hash_cache (ActiveSupport::CachingTools::HashCaching)

    # File lib/active_support/caching_tools.rb, line 31
          def hash_cache(method_name, options = {})
            selector = options[:as] || "#{method_name}_cache"
            method = self.instance_method(method_name)

            args = []
            code = "def #{selector}(); @#{selector} ||= "

            (1..method.arity).each do |n|
              args << "v#{n}"
              code << "Hash.new {|h#{n}, v#{n}| h#{n}[v#{n}] = "
            end

            # Add the method call with arguments, followed by closing braces and end.
            code << "#{method_name}(#{args * ', '}) #{'}' * method.arity} end"

            # Extract the line number information from the caller. Exceptions arising
            # in the generated code should point to the +hash_cache :...+ line.
            if caller[0] && /^(.*):(\d+)$/ =~ caller[0]
              file, line_number = $1, $2.to_i
            else # We can't give good trackback info; fallback to this line:
              file, line_number = __FILE__, __LINE__
            end

            # We use eval rather than building proc's because it allows us to avoid
            # linking the Hash's to this method's binding. Experience has shown that
            # doing so can cause obtuse memory leaks.
            class_eval code, file, line_number
          end