commonware_utils/

vec.rs

//! A vector type that guarantees at least one element.

use crate::TryFromIterator;
#[cfg(not(feature = "std"))]
use alloc::{vec, vec::Vec};
use bytes::{Buf, BufMut};
use commonware_codec::{EncodeSize, RangeCfg, Read, Write};
use core::{
    num::NonZeroUsize,
    ops::{Deref, DerefMut},
};
use thiserror::Error;

/// Errors that can occur when creating a [`NonEmptyVec`].
#[derive(Error, Debug, PartialEq, Eq)]
pub enum Error {
    /// The collection was empty.
    #[error("cannot create NonEmptyVec from empty collection")]
    Empty,
}

/// A vector that is guaranteed to contain at least one element.
#[derive(Clone, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub struct NonEmptyVec<T>(Vec<T>);

impl<T> NonEmptyVec<T> {
    /// Creates a new [`NonEmptyVec`] with a single element.
    pub fn new(first: T) -> Self {
        Self(vec![first])
    }

    /// Creates a [`NonEmptyVec`] from a [`Vec`] without returning a `Result`.
    ///
    /// # Panics
    ///
    /// Panics if the vector is empty.
    pub fn from_unchecked(vec: Vec<T>) -> Self {
        assert!(
            !vec.is_empty(),
            "NonEmptyVec::from_unchecked: vector is empty"
        );
        Self(vec)
    }

    /// Returns the number of elements in the vector.
    ///
    /// This is guaranteed to be at least 1.
    pub const fn len(&self) -> NonZeroUsize {
        NonZeroUsize::new(self.0.len()).unwrap()
    }

    /// Returns `true` if the vector contains exactly one element.
    pub const fn is_singleton(&self) -> bool {
        self.0.len() == 1
    }

    /// Returns a reference to the first element.
    ///
    /// Unlike [`slice::first`], this doesn't return an `Option`.
    pub fn first(&self) -> &T {
        self.0.first().unwrap()
    }

    /// Returns a mutable reference to the first element.
    ///
    /// Unlike [`slice::first_mut`], this doesn't return an `Option`.
    pub fn first_mut(&mut self) -> &mut T {
        self.0.first_mut().unwrap()
    }

    /// Returns a reference to the last element.
    ///
    /// Unlike [`slice::last`], this doesn't return an `Option`.
    pub fn last(&self) -> &T {
        self.0.last().unwrap()
    }

    /// Returns a mutable reference to the last element.
    ///
    /// Unlike [`slice::last_mut`], this doesn't return an `Option`.
    pub fn last_mut(&mut self) -> &mut T {
        self.0.last_mut().unwrap()
    }

    /// Maps each element to a new value, preserving the non-empty guarantee.
    pub fn map<U, F: FnMut(&T) -> U>(&self, f: F) -> NonEmptyVec<U> {
        NonEmptyVec(self.0.iter().map(f).collect())
    }

    /// Consumes the vector and maps each element to a new value.
    pub fn map_into<U, F: FnMut(T) -> U>(self, f: F) -> NonEmptyVec<U> {
        NonEmptyVec(self.0.into_iter().map(f).collect())
    }

    /// Appends an element to the back of the vector.
    pub fn push(&mut self, value: T) {
        self.0.push(value);
    }

    /// Inserts an element at position `index`.
    ///
    /// # Panics
    ///
    /// Panics if `index > len`.
    pub fn insert(&mut self, index: usize, element: T) {
        self.0.insert(index, element);
    }

    /// Extends the vector with the contents of an iterator.
    pub fn extend<I: IntoIterator<Item = T>>(&mut self, iter: I) {
        self.0.extend(iter);
    }

    /// Resizes the vector to the specified length.
    ///
    /// If `new_len` is greater than `len()`, the vector is extended with clones
    /// of `value`. If `new_len` is less than `len()`, the vector is truncated.
    ///
    /// Unlike [`Vec::resize`], this takes [`NonZeroUsize`] to maintain the
    /// non-empty guarantee.
    pub fn resize(&mut self, new_len: NonZeroUsize, value: T)
    where
        T: Clone,
    {
        self.0.resize(new_len.get(), value);
    }

    /// Resizes the vector to the specified length using a closure.
    ///
    /// If `new_len` is greater than `len()`, the vector is extended with values
    /// generated by calling `f` repeatedly. If `new_len` is less than `len()`,
    /// the vector is truncated.
    ///
    /// Unlike [`Vec::resize_with`], this takes [`NonZeroUsize`] to maintain the
    /// non-empty guarantee.
    pub fn resize_with<F>(&mut self, new_len: NonZeroUsize, f: F)
    where
        F: FnMut() -> T,
    {
        self.0.resize_with(new_len.get(), f);
    }

