module Support
  module Backends

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

    module ClassMethods
      attr_writer :backend
      attr_reader :queue_options

      def backend(queue_name=nil, args={})
        return @backend if @backend
        @queue_options = args
        @backend = queue_name and return if queue_name
        determine_backend
      end

      def available_backends
        @available_backends ||= begin
          backends = []
          backends << :girl_friday if defined? ::GirlFriday
          backends << :delayed_job if defined? ::Delayed::Job
          backends << :resque      if defined? ::Resque
          backends << :qu          if defined? ::Qu
          backends << :sidekiq     if defined? ::Sidekiq
          backends << :qc          if defined? ::QC
          backends << :immediate
        end
      end

      def enqueue_for_backend(worker, class_name, subject_id, mounted_as)
        case backend
        when :girl_friday
          @girl_friday_queue ||= GirlFriday::WorkQueue.new(queue_options.delete(:queue) || :carrierwave, queue_options) do |msg|
            worker = msg[:worker]
            worker.perform
          end
          @girl_friday_queue << { :worker => worker.new(class_name, subject_id, mounted_as) }
        when :delayed_job
          ::Delayed::Job.enqueue worker.new(class_name, subject_id, mounted_as), :queue => queue_options[:queue]
        when :resque
          worker.instance_variable_set('@queue', queue_options[:queue] || :carrierwave)
          ::Resque.enqueue worker, class_name, subject_id, mounted_as
        when :qu
          worker.instance_variable_set('@queue', queue_options[:queue] || :carrierwave)
          ::Qu.enqueue worker, class_name, subject_id, mounted_as
        when :sidekiq
          worker.extend ::Sidekiq::Worker
          worker.sidekiq_options queue_options[:queue] || :carrierwave
          ::Sidekiq::Client.enqueue worker, class_name, subject_id, mounted_as
        when :qc
          ::QC.enqueue "#{worker.name}.perform", class_name, subject_id, mounted_as.to_s
        when :immediate
          worker.new(class_name, subject_id, mounted_as).perform
        end
      end

      private

      def determine_backend
        @backend = if available_backends.empty?
          warn 'WARNING: No available queue backends found for CarrierWave::Backgrounder. Using the :immediate.'
          :immediate
        elsif available_backends.size > 1
          raise ::CarrierWave::Backgrounder::ToManyBackendsAvailableError,
            "You have to many backends available: #{available_backends.inspect}. Please specify which one to use in configuration block"
        else
          available_backends.first
        end
      end

    end
  end
end