use std::cmp::Ordering;

/// Sorts a slice of data using the merge sort algorithm with a custom comparator.
///
/// # Examples
///
/// ```
/// # use sort_steps::{merge_sort};
/// let numbers = [5, 9, 3, 6, 8, 2, 1, 7, 4];
/// println!("Heap Sort Steps:");
/// for (i, v) in merge_sort(&numbers).enumerate() {
///     println!("#{:02}: {:?}", i, v);
/// }
/// ```
pub fn merge_sort<T>(v: &[T]) -> impl Iterator<Item = Vec<T>>
where
    T: PartialOrd + Clone,
{
    merge_sort_by(v, |a, b| a.partial_cmp(b))
}

/// Sorts a slice of data using the heap sort algorithm with a custom comparator.
///
/// # Examples
///
/// ```
/// # use sort_steps::{merge_sort_by};
/// let numbers = [5, 9, 3, 6, 8, 2, 1, 7, 4];
/// println!("Merge Sort Steps:");
/// for (i, v) in merge_sort_by(&numbers, |a, b| a.partial_cmp(b)).enumerate() {
///     println!("#{:02}: {:?}", i, v);
/// }
/// ```
pub fn merge_sort_by<T, F>(v: &[T], compare: F) -> impl Iterator<Item = Vec<T>>
where
    F: Fn(&T, &T) -> Option<Ordering> + Copy,
    T: Clone,
{
    let mut steps = Vec::new();
    let mut sorted = v.to_vec();
    merge_sort_recursive(&mut sorted, compare, &mut steps);
    steps.into_iter()
}

// Helper function to merge two sorted slices
fn merge<T, F>(left: &[T], right: &[T], compare: F) -> Vec<T>
where
    F: Fn(&T, &T) -> Option<Ordering> + Copy,
    T: Clone,
{
    let mut result = Vec::with_capacity(left.len() + right.len());
    let mut left_index = 0;
    let mut right_index = 0;

    while left_index < left.len() && right_index < right.len() {
        match compare(&left[left_index], &right[right_index]) {
            Some(Ordering::Less) | None => {
                result.push(left[left_index].clone());
                left_index += 1;
            }
            _ => {
                result.push(right[right_index].clone());
                right_index += 1;
            }
        }
    }

    result.extend_from_slice(&left[left_index..]);
    result.extend_from_slice(&right[right_index..]);
    result
}

// Recursive merge sort function
fn merge_sort_recursive<T, F>(v: &mut [T], compare: F, steps: &mut Vec<Vec<T>>)
where
    F: Fn(&T, &T) -> Option<Ordering> + Copy,
    T: Clone,
{
    if v.len() <= 1 {
        return;
    }

    let mid = v.len() / 2;
    let mut left = v[..mid].to_vec();
    let mut right = v[mid..].to_vec();

    merge_sort_recursive(&mut left, compare, steps);
    merge_sort_recursive(&mut right, compare, steps);

    let merged = merge(&left, &right, compare);
    steps.push(merged.clone());

    v.clone_from_slice(&merged);
}