class PostImplication < ApplicationRecord
  self.primary_key = :post_id, :parent_post_id

  belongs_to :post, inverse_of: :parent_post_implications
  belongs_to :parent_post, class_name: 'Post', inverse_of: :child_post_implications

  validates :post_id, presence: true, uniqueness: { scope: :parent_post_id }
  validates :parent_post_id, presence: true

  validate :parent_post_mustnt_be_itself
  validate :parent_post_mustnt_create_cycle

  private

  def parent_post_mustnt_be_itself
    if parent_post_id == post_id
      errors.add :parent_post_id, '親投稿に同じ投稿を設定することはできません．'
    end
  end

  def parent_post_mustnt_create_cycle
    return if post_id.blank? || parent_post_id.blank?
    return if errors[:parent_post_id].present?

    ancestor_ids = Set.new
    frontier_ids = [parent_post_id]

    until frontier_ids.empty?
      if frontier_ids.include?(post_id)
        errors.add :parent_post_id, :invalid
        return
      end

      ancestor_ids.merge(frontier_ids)
      frontier_ids = PostImplication.where(post_id: frontier_ids)
          .distinct.pluck(:parent_post_id)
          .reject { ancestor_ids.include?(_1) }
    end
  end
end