require 'time'
require 'oj'

module Ezlog
  class LoggingLayout < ::Logging::Layout
    def initialize(context = {}, options = {})
      @initial_context = context
      @level_formatter = options.fetch(:level_formatter, ->(numeric_level) { ::Logging::LNAMES[numeric_level] })
    end

    def format(event)
      log_entry = basic_information_for event
      add_initial_context_to log_entry
      add_logging_context_to log_entry
      add_event_information_to log_entry, event
      ::Oj.dump(log_entry, mode: :json) + "\n"
    end

    private

    def basic_information_for(event)
      {
        'logger' => event.logger,
        'timestamp' => event.time.iso8601(3),
        'level' => @level_formatter.call(event.level),
        'hostname' => Socket.gethostname,
        'pid' => Process.pid
      }
    end

    def add_initial_context_to(log_entry)
      log_entry.merge! @initial_context
    end

    def add_logging_context_to(log_entry)
      log_entry.merge! ::Logging.mdc.context
    end

    def add_event_information_to(log_entry, event)
      log_entry.merge! hash_from(event.data)
    end

    def hash_from(obj)
      case obj
      when Exception
        exception_message_by(obj)
      when Hash
        obj
      else
        { 'message' => obj }
      end
    end

    def exception_message_by(exception)
      {
        'message' => exception.message,
        'error' => {
          'class' => exception.class.name,
          'message' => exception.message,
          'backtrace' => exception.backtrace&.first(20)
        }
      }
    end
  end
end