nth_child

# File actionpack/lib/action_controller/vendor/html-scanner/html/selector.rb, line 722
    def nth_child(a, b, of_type, reverse)
      # a = 0 means select at index b, if b = 0 nothing selected
      return lambda { |element| false } if a == 0 && b == 0
      # a < 0 and b < 0 will never match against an index
      return lambda { |element| false } if a < 0 && b < 0
      b = a + b + 1 if b < 0   # b < 0 just picks last element from each group
      b -= 1 unless b == 0  # b == 0 is same as b == 1, otherwise zero based
      lambda do |element|
        # Element must be inside parent element.
        return false unless element.parent && element.parent.tag?
        index = 0
        # Get siblings, reverse if counting from last.
        siblings = element.parent.children
        siblings = siblings.reverse if reverse
        # Match element name if of-type, otherwise ignore name.
        name = of_type ? element.name : nil
        found = false
        for child in siblings
          # Skip text nodes/comments.
          if child.tag? && (name == nil || child.name == name)
            if a == 0
              # Shortcut when a == 0 no need to go past count
              if index == b
                found = child.equal?(element)
                break
              end
            elsif a < 0
              # Only look for first b elements
              break if index > b
              if child.equal?(element)
                found = (index % a) == 0
                break
              end
            else
              # Otherwise, break if child found and count ==  an+b
              if child.equal?(element)
                found = (index % a) == b
                break
              end
            end
            index += 1
          end
        end
        found
      end
    end