    /// Removes the last element and returns it, or `None` if there is only one
    /// element.
    ///
    /// This ensures the vector always has at least one element.
    pub fn pop(&mut self) -> Option<T> {
        if self.0.len() > 1 {
            self.0.pop()
        } else {
            None
        }
    }

    /// Removes and returns the element at position `index`, or `None` if
    /// removing would leave the vector empty.
    ///
    /// This ensures the vector always has at least one element.
    ///
    /// # Panics
    ///
    /// Panics if `index >= len`.
    pub fn remove(&mut self, index: usize) -> Option<T> {
        assert!(index < self.0.len(), "index out of bounds");
        if self.0.len() > 1 {
            Some(self.0.remove(index))
        } else {
            None
        }
    }

    /// Provides mutable access to the underlying vector via a closure.
    ///
    /// This is an escape hatch for operations not directly exposed by `NonEmptyVec`.
    ///
    /// # Panics
    ///
    /// Panics if the closure leaves the vector empty.
    ///
    /// # Examples
    ///
    /// ```
    /// use commonware_utils::non_empty_vec;
    ///
    /// let mut v = non_empty_vec![3, 1, 2];
    /// v.mutate(|vec| vec.sort());
    /// assert_eq!(v.first(), &1);
    /// ```
    pub fn mutate<F, R>(&mut self, f: F) -> R
    where
        F: FnOnce(&mut Vec<T>) -> R,
    {
        let result = f(&mut self.0);
        assert!(
            !self.0.is_empty(),
            "NonEmptyVec::mutate: closure left vector empty"
        );
        result
    }

    /// Consumes the [`NonEmptyVec`] and returns the underlying [`Vec`].
    pub fn into_vec(self) -> Vec<T> {
        self.0
    }
}

impl<T> Deref for NonEmptyVec<T> {
    type Target = [T];

    fn deref(&self) -> &Self::Target {
        &self.0
    }
}

impl<T> DerefMut for NonEmptyVec<T> {
    fn deref_mut(&mut self) -> &mut Self::Target {
        &mut self.0
    }
}

impl<T> AsRef<[T]> for NonEmptyVec<T> {
    fn as_ref(&self) -> &[T] {
        &self.0
    }
}

impl<T> AsRef<Vec<T>> for NonEmptyVec<T> {
    fn as_ref(&self) -> &Vec<T> {
        &self.0
    }
}

impl<T> From<NonEmptyVec<T>> for Vec<T> {
    fn from(vec: NonEmptyVec<T>) -> Self {
        vec.0
    }
}

impl<T> TryFrom<Vec<T>> for NonEmptyVec<T> {
    type Error = Error;

    fn try_from(vec: Vec<T>) -> Result<Self, Self::Error> {
        if vec.is_empty() {
            Err(Error::Empty)
        } else {
            Ok(Self(vec))
        }
    }
}

impl<T: Clone> TryFrom<&[T]> for NonEmptyVec<T> {
    type Error = Error;

    fn try_from(slice: &[T]) -> Result<Self, Self::Error> {
        if slice.is_empty() {
            Err(Error::Empty)
        } else {
            Ok(Self(slice.to_vec()))
        }
    }
}

impl<T, const N: usize> TryFrom<[T; N]> for NonEmptyVec<T> {
    type Error = Error;

    fn try_from(arr: [T; N]) -> Result<Self, Self::Error> {
        if N == 0 {
            Err(Error::Empty)
        } else {
            Ok(Self(arr.into()))
        }
    }
}

impl<T: Clone, const N: usize> TryFrom<&[T; N]> for NonEmptyVec<T> {
    type Error = Error;

    fn try_from(arr: &[T; N]) -> Result<Self, Self::Error> {
        Self::try_from(arr.as_slice())
    }
}

impl<T> TryFromIterator<T> for NonEmptyVec<T> {
    type Error = Error;

    fn try_from_iter<I: IntoIterator<Item = T>>(iter: I) -> Result<Self, Self::Error> {
        let vec: Vec<T> = iter.into_iter().collect();
        Self::try_from(vec)
    }
}

impl<T> IntoIterator for NonEmptyVec<T> {
    type Item = T;
    type IntoIter = <Vec<T> as IntoIterator>::IntoIter;

