gistfile1.rb

preprocess do
  # Find items to put in archives
  items_to_archive = items.select { |i| i.identifier =~ /^\/files/ }

  # Group them by extension
  item_groups = items_to_archive.group_by { |i| i[:extension] }

  # For each extension (and their items)...
  item_groups.each_pair do |extension, items|
    # ... create a new item
    items << Nanoc3::Item.new(
      '',
      { :identifiers => items.map { |i| i.identifier } },
      "/archives/#{extension}.tar.bz2"
    )
end

compile '/archives/*/' do
  filter :tar
end

route '/archives/*/' do
  item.identifier.chop + '.tar.b2'
end