#!/usr/bin/env ruby
#
# tree_merge_handler.rb
#
# Author:  Anupam Sengupta
# Time-stamp: <2022-06-20 22:17:12 anupam>
#
# Copyright (C) 2013, 2015, 2022 Anupam Sengupta (anupamsg@gmail.com)
#
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# frozen_string_literal: true

# Provides utility methods to merge two {Tree::TreeNode} based trees.
# @since 0.9.0
module Tree
  module Utils
    # Handles merging of two trees.
    module TreeMergeHandler
      # @!group Merging Trees

      # Merge two trees that share the same root node and returns <em>a new
      # tree</em>.
      #
      # The new tree contains the contents of the merge between _other_tree_ and
      # self. Duplicate nodes (coming from _other_tree_) will *NOT* be overwritten
      # in self.
      #
      # @author Darren Oakley (https://github.com/dazoakley)
      #
      # @param [Tree::TreeNode] other_tree The other tree to merge with.
      # @return [Tree::TreeNode] the resulting tree following the merge.
      #
      # @raise [TypeError] This exception is raised if _other_tree_ is not a
      #                    {Tree::TreeNode}.
      #
      # @raise [ArgumentError] This exception is raised if _other_tree_ does not
      #                        have the same root node as self.
      def merge(other_tree)
        check_merge_prerequisites(other_tree)
        merge_trees(root.dup, other_tree.root)
      end

      # Merge in another tree (that shares the same root node) into +this+ tree.
      # Duplicate nodes (coming from _other_tree_) will NOT be overwritten in
      # self.
      #
      # @author Darren Oakley (https://github.com/dazoakley)
      #
      # @param [Tree::TreeNode] other_tree The other tree to merge with.
      #
      # @raise [TypeError] This exception is raised if _other_tree_ is not a
      #                    {Tree::TreeNode}.
      #
      # @raise [ArgumentError] This exception is raised if _other_tree_ does not
      #                        have the same root node as self.
      def merge!(other_tree)
        check_merge_prerequisites(other_tree)
        merge_trees(root, other_tree.root)
      end

      private

      # Utility function to check that the conditions for a tree merge are met.
      #
      # @author Darren Oakley (https://github.com/dazoakley)
      #
      # @see #merge
      # @see #merge!
      def check_merge_prerequisites(other_tree)
        raise TypeError, 'You can only merge in another instance of Tree::TreeNode' \
          unless other_tree.is_a?(Tree::TreeNode)

        raise ArgumentError, 'Unable to merge trees as they do not share the same root' \
          unless root.name == other_tree.root.name
      end

      # Utility function to recursively merge two subtrees.
      #
      # @author Darren Oakley (https://github.com/dazoakley)
      #
      # @param [Tree::TreeNode] tree1 The target tree to merge into.
      # @param [Tree::TreeNode] tree2 The donor tree (that will be merged
      #                               into target).
      # @raise [Tree::TreeNode] The merged tree.
      def merge_trees(tree1, tree2)
        names1 = tree1.children? ? tree1.children.map(&:name) : []
        names2 = tree2.children? ? tree2.children.map(&:name) : []

        names_to_merge = names2 - names1
        names_to_merge.each do |name|
          tree1 << tree2[name].detached_subtree_copy
        end

        tree1.children.each do |child|
          merge_trees(child, tree2[child.name]) unless tree2[child.name].nil?
        end

        tree1
      end
    end
  end
end
