIter: NormalizedRangeIter> {}

impl<T: IntoIterator<IntoIter: NormalizedRangeIter>> IntoNormalizedRangeIter for T {}

impl<T: Endpoint> private::Sealed for (T, T) {}

impl<T: Endpoint> ClosedRange for (T, T) {
    type EndT = T;

    #[inline(always)]
    fn get(self) -> (T, T) {
        self
    }
}

impl<T: Endpoint> private::Sealed for &(T, T) {}

impl<T: Endpoint> ClosedRange for &(T, T) {
    type EndT = T;

    #[inline(always)]
    fn get(self) -> (T, T) {
        *self
    }
}

/// The endpoints of the closed range.
type ClosedRangeEnd<T> = <T as ClosedRange>::EndT;
/// The return type of `ClosedRange::get()`.
type ClosedRangeVal<T> = Pair<ClosedRangeEnd<T>>;

#[cfg(test)]
#[cfg_attr(coverage_nightly, coverage(off))]
fn ranges_to_bits(ranges: &[(u8, u8)]) -> alloc::vec::Vec<bool> {
    use alloc::vec;

    let mut marks = vec![false; 256];

    for (start, stop) in ranges.iter().copied() {
        if start <= stop {
            for i in start..=stop {
                marks[i as usize] = true;
            }
        }
    }

    marks
}

#[cfg(test)]
#[cfg_attr(coverage_nightly, coverage(off))]
mod test {
    use super::*;
    use alloc::vec;
    use alloc::vec::Vec;

    #[test]
    fn test_min_max() {
        assert_eq!(<u8 as Endpoint>::min_value(), 0);
        assert_eq!(<u8 as Endpoint>::max_value(), 255);

        assert_eq!(<i8 as Endpoint>::min_value(), -128);
        assert_eq!(<i8 as Endpoint>::max_value(), 127);

        assert_eq!(<i32 as Endpoint>::min_value(), i32::MIN);
        assert_eq!(<i32 as Endpoint>::max_value(), i32::MAX);

        assert_eq!(<isize as Endpoint>::min_value(), isize::MIN);
        assert_eq!(<isize as Endpoint>::max_value(), isize::MAX);

        assert_eq!(<usize as Endpoint>::min_value(), usize::MIN);
        assert_eq!(<usize as Endpoint>::max_value(), usize::MAX);
    }

    #[test]
    fn test_prev_next_u64() {
        assert_eq!(0u64.prev_before(), None);
        assert_eq!(0u64.next_after(), Some(1));

        assert_eq!(u64::MAX.prev_before(), Some(u64::MAX - 1));
        assert_eq!(u64::MAX.next_after(), None);

        assert_eq!(0u64.decrease_toward(0u64), None);
        assert_eq!(0u64.increase_toward(10u64), Some(1));

        assert_eq!(1u64.decrease_toward(0u64), Some(0u64));
        assert_eq!(1u64.decrease_toward(1u64), None);
        assert_eq!(1u64.decrease_toward(2u64), None);

        assert_eq!(1u64.increase_toward(0u64), None);
        assert_eq!(1u64.increase_toward(1u64), None);
        assert_eq!(1u64.increase_toward(2u64), Some(2u64));

        assert_eq!(u64::MAX.increase_toward(u64::MAX), None);
        assert_eq!(u64::MAX.decrease_toward(0), Some(u64::MAX - 1));
    }

    #[test]
    fn test_closed_range() {
        let ranges = vec![(1u8, 2u8), (10u8, 4u8)];

        assert_eq!(
            &ranges.iter().map(ClosedRange::get).collect::<Vec<_>>(),
            &ranges
        );
        assert_eq!(
            ranges
                .clone()
                .into_iter()
                .map(ClosedRange::get)
                .collect::<Vec<_>>(),
            ranges
        );
    }

    #[test]
    fn test_chain_boxed_iter() {
        let mut acc: Option<Box<dyn NormalizedRangeIter<Item = (u8, u8)>>> = None;

        for i in 1u8..=4u8 {
            let vec = RangeVec::from_vec(vec![(2 * i, 10 * i)]);

            acc = match acc.take() {
                None => Some(Box::new(vec.into_iter())),
                Some(acc) if i % 2 == 0 => Some(Box::new(acc.intersect(vec.into_iter()))),
                Some(acc) => Some(Box::new(acc.intersect_vec(Box::leak(Box::new(vec))))),
            };
        }

        // Intersection is (8u8, 10u8); union with [0, 6]
        let singleton: SmallVec<[(u8, u8); 1]> = smallvec::smallvec![(0, 6)];
        acc = Some(Box::new(
            acc.unwrap().union(RangeVec::from_smallvec(singleton)),
        ));

        let expected = RangeVec::from_vec(vec![(7u8, 7u8), (11u8, 255u8)]);
        assert!(acc.unwrap().complement().eqv(expected));
    }

    proptest::proptest! {
        #[test]
        fn test_increase(x: u8) {
            assert_eq!(<u8 as Endpoint>::max_value(), u8::MAX);

            if x != u8::MAX {
                assert_eq!(x.next_after(), Some(x + 1));
            } else {
                assert_eq!(x.next_after(), None);
            }
        }

        #[test]
        fn test_decrease(x: u8) {
            assert_eq!(<u8 as Endpoint>::min_value(), 0u8);

            if x != 0u8 {
                assert_eq!(x.prev_before(), Some(x - 1));
            } else {
                assert_eq!(x.prev_before(), None);
            }
        }

