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

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

    // Merge both sorted halves into a temporary buffer.
    let mut merged = Vec::with_capacity(values.len());
    let (left, right) = values.split_at(middle);
    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);
}

fn main() {
    let mut numbers = [8, 3, 5, 1, 9, 2];
    merge_sort(&mut numbers);
    println!("{numbers:?}");
}

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

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

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

    #[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]);
    }
}