# frozen_string_literal: true

module CoreExtensions
  # N-ary tree data structure.
  # Each node of the tree contains an +Object+ (the node value) and can be accessed from its parent
  # node using an identifier +Object+. A node can be identified uniquely by the succession of
  # identifiers that lead from the tree root to itself.
  class Tree
    # The value of the current tree node.
    attr_accessor :value

    # The child nodes of the current tree node.
    # @return [Hash]
    attr_reader :children_nodes

    # Creates a new tree with a root element and no child nodes.
    # @param node_value [Object] The value associated to the root element
    def initialize(node_value = nil)
      @value = node_value
      @children_nodes = {}
    end

    # Queries

    # Check if the current Tree node has children.
    def children?
      !@children_nodes.empty?
    end

    # Tests whether the current tree node has a specific child.
    # @param child_id [Object] The unique identifier of the child.
    # @return [Boolean] True if the child node identified by +child_id+ exists, False otherwise.
    def child_exists?(child_id)
      child_node(child_id)
      true
    rescue KeyError
      false
    end

    # Retrieves the child of the current tree node.
    # @param child_id [Object] The unique identifier of the child.
    # @return [Object] The node value of the child identified by +child_id+.
    # @raise [KeyError] If the child node identified by +child_id+ does not exist.
    def child_node(child_id)
      @children_nodes.fetch(child_id)
    end

    # Returns a hash representation of the current tree node and all its children, recursively
    # (depth-first).
    # @param separator [String] The separator to be used between node identifiers.
    # @return [Hash]
    def to_h(separator = '/', id_prefix = '')
      hash = {}
      id_prefix.empty? ? hash[separator] = @value : hash[id_prefix] = @value
      @children_nodes.each do |id, node|
        hash.merge!(node.to_h(separator, id_prefix + separator + id.to_s))
      end
      hash
    end

    # Returns the string representation of the current tree node and all its children, recursively
    # (depth-first).
    # @return [String]
    def to_s(offset = '')
      str = offset + "VALUE: #{@value}\n"
      @children_nodes.each do |id, node|
        str += offset + " * ID:   '#{id}'\n"
        str += node.to_s("#{offset}   ")
      end
      str
    end

    # Retrieves the value associated to a child node of the current tree node, inserting it first if
    # it does not exist.
    # @param child_id [Object] The unique identifier for the child node to retrieve or insert.
    # @param child_value [Object] The value associated to the child node to insert. Ignored if a
    #                             child node identified by the given +child_id+ already exists.
    # @return [Object] The value associated to the child node identified by the given +child_id+.
    def add_child_node(child_id, child_value = nil)
      @children_nodes[child_id] = Tree.new(child_value) if @children_nodes[child_id].nil?
      @children_nodes[child_id]
    end
  end
end