# In order to build the tree structure, we select comments that are only root nodes,
# i.e. comments that are not a reply. From there, we can build the tree structure based
# on their children, which we collected earlier.
transform_node_index(node_index.select { |node_id, data| data[:parent].nil? }.values)

# This is a recursive method that will traverse the comment structure, and run for
# the children of each comment. In the end, once the recursion finishes, a tree pops out.
def transform_node_index(nodes)
  nodes.map do |data|
    {
      comment: data[:node],
      reply_to: data[:reply_to],
      children: transform_node_index(data[:children])
    }
  end
end