brew/Library/Homebrew/os/mac/version.rb

require 'version'

module MacOS
  class Version < ::Version
    def <=>(other)
      v = case other
          when :mountain_lion then 10.8
          when :lion          then 10.7
          when :snow_leopard  then 10.6
          when :leopard       then 10.5
          when :tiger         then 10.4
          else other.to_s
          end
      super(Version.new(v))
    end

    def pretty_name
      case @version
      when "10.9" then "Mavericks"
      when "10.8" then "Mountain Lion"
      when "10.7" then "Lion"
      when "10.6" then "Snow Leopard"
      when "10.5" then "Leopard"
      when "10.4" then "Tiger"
      else @version
      end
    end
  end
end