class Jekyll::IncludeTag

def initialize(tag_name, file, tokens)
  super
  @file = file.strip
end

def render(context)
  if @file !~ /^[a-zA-Z0-9_\/\.-]+$/ || @file =~ /\.\// || @file =~ /\/\./
    return "Include file '#{@file}' contains invalid characters or sequences"
  end
  Dir.chdir(File.join(context.registers[:site].source, '_includes')) do
    choices = Dir['**/*'].reject { |x| File.symlink?(x) }
    if choices.include?(@file)
      source = File.read(@file)
      partial = Liquid::Template.parse(source)
      context.stack do
        partial.render(context)
      end
    else
      "Included file '#{@file}' not found in _includes directory"
    end
  end
end