    fn into_iter(self) -> Self::IntoIter {
        self.0.into_iter()
    }
}

impl<'a, T> IntoIterator for &'a NonEmptyVec<T> {
    type Item = &'a T;
    type IntoIter = core::slice::Iter<'a, T>;

    fn into_iter(self) -> Self::IntoIter {
        self.0.iter()
    }
}

impl<'a, T> IntoIterator for &'a mut NonEmptyVec<T> {
    type Item = &'a mut T;
    type IntoIter = core::slice::IterMut<'a, T>;

    fn into_iter(self) -> Self::IntoIter {
        self.0.iter_mut()
    }
}

impl<T: Write> Write for NonEmptyVec<T> {
    fn write(&self, buf: &mut impl BufMut) {
        self.0.write(buf);
    }
}

impl<T: EncodeSize> EncodeSize for NonEmptyVec<T> {
    fn encode_size(&self) -> usize {
        self.0.encode_size()
    }
}

impl<T: Read> Read for NonEmptyVec<T> {
    type Cfg = (RangeCfg<NonZeroUsize>, T::Cfg);

    fn read_cfg(buf: &mut impl Buf, cfg: &Self::Cfg) -> Result<Self, commonware_codec::Error> {
        let items = Vec::read_cfg(buf, &(cfg.0.into(), cfg.1.clone()))?;
        if items.is_empty() {
            return Err(commonware_codec::Error::Invalid(
                "NonEmptyVec",
                "cannot decode empty vector",
            ));
        }
        Ok(Self(items))
    }
}

#[cfg(feature = "arbitrary")]
impl<'a, T: arbitrary::Arbitrary<'a>> arbitrary::Arbitrary<'a> for NonEmptyVec<T> {
    fn arbitrary(u: &mut arbitrary::Unstructured<'a>) -> arbitrary::Result<Self> {
        let first: T = u.arbitrary()?;
        let rest: Vec<T> = u.arbitrary()?;
        let mut vec = Vec::with_capacity(1 + rest.len());
        vec.push(first);
        vec.extend(rest);
        Ok(Self(vec))
    }
}

/// Creates a [`NonEmptyVec`] containing the given elements.
///
/// # Forms
///
/// | Syntax | Count type | Guarantee |
/// |--------|------------|-----------|
/// | `non_empty_vec![a, b, c]` | - | Compile-time (at least one element required) |
/// | `non_empty_vec![elem; N]` | const `usize` | Compile-time (N must be const and > 0) |
/// | `non_empty_vec![elem; NZUsize!(N)]` | [`NZUsize!`] | Runtime (panics if N == 0) |
/// | `non_empty_vec![elem; @n]` | [`NonZeroUsize`] | Type-safe (n is already non-zero) |
/// | `non_empty_vec![@v]` | - | Runtime (panics if v is empty) |
///
/// The `@` marker is required for runtime [`NonZeroUsize`] values to distinguish
/// them from const `usize` values, since declarative macros cannot inspect types.
///
/// [`NZUsize!`]: crate::NZUsize
///
/// # Examples
///
/// ```
/// use commonware_utils::{non_empty_vec, NZUsize};
///
/// // List form
/// let v = non_empty_vec![1, 2, 3];
/// assert_eq!(v.len().get(), 3);
///
/// // Const repeat: N must be a const expression > 0
/// let v = non_empty_vec![42; 5];
/// assert_eq!(v.len().get(), 5);
///
/// // NZUsize! form: convenient for inline literals
/// let v = non_empty_vec![42; NZUsize!(3)];
/// assert_eq!(v.len().get(), 3);
///
/// // Runtime form: use @ with any NonZeroUsize expression
/// let n = NZUsize!(2);
/// let v = non_empty_vec![42; @n];
/// assert_eq!(v.len().get(), 2);
///
/// // Vec form: wrap an existing Vec (panics if empty)
/// let vec = vec![1, 2, 3];
/// let v = non_empty_vec![@vec];
/// assert_eq!(v.len().get(), 3);
/// ```
///
/// # Compile Errors
///
/// ```compile_fail
/// use commonware_utils::non_empty_vec;
/// let empty = non_empty_vec![]; // error: no elements
/// ```
///
/// ```compile_fail
/// use commonware_utils::non_empty_vec;
/// let zero = non_empty_vec![42; 0]; // error: count is 0
/// ```
///
/// ```compile_fail
/// use commonware_utils::non_empty_vec;
/// let n: usize = 5;
/// let v = non_empty_vec![42; n]; // error: n is not const (use @n with NonZeroUsize)
/// ```
#[macro_export]
macro_rules! non_empty_vec {
    (@$vec:expr) => {{
        $crate::vec::NonEmptyVec::from_unchecked($vec)
    }};
    ($elem:expr; NZUsize!($n:expr)) => {{
        $crate::vec::NonEmptyVec::from_unchecked(vec![$elem; $crate::NZUsize!($n).get()])
    }};
    ($elem:expr; @$n:expr) => {{
        let n: core::num::NonZeroUsize = $n;
        $crate::vec::NonEmptyVec::from_unchecked(vec![$elem; n.get()])
    }};
    ($elem:expr; $n:expr) => {{
        const N: usize = $n;
        const _: () = assert!(N > 0, "count must be greater than 0");
        $crate::vec::NonEmptyVec::from_unchecked(vec![$elem; N])
    }};
    ($first:expr $(, $rest:expr)* $(,)?) => {
        $crate::vec::NonEmptyVec::from_unchecked(vec![$first $(, $rest)*])
    };
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::{NZUsize, TryCollect};
    use commonware_codec::Error as CodecError;
    use std::num::NonZeroUsize;

