Sass function that returns a unique value each time it is called within the scope of a single css file compilation.

unique.rb

module Sass::Script::Functions
  def unique
    options[:unique_values] ||= Set.new
    r = rand(100000)
    r = rand(100000) while options[:unique_values].include?(r)
    options[:unique_values] << r
    Sass::Script::String.new("u#{r.to_s(36)}") #we add a u to the font to ensure this is a legal css identifier.
  end
end

This is useful for doing things like generating placeholder classes that are unique within a single mixin include.