require 'enumerator'

module HTML

  class Generator

    attr_accessor :indentation, :stack, :buffer
    
    def initialize(indentation="  ")
      self.indentation=indentation
      self.stack=[]
      self.buffer=""
    end
    
    def write(text)    
      whitespace=(1..self.stack.size).collect{self.indentation}.join
      self.buffer << whitespace+text.gsub("\n", "\n#{whitespace}")+"\n"
    end
    
    def start_tag(tag)
      text="<#{tag}>"
      self.write(text)
      self.stack << tag
    end
    
    def text(text); self.write("#{text}"); end
    
    def end_tag
      tag=self.stack.pop()
      text="</#{tag}>"
      self.write(text)
    end
    
    def process(obj)
      proc=Proc.new do |head, tail|
        if tail.class==Array        
          text=(0!=tail.size%2) ? tail.pop() : nil
          if tail.empty?
            self.start_tag(head)
            self.text(text) unless text==nil
            self.end_tag
          else
            self.start_tag(head)
            tail.each_slice(2){|h, t|proc.call(h, t)}
            self.text(text) unless text==nil
            self.end_tag
          end
        else
          self.start_tag(head)
          self.text(tail)
          self.end_tag
        end
      end
      proc.call(obj[0], obj[1])
    end
    
  end

  def HTML.generate(obj)
    generator=HTML::Generator.new
    generator.process(obj)
    generator.buffer
  end

end

puts HTML.generate(['html', ['head', ['title', "Here's the title"], 'body', ['h1', "Here's the body"]]])