        #[test]
        fn test_toward(x: u8, y: u8) {
            let (x, y) = (x.min(y), x.max(y));

            assert_eq!(x.decrease_toward(y), None);
            assert_eq!(y.increase_toward(x), None);

            if x == y {
                assert_eq!(x.increase_toward(y), None);
                assert_eq!(x.decrease_toward(y), None);
                assert_eq!(y.increase_toward(x), None);
                assert_eq!(y.decrease_toward(x), None);
            } else {
                assert_eq!(x.increase_toward(y), Some(x + 1));
                assert_eq!(y.decrease_toward(x), Some(y - 1));
            }
        }

        #[test]
        fn test_top_bot(x: u8, y: u8) {
            assert_eq!(x.bot_end(y), x.min(y));
            assert_eq!(y.bot_end(x), x.min(y));

            assert_eq!(x.top_end(y), x.max(y));
            assert_eq!(y.top_end(x), x.max(y));
        }

        #[test]
        fn test_cmp(x: u8, y: u8) {
            assert_eq!(x.cmp_end(y), x.cmp(&y));
            assert_eq!(y.cmp_end(x), y.cmp(&x));
        }

        #[test]
        fn test_cmp_range(x: (u8, u8), y: (u8, u8)) {
            assert_eq!(u8::cmp_range(x, y), x.cmp(&y));
            assert_eq!(u8::cmp_range(y, x), y.cmp(&x));
        }

        // Smoke test isize and usize: they're the same as one of the
        // regular integer types, so not worth fuzzing individually.
        // However, we still want some coverage.
        #[test]
        fn test_increase_isize(x: isize) {
            assert_eq!(<isize as Endpoint>::max_value(), isize::MAX);

            if x != isize::MAX {
                assert_eq!(x.next_after(), Some(x + 1));
            } else {
                assert_eq!(x.next_after(), None);
            }
        }

        #[test]
        fn test_decrease_isize(x: isize) {
            assert_eq!(<isize as Endpoint>::min_value(), isize::MIN);

            if x != isize::MIN {
                assert_eq!(x.prev_before(), Some(x - 1));
            } else {
                assert_eq!(x.prev_before(), None);
            }
        }

        #[test]
        fn test_toward_isize(x: isize, y: isize) {
            let (x, y) = (x.min(y), x.max(y));

            assert_eq!(x.decrease_toward(y), None);
            assert_eq!(y.increase_toward(x), None);

            if x == y {
                assert_eq!(x.increase_toward(y), None);
                assert_eq!(x.decrease_toward(y), None);
                assert_eq!(y.increase_toward(x), None);
                assert_eq!(y.decrease_toward(x), None);
            } else {
                assert_eq!(x.increase_toward(y), Some(x + 1));
                assert_eq!(y.decrease_toward(x), Some(y - 1));
            }
        }

        #[test]
        fn test_top_bot_isize(x: isize, y: isize) {
            assert_eq!(x.bot_end(y), x.min(y));
            assert_eq!(y.bot_end(x), x.min(y));

            assert_eq!(x.top_end(y), x.max(y));
            assert_eq!(y.top_end(x), x.max(y));
        }

        #[test]
        fn test_cmp_isize(x: isize, y: isize) {
            assert_eq!(x.cmp_end(y), x.cmp(&y));
            assert_eq!(y.cmp_end(x), y.cmp(&x));
        }

        #[test]
        fn test_cmp_range_isize(x: (isize, isize), y: (isize, isize)) {
            assert_eq!(isize::cmp_range(x, y), x.cmp(&y));
            assert_eq!(isize::cmp_range(y, x), y.cmp(&x));
        }

        #[test]
        fn test_increase_usize(x: usize) {
            assert_eq!(<usize as Endpoint>::max_value(), usize::MAX);

            if x != usize::MAX {
                assert_eq!(x.next_after(), Some(x + 1));
            } else {
                assert_eq!(x.next_after(), None);
            }
        }

        #[test]
        fn test_decrease_usize(x: usize) {
            assert_eq!(<usize as Endpoint>::min_value(), 0usize);

            if x != usize::MIN {
                assert_eq!(x.prev_before(), Some(x - 1));
            } else {
                assert_eq!(x.prev_before(), None);
            }
        }

        #[test]
        fn test_toward_usize(x: usize, y: usize) {
            let (x, y) = (x.min(y), x.max(y));

            assert_eq!(x.decrease_toward(y), None);
            assert_eq!(y.increase_toward(x), None);

            if x == y {
                assert_eq!(x.increase_toward(y), None);
                assert_eq!(x.decrease_toward(y), None);
                assert_eq!(y.increase_toward(x), None);
                assert_eq!(y.decrease_toward(x), None);
            } else {
                assert_eq!(x.increase_toward(y), Some(x + 1));
                assert_eq!(y.decrease_toward(x), Some(y - 1));
            }
        }

        #[test]
        fn test_top_bot_usize(x: usize, y: usize) {
            assert_eq!(x.bot_end(y), x.min(y));
            assert_eq!(y.bot_end(x), x.min(y));

            assert_eq!(x.top_end(y), x.max(y));
            assert_eq!(y.top_end(x), x.max(y));
        }

        #[test]
        fn test_cmp_usize(x: usize, y: usize) {
            assert_eq!(x.cmp_end(y), x.cmp(&y));
            assert_eq!(y.cmp_end(x), y.cmp(&x));
        }

        #[test]
        fn test_cmp_range_usize(x: (usize, usize), y: (usize, usize)) {
            assert_eq!(usize::cmp_range(x, y), x.cmp(&y));
            assert_eq!(usize::cmp_range(y, x), y.cmp(&x));
        }
    }
}