module Immutable
  class CannotOverrideMethod < StandardError; end

  # Random ID changed at each interpreter load
  UNIQ = "_#{object_id.abs}"

  def self.included(mod)
    mod.extend(ClassMethods)
  end

  module ClassMethods
    def immutable_method(*args)
      # Initialize variables
      @immutable_methods = [] if @immutable_methods.nil?
      @silent_immutable_methods = [] if @silent_immutable_methods.nil?
      instance_variable_set("@#{UNIQ}_in_method_added", false)

      opts = args.last.is_a?(Hash) ? args.pop : {}
      
      args.each do |method|
        alias_method "#{UNIQ}_old_#{method}", method
      end
      
      # Build list of immutable methods
      @immutable_methods += args
      @silent_immutable_methods += args if opts[:silent]

      @opts = opts
      module_eval do
        def self.method_added(sym)
          if @immutable_methods
            @immutable_methods.each do |method|
              if method && sym == method.to_sym && !in_method_added?
                unless @silent_immutable_methods.include?(method)
                  raise CannotOverrideMethod, "Cannot override the immutable method: #{sym}"
                end
                
                allow_method_override do
                  self.module_eval <<-"end;"
                    def #{method}(*args, &block)
                      #{UNIQ}_old_#{method}(*args, &block)
                    end
                  end;
                end
              end 
            end # @args.each
          end # @immutable_methods
        end # def self.method_added()

        def self.method_undefined(sym)
          method_added(sym)
        end

        def self.method_removed(sym)
          method_added(sym)
        end
      end # module_eval

      def self.allow_method_override
        instance_variable_set("@#{UNIQ}_in_method_added", true)
        yield
      ensure
        instance_variable_set("@#{UNIQ}_in_method_added", false)
      end

      def self.in_method_added?
        instance_variable_get("@#{UNIQ}_in_method_added")
      end
    end # def immutable_method()

    alias immutable_methods immutable_method

  end # module ClassMethods
end # module Immutable