module Internals

  module Query

    # Combinations are a number of Combination-s.
    #
    # They are the core of an allocation.
    # An allocation consists of a number of combinations.
    #
    module Combinations # :nodoc:all

      # Redis Combinations contain specific methods for
      # calculating score and ids in memory.
      #
      class Redis < Base

        # Connect to the backend.
        #
        # TODO Use specific Picky Redis wrapper.
        #
        def self.redis
          @redis ||= ::Redis.new :db => 15
        end

        attr_reader :redis

        #
        #
        def initialize combinations
          super combinations

          @redis = self.class.redis
        end

        # Returns the result ids for the allocation.
        #
        def ids amount, offset
          return [] if @combinations.empty?

          identifiers = @combinations.inject([]) do |identifiers, combination|
            identifiers << "#{combination.identifier}"
          end

          result_id = generate_intermediate_result_id

          # Intersect and store.
          #
          redis.zinterstore result_id, identifiers

          # Get the stored result.
          #
          results = redis.zrange result_id, offset, (offset + amount)

          # Delete the stored result as it was only for temporary purposes.
          #
          # Note: I could also not delete it, but that would not be clean at all.
          #
          redis.del result_id

          results
        end

        # Generate a multiple host/process safe result id.
        #
        # Note: Generated when this class loads.
        #
        require 'socket'
        def self.extract_host
          @host ||= Socket.gethostname
        end
        def host
          self.class.extract_host
        end
        extract_host
        def pid
          @pid ||= Process.pid
        end
        # Use the host and pid (generated lazily in child processes) for the result.
        #
        def generate_intermediate_result_id
          :"#{host}:#{pid}:picky:result"
        end

      end

    end

  end

end