require 'cgi'
require 'uri'

class RewriteLinks
=begin
  def initialize(html)
    @doc = REXML::Document.new("<div>" + html + "</div>")
  end

  def relative_to(url)
    @doc.elements.each("//img") { |element|
      if element.attributes["src"] !~ %r{\A\w+://}
        element.attributes["src"] = url + "/" + element.attributes["src"]
      end
    }
    @doc.elements.each("//a") { |element|
      if element.attributes["href"] !~ %r{\A\w+://}
        element.attributes["href"] = url + "/" + element.attributes["href"]
      end
    }
    @doc.root[0..-1].to_s       # Drop <div>
  end
=end

  def initialize(html)
    @html = html
  end

  def relative_to(root)
    @html.gsub(/(<(img|a)\b[^>]*(src|href)=)(["'])(.*?)\4/) {
      md = $~

      url = URI.parse CGI.unescapeHTML(md[5])

      if url.relative? && url.path !~ /^\//
        md[1] + md[4] + root + "/" + md[5] + md[4]
      else
        md.to_s
      end
    }
  end
end