convert html to markdown, forked from https://github.com/xijo/reverse-markdown/blob/master/reverse_markdown.rb

reverse-markdown

# it is forked from https://github.com/xijo/reverse-markdown/blob/master/reverse_markdown.rb
require 'rexml/document'
require 'benchmark'
include REXML
include Benchmark

class ReverseMarkdown
  
  # set basic variables:
  # - @li_counter: numbering list item (li) tags in an ordered list (ol)
  # - @links:      hold the links for adding them to the bottom of the @output
  #                this means 'reference style', please take a look at http://daringfireball.net/projects/markdown/syntax#link
  # - @outout:     fancy markdown code in here!
  # - @indent:     control indention level for nested lists
  # - @errors:     appearing errors, like unknown tags, go into this array
  def initialize()
    @li_counter = 0
    @links = []
    @output = ""
    @indent = 0
    @errors = []
  end
  
  # Invokes the HTML parsing by using a string. Returns the markdown code in @output.
  # To garantuee well-formed xml for REXML a <root> element will be added, but has no effect.
  # After parsing all elements, the 'reference style'-links will be inserted.
  def parse_string(string)
    doc = Document.new("<root>\n"+string+"\n</root>")
    root = doc.root
    root.elements.each do |element|
      parse_element(element, :root)
    end
    insert_links()
    @output
  end
  
  # Parsing an element and its children (recursive) and writing its markdown code to @output
  # 1. do indent for nested list items
  # 2. add the markdown opening tag for this element
  # 3a. if element only contains text, handle it like a text node
  # 3b. if element is a container handle its children, which may be text- or element nodes
  # 4. finally add the markdown ending tag for this element
  def parse_element(element, parent)
    name = element.name.to_sym
    # 1.
    @output << indent() if name.eql?(:li)
    # 2.
    @output << opening(element, parent)
    
    # 3a.
    if (element.has_text? and element.children.size < 2)
      @output << text_node(element, parent)
    end
    
    # 3b.
    if element.has_elements?
      element.children.each do |child|
        # increase indent if nested list
        @indent += 1 if element.name=~/(ul|ol)/ and parent.eql?(:li)
        
        if child.node_type.eql?(:element)
          parse_element(child, element.name.to_sym)
        else
          if parent.eql?(:blockquote)
            @output << child.to_s.gsub("\n ", "\n>")
          else
            @output << child.to_s
          end
        end
        
        # decrease indent if end of nested list
        @indent -= 1 if element.name=~/(ul|ol)/ and parent.eql?(:li)
      end
    end
    
    # 4.
    @output << ending(element, parent)
  end

  # Returns opening markdown tag for the element. Its parent matters sometimes!
  def opening(type, parent)
    case type.name.to_sym
      when :h1
        "# "
      when :li
        parent.eql?(:ul) ? " - " : " "+(@li_counter+=1).to_s+". "
      when :ol
        @li_counter = 0
        ""
      when :ul
        ""
      when :h2
        "## "
      when :em
        "*"
      when :strong
        "**"
      when :blockquote
        # remove leading newline
        type.children.first.value = ""
        "> "
      when :code
        parent.eql?(:pre) ? "    " : "`"
      when :a
        "["
      when :img
        "!["
      when :hr
        "----------\n\n"
      else
        @errors << "unknown start tag: "+type.name.to_s
        ""
    end
  end
  
  # Returns the closing markdown tag, like opening()
  def ending(type, parent)
    case type.name.to_sym
      when :h1
        " #\n\n"
      when :h2
        " ##\n\n"
      when :p
        parent.eql?(:root) ? "\n\n" : "\n"
      when :ol
        parent.eql?(:li) ? "" : "\n"
      when :ul
        parent.eql?(:li) ? "" : "\n"
      when :em
        "*"
      when :strong
        "**"
      when :li
        ""
      when :blockquote
        ""
      when :code
        parent.eql?(:pre) ? "" : "`"
      when :a
        @links << type.attribute('href').to_s
        "][" + @links.size.to_s + "] "
      when :img
        @links << type.attribute('src').to_s
        "" + type.attribute('alt').to_s + "][" + @links.size.to_s + "] "
        "#{type.attribute('alt')}][#{@links.size}] "
      else
        @errors << "  unknown end tag: "+type.name.to_s
        ""
    end
  end
  
  # Perform indent: two space, @indent times - quite simple! :)
  def indent
    str = ""
    @indent.times do
      str << "  "
    end
    str
  end
  
  # Return the content of element, which should be just text.
  # If its a code block to indent of 4 spaces.
  # For block quotation add a leading '>'
  def text_node(element, parent)
    if element.name.to_sym.eql?(:code) and parent.eql?(:pre)
      element.text.gsub("\n","\n    ") << "\n"
    elsif parent.eql?(:blockquote)
      element.text.gsub!("\n ","\n>")
    else
      element.text
    end
  end
  
  # Insert the mentioned reference style links.
  def insert_links
    @output << "\n"
    @links.each_index do |index|
      @output << "  [#{index+1}]: #{@links[index]}\n"
    end
  end
  
  # Print out all errors, that occured and have been written to @errors.
  def print_errors
    @errors.each do |error|
      puts error
    end
  end
  
  # Perform a benchmark on a given string n-times.
  def speed_benchmark(string, n)
    initialize()
    bm(15) do |test|
      test.report("reverse markdown:")    { n.times do; parse_string(string); initialize(); end; }
    end
  end
  
end