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# /lib/helpers/markdown_renderer_with_special_links.rb
class MarkdownRendererWithSpecialLinks < Redcarpet::Render::HTML
  def autolink(link, link_type)
    case link_type
      when :url then url_link(link)
      when :email then email_link(link)
    end
  end
  def url_link(link)
    case link
      when /^http://youtube/ then youtube_link(link)
      else normal_link(link)
    end
  end
  def youtube_link(link)
    parameters_start = link.index('?')
    video_id = link[15..(parameters_start ? parameters_start-1 : -1)]
    "<iframe width="560" height="315" src="//www.youtube.com/embed/#{video_id}?rel=0" frameborder="0" allowfullscreen></iframe>"
  end
  def normal_link(link)
    "<a href="#{link}">#{link}</a>"
  end
  def email_link(email)
    "<a href="mailto:#{email}">#{email}</a>"
  end
end

# /app/helpers/application_helper.rb
module ApplicationHelper
  require './lib/helpers/markdown_renderer_with_special_links'
  def markdown(content)
    @markdown ||= Redcarpet::Markdown.new(MarkdownRendererWithSpecialLinks, autolink: true, space_after_headers: true, fenced_code_blocks: true)
    @markdown.render(content).html_safe
  end
end