https://github.com/gjtorikian/commonmarker 使って Markdown のチェックボックス一覧取得したり、チェックボックス toggle したりできるやつ

tasklist.rb

class TaskList
  attr_reader :markdown

  # @param markdown [String]
  def initialize(markdown)
    @markdown = markdown
  end

  # @return [CommonMarker::Node]
  def document
    @document ||= CommonMarker.render_doc(markdown, :DEFAULT, [:tasklist])
  end

  # @return [Array<Checkbox>]
  def checkboxes
    @checkboxes ||= document.walk.select { |node| node.type_string == 'tasklist' }.map { |node| Checkbox.new(node) }
  end

  # @return [String]
  def to_plaintext
    checkboxes.select(&:modified?).reduce(markdown) do |text, checkbox|
      start_pos = index_from(line: checkbox.sourcepos[:start_line], column: checkbox.sourcepos[:start_column])
      checkbox_text = checkbox.to_plaintext
      after_start_pos = start_pos + checkbox_text.length
      text.slice(0...start_pos) + checkbox_text + (text.slice(after_start_pos..-1) || '')
    end
  end

  private

  # @param line [Integer] 1-indexed line number.
  # @param column [Integer] 1-indexed column number.
  # @return [Integer] the index of the head of the given point.
  def index_from(line:, column:)
    lines = @markdown.lines
    above_lines = lines.slice(0, line - 1)
    current_line = lines[line - 1]
    ahead_source = above_lines.join('') + current_line.slice(0, column - 1)
    ahead_source.length
  end

  class Checkbox
    # @param node [CommonMarker::Node]
    def initialize(node)
      @node = node
      @check_state = nil
    end

    def checked?
      @check_state.nil? ? literally_checked? : @check_state
    end

    def check
      @check_state = true
    end

    def uncheck
      @check_state = false
    end

    def sourcepos
      @node.sourcepos
    end

    def modified?
      !@check_state.nil? && (checked? != literally_checked?)
    end

    def to_plaintext
      if @check_state.nil?
        @node.to_plaintext
      else
        @node.to_plaintext.sub(/\[( |x)\]/, checked? ? '[x]' : '[ ]')
      end
    end

    def content
      @node.to_a.first.to_plaintext
    end

    def literally_checked?
      @node.tasklist_state == 'checked'
    end
  end
end