class Logger
  # Default formatter for log messages.
  class Formatter
    Format = "%s, [%s#%d] %5s -- %s: %s\n".freeze

    attr_accessor :datetime_format

    def initialize
      @datetime_format = nil
    end

    def call(severity, time, progname, msg)
      Format % [severity[0..0], format_datetime(time), $$, severity, progname,
        msg2str(msg)]
    end

  private

    def format_datetime(time)
      time.strftime(@datetime_format || "%Y-%m-%dT%H:%M:%S.%6N ".freeze)
    end

    def msg2str(msg)
      case msg
      when ::String
        msg
      when ::Exception
        "#{ msg.message } (#{ msg.class })\n" <<
          (msg.backtrace || []).join("\n")
      else
        msg.inspect
      end
    end
  end
end