    #[test]
    fn test_new() {
        let v = NonEmptyVec::new(42);
        assert_eq!(v.len().get(), 1);
        assert_eq!(v.first(), &42);
        assert_eq!(v.last(), &42);
    }

    #[test]
    #[should_panic(expected = "vector is empty")]
    fn test_from_unchecked_panics_on_empty() {
        let _: NonEmptyVec<i32> = NonEmptyVec::from_unchecked(vec![]);
    }

    #[test]
    fn test_is_singleton() {
        let v = non_empty_vec![42];
        assert!(v.is_singleton());

        let v = non_empty_vec![1, 2];
        assert!(!v.is_singleton());

        let v = non_empty_vec![1, 2, 3];
        assert!(!v.is_singleton());
    }

    #[test]
    fn test_macro() {
        let v = non_empty_vec![1, 2, 3];
        assert_eq!(v.len().get(), 3);
        assert_eq!(v.first(), &1);
        assert_eq!(v.last(), &3);

        let v = non_empty_vec![42];
        assert_eq!(v.len().get(), 1);
        assert_eq!(v.first(), &42);

        // Trailing comma support
        let v = non_empty_vec![1, 2, 3,];
        assert_eq!(v.len().get(), 3);

        // Const repeat syntax
        let v = non_empty_vec![42; 5];
        assert_eq!(v.len().get(), 5);
        assert!(v.iter().all(|&x| x == 42));

        let v = non_empty_vec![0; 1];
        assert_eq!(v.len().get(), 1);
        assert_eq!(v.first(), &0);

        // NZUsize! macro form
        let v = non_empty_vec![99; NZUsize!(3)];
        assert_eq!(v.len().get(), 3);
        assert!(v.iter().all(|&x| x == 99));

        // Runtime repeat syntax with NonZeroUsize variable
        let n = NonZeroUsize::new(4).unwrap();
        let v = non_empty_vec![7; @n];
        assert_eq!(v.len().get(), 4);
        assert!(v.iter().all(|&x| x == 7));

        // Vec wrap syntax
        let vec = vec![1, 2, 3];
        let v = non_empty_vec![@vec];
        assert_eq!(v.len().get(), 3);
        assert_eq!(&*v, &[1, 2, 3]);
    }

    #[test]
    fn test_try_from_vec() {
        let v: NonEmptyVec<i32> = vec![1, 2, 3].try_into().unwrap();
        assert_eq!(v.len().get(), 3);

        let result: Result<NonEmptyVec<i32>, _> = Vec::new().try_into();
        assert_eq!(result, Err(Error::Empty));
    }

    #[test]
    fn test_try_from_slice() {
        let v: NonEmptyVec<i32> = [1, 2, 3].as_slice().try_into().unwrap();
        assert_eq!(v.len().get(), 3);

        let empty: &[i32] = &[];
        let result: Result<NonEmptyVec<i32>, _> = empty.try_into();
        assert_eq!(result, Err(Error::Empty));
    }

    #[test]
    fn test_try_from_array() {
        let v: NonEmptyVec<i32> = [1, 2, 3].try_into().unwrap();
        assert_eq!(v.len().get(), 3);

        let result: Result<NonEmptyVec<i32>, _> = [0i32; 0].try_into();
        assert_eq!(result, Err(Error::Empty));
    }

