class Nokogiri::CSS::Node

def preprocess!
  ### Deal with nth-child
  matches = find_by_type(
    [:CONDITIONAL_SELECTOR,
      [:ELEMENT_NAME],
      [:PSEUDO_CLASS,
        [:FUNCTION]
      ]
    ]
  )
  matches.each do |match|
    if match.value[1].value[0].value[0] =~ /^nth-child/
      tag_name = match.value[0].value.first
      match.value[0].value = ['*']
      match.value[1] = Node.new(:COMBINATOR, [
        match.value[1].value[0],
        Node.new(:FUNCTION, ['self(', tag_name])
      ])
    end
    if match.value[1].value[0].value[0] =~ /^nth-last-child/
      tag_name = match.value[0].value.first
      match.value[0].value = ['*']
      match.value[1] = Node.new(:COMBINATOR, [
        match.value[1].value[0],
        Node.new(:FUNCTION, ['self(', tag_name])
      ])
    end
  end
  ### Deal with first-child, last-child
  matches = find_by_type(
    [:CONDITIONAL_SELECTOR,
      [:ELEMENT_NAME], [:PSEUDO_CLASS]
  ])
  matches.each do |match|
    if ['first-child', 'last-child'].include?(match.value[1].value.first)
      which = match.value[1].value.first.gsub(/-\w*$/, '')
      tag_name = match.value[0].value.first
      match.value[0].value = ['*']
      match.value[1] = Node.new(:COMBINATOR, [
        Node.new(:FUNCTION, ["#{which}("]),
        Node.new(:FUNCTION, ['self(', tag_name])
      ])
    elsif 'only-child' == match.value[1].value.first
      tag_name = match.value[0].value.first
      match.value[0].value = ['*']
      match.value[1] = Node.new(:COMBINATOR, [
        Node.new(:FUNCTION, ["#{match.value[1].value.first}("]),
        Node.new(:FUNCTION, ['self(', tag_name])
      ])                                        
    end
  end
  self
end