require 'logger'
require 'rainbow'

module LoggingLibrary
  #
  # Handles log formatting. Not intended to be used from user code.
  #
  class CustomFormatter < ::Logger::Formatter
    DATE_PATTERN = '%Y-%m-%d %H:%M:%S.%L'.freeze

    LogMessage = Struct.new(:severity, :time, :logger_name, :message) do
      def to_formatted_string
        if show_time?
          format("%s %s %s %s\n", formatted_colored_severity, formatted_colored_time,
                 formatted_colored_logger_name, colored_message)
        else
          format("%-5s %s %s\n", formatted_colored_severity, formatted_colored_logger_name, colored_message)
        end
      end

      private

      LINE_PREPEND = ' ' * 8

      def colored_message
        return formatted_message unless Rainbow.enabled
        Rainbow(formatted_message).color(text_color_for_severity)
      end

      # Converts some argument to a Logger.severity() call to a string.
      # Regular strings pass through like normal, Exceptions get formatted as
      # "message (class)\nbacktrace", and other random stuff gets put through
      # "object.inspect"

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

      def formatted_colored_severity
        if Rainbow.enabled
          Rainbow(formatted_severity).color(color_for_severity)
        else
          formatted_severity
        end
      end

      def formatted_colored_time
        if Rainbow.enabled
          Rainbow(formatted_time).color(time_color_for_severity)
        else
          formatted_time
        end
      end

      def formatted_colored_logger_name
        return formatted_logger_name unless Rainbow.enabled

        Rainbow(formatted_logger_name).color(text_color_for_severity)
      end

      def formatted_severity
        format('%-5s', severity)
      end

      def formatted_time
        '[' + time.strftime(DATE_PATTERN) + ']'
      end

      def formatted_logger_name
        logger_name + ':'
      end

      def color_for_severity
        if show_time?
          text_color_for_severity
        else
          color_for_severity_lighter
        end
      end

      def text_color_for_severity
        case severity.downcase.to_sym
        when :fatal then :magenta
        when :error then :red
        when :warn  then :yellow
        when :info  then :gray
        when :debug then '#606060'
        else :gray
        end
      end

      # Returns a somewhat lighter color for the time, to make it stand out
      # in the presentation.
      def time_color_for_severity
        case severity.downcase.to_sym
        when :fatal then '#FF00FF'
        when :error then '#FF0000'
        when :warn  then '#FFFF00'
        when :info  then '#FFFFFF'
        when :debug then '#808080'
        else :gray
        end
      end

      # We want "something" to stand out. If time isn't being shown, we
      # utilize its color for displaying the severity.
      alias_method :color_for_severity_lighter, :time_color_for_severity

      def show_time?
        # When STDERR is redirected, we are likely running as a service with a
        # syslog daemon already appending a timestamp to the line (and two
        # timestamps is redundant).
        tty? && !disable_timestamps?
      end

      def disable_timestamps?
        ENV['LOGGING_LIBRARY_DISABLE_TIMESTAMPS']
      end

      def tty?
        LoggingLibrary.output_device.tty?
      end
    end

    def call(severity, time, progname, message)
      LogMessage.new(severity, time, progname, message).to_formatted_string
    end
  end
end