pub fn merge_sort<T>(arr: &mut [T])
where
    T: PartialOrd + Clone + Default,
{
    let len = arr.len();
    if len > 2 {
        merge_sort_range(arr, 0, len - 1);
    }
}

fn merge_sort_range<T>(arr: &mut [T], low: usize, high: usize)
where  
    T: PartialOrd + Clone + Default 
{
    // 当前可能递归，区分边界 
    if low < high {
        let mid = low + ((high - low) >> 1);
        merge_sort_range(arr, low, mid);
        merge_sort_range(arr, mid + 1, high);
        merge_sort_array(arr, low, mid, high);
    }
}

fn merge_sort_array<T>(arr: &mut[T], low: usize, mid: usize, high: usize)
where 
    T: PartialOrd + Clone + Default 
{
    let mut left_arr = arr[low..=mid].to_vec();
    let mut right_arr = arr[mid+1..=high].to_vec();
    let (
        left_size,
        right_size,
        mut left_index,
        mut right_index,
        mut current_index
    ) = (
        left_arr.len(),
        right_arr.len(),
        0,
        0,
        low
    );

    // 双数组合并 
    while left_index < left_size && right_index < right_size {
        if left_arr[left_index] < right_arr[right_index] {
            arr[current_index] = std::mem::take(&mut left_arr[left_index]);
            left_index += 1;
        } else {
            arr[current_index] = std::mem::take(&mut right_arr[right_index]);
            right_index += 1;
        }
        current_index += 1;
    }
    // 单边收纳 
    while left_index < left_size {
        arr[current_index] = std::mem::take(&mut left_arr[left_index]);
        left_index += 1;
        current_index += 1;
    }
    while right_index < right_size {
        arr[current_index] = std::mem::take(&mut right_arr[right_index]);
        right_index += 1;
        current_index += 1;
    }
}