    #[test]
    fn test_try_from_iterator() {
        let v: NonEmptyVec<i32> = (1..=3).try_collect().unwrap();
        assert_eq!(v.len().get(), 3);

        let result: Result<NonEmptyVec<i32>, _> = core::iter::empty().try_collect();
        assert_eq!(result, Err(Error::Empty));
    }

    #[test]
    fn test_first_last() {
        let mut v = non_empty_vec![1, 2, 3];

        assert_eq!(v.first(), &1);
        assert_eq!(v.last(), &3);

        *v.first_mut() = 10;
        *v.last_mut() = 30;

        assert_eq!(v.first(), &10);
        assert_eq!(v.last(), &30);
    }

    #[test]
    fn test_push() {
        let mut v = non_empty_vec![1];
        v.push(2);
        v.push(3);
        assert_eq!(v.len().get(), 3);
        assert_eq!(v.last(), &3);
    }

    #[test]
    fn test_insert() {
        let mut v = non_empty_vec![1, 3];
        v.insert(1, 2);
        assert_eq!(&*v, &[1, 2, 3]);
    }

    #[test]
    fn test_extend() {
        let mut v = non_empty_vec![1];
        v.extend([2, 3, 4]);
        assert_eq!(v.len().get(), 4);
        assert_eq!(&*v, &[1, 2, 3, 4]);
    }

    #[test]
    fn test_resize() {
        // Grow
        let mut v = non_empty_vec![1, 2];
        v.resize(NonZeroUsize::new(5).unwrap(), 0);
        assert_eq!(&*v, &[1, 2, 0, 0, 0]);

        // Shrink
        v.resize(NonZeroUsize::new(2).unwrap(), 0);
        assert_eq!(&*v, &[1, 2]);

        // Shrink to 1 (minimum)
        v.resize(NonZeroUsize::new(1).unwrap(), 0);
        assert_eq!(&*v, &[1]);
    }

    #[test]
    fn test_resize_with() {
        let mut counter = 0;
        let mut v = non_empty_vec![1];
        v.resize_with(NonZeroUsize::new(4).unwrap(), || {
            counter += 1;
            counter * 10
        });
        assert_eq!(&*v, &[1, 10, 20, 30]);

        // Shrink (closure not called)
        v.resize_with(NonZeroUsize::new(2).unwrap(), || {
            panic!("should not be called")
        });
        assert_eq!(&*v, &[1, 10]);
    }

    #[test]
    fn test_pop() {
        let mut v = non_empty_vec![1, 2, 3];

        assert_eq!(v.pop(), Some(3));
        assert_eq!(v.len().get(), 2);

        assert_eq!(v.pop(), Some(2));
        assert_eq!(v.len().get(), 1);

        // Cannot pop the last element
        assert_eq!(v.pop(), None);
        assert_eq!(v.len().get(), 1);
        assert_eq!(v.first(), &1);
    }

    #[test]
    fn test_remove() {
        let mut v = non_empty_vec![1, 2, 3];

        assert_eq!(v.remove(1), Some(2));
        assert_eq!(&*v, &[1, 3]);

        assert_eq!(v.remove(0), Some(1));
        assert_eq!(&*v, &[3]);

        // Cannot remove the last element
        assert_eq!(v.remove(0), None);
        assert_eq!(&*v, &[3]);
    }

    #[test]
    fn test_mutate() {
        let mut v = non_empty_vec![3, 1, 2, 1];
        v.mutate(|vec| {
            vec.sort();
            vec.dedup();
        });
        assert_eq!(&*v, &[1, 2, 3]);

        // Test that return value is propagated
        let mut v = non_empty_vec![1, 2, 3];
        let sum: i32 = v.mutate(|vec| vec.iter().sum());
        assert_eq!(sum, 6);
    }

    #[test]
    #[should_panic(expected = "closure left vector empty")]
    fn test_mutate_panics_on_empty() {
        let mut v = non_empty_vec![1];
        v.mutate(|vec| vec.clear());
    }

    #[test]
    fn test_deref() {
        let v = non_empty_vec![3, 1, 2];

        // slice methods via Deref
        assert_eq!(v.len().get(), 3);
        assert!(v.contains(&2));
        assert_eq!(v.get(1), Some(&1));
    }

    #[test]
    fn test_deref_mut() {
        let mut v = non_empty_vec![3, 1, 2];

