Extension for numbering figures

extension.rb

require 'asciidoctor/extensions' unless RUBY_ENGINE == 'opal'

include Asciidoctor

Extensions.register do
  treeprocessor do
    process do |document|
      figureNumbers = {}
      document.find_by(context: :section) {|sect|
        if sect.level == 1
          figureNumbers[sect.level] = 0
          sect.find_by(context: :image) {|image|
            figureNumbers[sect.level] += 1

            image.caption = 'Figure ' + sect.sectnum.to_s + figureNumbers[sect.level].to_s + '. '
            image.attributes['reftext'] = image.caption
          }
        end
      }
    end
    nil
  end
end