/// Sorts a mutable slice in ascending order using merge sort.
pub fn merge_sort<T: Ord + Clone>(values: &mut [T]) {
    let len = values.len();
    if len <= 1 {
        return;
    }

    let mid = len / 2;
    merge_sort(&mut values[..mid]);
    merge_sort(&mut values[mid..]);

    // Merge both sorted halves into temporary storage.
    let mut merged = Vec::with_capacity(len);
    let (left, right) = values.split_at(mid);
    let (mut i, mut j) = (0, 0);

    while i < left.len() && j < right.len() {
        if left[i] <= right[j] {
            merged.push(left[i].clone());
            i += 1;
        } else {
            merged.push(right[j].clone());
            j += 1;
        }
    }

    merged.extend_from_slice(&left[i..]);
    merged.extend_from_slice(&right[j..]);
    values.clone_from_slice(&merged);
}

#[cfg(test)]
mod tests {
    use super::merge_sort;

    #[test]
    fn sorts_integers() {
        let mut values = [8, 3, 5, 1, 9, 3, -2];
        merge_sort(&mut values);
        assert_eq!(values, [-2, 1, 3, 3, 5, 8, 9]);
    }

    #[test]
    fn handles_empty_and_single_element_slices() {
        let mut empty: [i32; 0] = [];
        let mut single = [42];

        merge_sort(&mut empty);
        merge_sort(&mut single);

        assert_eq!(empty, []);
        assert_eq!(single, [42]);
    }

    #[test]
    fn sorts_strings() {
        let mut values = vec!["pear", "apple", "orange", "banana"];
        merge_sort(&mut values);
        assert_eq!(values, ["apple", "banana", "orange", "pear"]);
    }
}