        // Mutable slice methods via DerefMut
        v.sort();
        assert_eq!(&*v, &[1, 2, 3]);

        v.reverse();
        assert_eq!(&*v, &[3, 2, 1]);

        v.swap(0, 2);
        assert_eq!(&*v, &[1, 2, 3]);
    }

    #[test]
    fn test_into_vec() {
        let v = non_empty_vec![1, 2, 3];
        let vec: Vec<i32> = v.into_vec();
        assert_eq!(vec, vec![1, 2, 3]);
    }

    #[test]
    fn test_map() {
        let v = non_empty_vec![1, 2, 3];
        let doubled = v.map(|x| x * 2);
        assert_eq!(&*doubled, &[2, 4, 6]);

        // Original unchanged
        assert_eq!(&*v, &[1, 2, 3]);
    }

    #[test]
    fn test_map_into() {
        let v = non_empty_vec![1, 2, 3];
        let doubled = v.map_into(|x| x * 2);
        assert_eq!(&*doubled, &[2, 4, 6]);
    }

    #[test]
    fn test_from_non_empty_vec() {
        let v = non_empty_vec![1, 2, 3];
        let vec: Vec<i32> = v.into();
        assert_eq!(vec, vec![1, 2, 3]);
    }

    #[test]
    fn test_index() {
        let v = non_empty_vec![1, 2, 3];
        assert_eq!(v[0], 1);
        assert_eq!(v[1], 2);
        assert_eq!(v[2], 3);
        assert_eq!(&v[0..2], &[1, 2]);

        // IndexMut
        let mut v = non_empty_vec![1, 2, 3];
        v[0] = 10;
        v[1..3].copy_from_slice(&[20, 30]);
        assert_eq!(&*v, &[10, 20, 30]);
    }

    #[test]
    fn test_iterators() {
        let v = non_empty_vec![1, 2, 3];

        // iter()
        let sum: i32 = v.iter().sum();
        assert_eq!(sum, 6);

        // iter_mut()
        let mut v = non_empty_vec![1, 2, 3];
        for x in v.iter_mut() {
            *x *= 2;
        }
        assert_eq!(&*v, &[2, 4, 6]);

        // IntoIterator (owned)
        let v = non_empty_vec![1, 2, 3];
        let collected: Vec<_> = v.into_iter().collect();
        assert_eq!(collected, vec![1, 2, 3]);

        // IntoIterator (borrowed)
        let v = non_empty_vec![1, 2, 3];
        let collected: Vec<_> = (&v).into_iter().copied().collect();
        assert_eq!(collected, vec![1, 2, 3]);

        // IntoIterator (borrowed mut)
        let mut v = non_empty_vec![1, 2, 3];
        for x in &mut v {
            *x += 10;
        }
        assert_eq!(&*v, &[11, 12, 13]);
    }

    #[test]
    fn test_codec_roundtrip() {
        let v = non_empty_vec![1u8, 2, 3];

        let mut buf = Vec::with_capacity(v.encode_size());
        v.write(&mut buf);

        let decoded = NonEmptyVec::<u8>::read_cfg(
            &mut buf.as_slice(),
            &(RangeCfg::from(NZUsize!(1)..=NZUsize!(10)), ()),
        )
        .unwrap();

        assert_eq!(v, decoded);
    }

    #[test]
    fn test_codec_rejects_empty() {
        let empty: Vec<u8> = vec![];
        let mut buf = Vec::new();
        empty.write(&mut buf);

        let result = NonEmptyVec::<u8>::read_cfg(&mut buf.as_slice(), &(RangeCfg::from(..), ()));
        assert!(matches!(
            result,
            Err(CodecError::Invalid(
                "NonEmptyVec",
                "cannot decode empty vector"
            ))
        ));

        let result =
            NonEmptyVec::<u8>::read_cfg(&mut buf.as_slice(), &(RangeCfg::from(..NZUsize!(10)), ()));
        assert!(matches!(
            result,
            Err(CodecError::Invalid(
                "NonEmptyVec",
                "cannot decode empty vector"
            ))
        ));

        let result = NonEmptyVec::<u8>::read_cfg(
            &mut buf.as_slice(),
            &(RangeCfg::from(NZUsize!(1)..NZUsize!(10)), ()),
        );
        assert!(matches!(result, Err(CodecError::InvalidLength(0))));
    }

    #[test]
    fn test_as_ref() {
        let v = non_empty_vec![1, 2, 3];

        let slice: &[i32] = v.as_ref();
        assert_eq!(slice, &[1, 2, 3]);

        let vec_ref: &Vec<i32> = v.as_ref();
        assert_eq!(vec_ref, &vec![1, 2, 3]);
    }
}