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module Picky

  module Backends

    #
    #
    class Redis < Backend

      attr_reader :client,
                  :realtime

      def initialize options = {}
        maybe_load_hiredis
        check_hiredis_gem
        check_redis_gem

        @client = options[:client] || ::Redis.new(:db => (options[:db] || 15))
        @realtime = options[:realtime]
      end
      def maybe_load_hiredis
        require 'hiredis'
      rescue LoadError
        # It's ok.
      end
      def check_hiredis_gem
        require 'redis/connection/hiredis'
      rescue LoadError
        # It's ok, the next check will fail if this one does.
      end
      def check_redis_gem
        require 'redis'
      rescue LoadError => e
        warn_gem_missing 'redis', 'the Redis client'
      end

      # Returns an object that on #initial, #load returns an object that responds to:
      # [:token] # => [id, id, id, id, id] (an array of ids)
      #
      def create_inverted bundle
        List.new client, "#{PICKY_ENVIRONMENT}:#{bundle.identifier}:inverted", realtime: realtime
      end
      # Returns an object that on #initial, #load returns an object that responds to:
      # [:token] # => 1.23 (a weight)
      #
      def create_weights bundle
        Float.new client, "#{PICKY_ENVIRONMENT}:#{bundle.identifier}:weights", realtime: realtime
      end
      # Returns an object that on #initial, #load returns an object that responds to:
      # [:encoded] # => [:original, :original] (an array of original symbols this similarity encoded thing maps to)
      #
      def create_similarity bundle
        List.new client, "#{PICKY_ENVIRONMENT}:#{bundle.identifier}:similarity", realtime: realtime
      end
      # Returns an object that on #initial, #load returns an object that responds to:
      # [:key] # => value (a value for this config key)
      #
      def create_configuration bundle
        String.new client, "#{PICKY_ENVIRONMENT}:#{bundle.identifier}:configuration", realtime: realtime
      end
      # Returns an object that on #initial, #load returns an object that responds to:
      # [id] # => [:sym1, :sym2]
      #
      def create_realtime bundle
        List.new client, "#{bundle.identifier}:realtime", realtime: realtime
      end

      # Does the Redis version already include
      # scripting support?
      #
      def redis_with_scripting?
        at_least_version redis_version, [2, 6, 0]
      end

      # Compares two versions each in an array [major, minor, patch]
      # format and returns true if the first version is higher
      # or the same as the second one. False if not.
      #
      # Note: Destructive.
      #
      def at_least_version major_minor_patch, should_be
        3.times { return false if major_minor_patch.shift < should_be.shift }
        true
      end

      # Returns an array describing the
      # current Redis version.
      #
      # Note: This method assumes that clients answer
      # to #info with a hash (string/symbol keys)
      # detailing the infos.
      #
      # Example:
      # backend.redis_version # => [2, 4, 1]
      #
      def redis_version
        infos = client.info
        version_string = infos['redis_version'] || infos[:redis_version]
        version_string.split('.').map &:to_i
      end

      # Returns the total weight for the combinations.
      #
      def weight combinations
        # Note: A nice experiment that generated far too many strings.
        #
        # if redis_with_scripting?
        # @@weight_script = "local sum = 0; for i=1,#(KEYS),2 do local value = redis.call('hget', KEYS[i], KEYS[i+1]); if value then sum = sum + value end end return sum;"
        #
        # require 'digest/sha1'
        # @@weight_sent_once = nil
        #
        # # Scripting version of #ids.
        # #
        # class << self
        # def weight combinations
        # namespaces_keys = combinations.inject([]) do |namespaces_keys, combination|
        # namespaces_keys << "#{combination.bundle.identifier}:weights"
        # namespaces_keys << combination.token.text
        # end
        #
        # # Assume it's using EVALSHA.
        # #
        # begin
        # client.evalsha @@weight_sent_once,
        # namespaces_keys.size,
        # *namespaces_keys
        # rescue RuntimeError => e
        # # Make the server have a SHA-1 for the script.
        # #
        # @@weight_sent_once = Digest::SHA1.hexdigest @@weight_script
        # client.eval @@weight_script,
        # namespaces_keys.size,
        # *namespaces_keys
        # end
        # end
        # end
        # else
        # class << self
        # def weight combinations
              combinations.score
        # end
        # end
        # end
        # # Call the newly installed version.
        # #
        # weight combinations
      end

      # Returns the result ids for the allocation.
      #
      # Developers wanting to program fast intersection
      # routines, can do so analogue to this in their own
      # backend implementations.
      #
      # Note: We use the amount and offset hints to speed Redis up.
      #
      def ids combinations, amount, offset
        # TODO This is actually not correct:
        # A dumped/loaded Redis backend should use
        # the Redis backend calculation method.
        # So loaded? would be more appropriate.
        #
        if realtime
          # Just checked once on the first call.
          #
          if redis_with_scripting?
            @@ids_script = "local intersected = redis.call('zinterstore', ARGV[1], #(KEYS), unpack(KEYS)); if intersected == 0 then redis.call('del', ARGV[1]); return {}; end local results = redis.call('zrange', ARGV[1], tonumber(ARGV[2]), tonumber(ARGV[3])); redis.call('del', ARGV[1]); return results;"

            require 'digest/sha1'
            @@ids_sent_once = nil

            # Overrides _this_ method.
            #
            extend Scripting
          else
            # Overrides _this_ method.
            #
            extend NonScripting
          end
        else
          # Remove _this_ method and use the super
          # class method from now on.
          #
          # Note: This fails if there are multiple
          # Redis backends with different versions.
          #
          self.class.send :remove_method, __method__
        end
        # Call the newly installed / super class version.
        #
        ids combinations, amount, offset
      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
        @intermediate_result_id ||= "#{host}:#{pid}:picky:result"
      end
      
      # Uses Lua scripting on Redis 2.6.
      #
      module Scripting
        def ids combinations, amount, offset
          identifiers = combinations.inject([]) do |identifiers, combination|
            identifiers << "#{combination.identifier}"
          end

          # Assume it's using EVALSHA.
          #
          begin
            if identifiers.size > 1
              client.evalsha @@ids_sent_once,
                             identifiers.size,
                             *identifiers,
                             generate_intermediate_result_id,
                             offset,
                             (offset + amount)
            else
              client.zrange identifiers.first,
                            offset,
                            (offset + amount)
            end
          rescue RuntimeError => e
            # Make the server have a SHA-1 for the script.
            #
            @@ids_sent_once = Digest::SHA1.hexdigest @@ids_script
            client.eval @@ids_script,
                        identifiers.size,
                        *identifiers,
                        generate_intermediate_result_id,
                        offset,
                        (offset + amount)
          end
        end
      end
      
      # Does not use Lua scripting, < Redis 2.6.
      #
      module NonScripting
        def ids combinations, amount, offset
          identifiers = combinations.inject([]) do |identifiers, combination|
            identifiers << "#{combination.identifier}"
          end

          result_id = generate_intermediate_result_id

          # Little optimization.
          #
          if identifiers.size > 1
            # Intersect and store.
            #
            intersected = client.zinterstore result_id, identifiers

            # Return clean and early if there has been no intersection.
            #
            if intersected.zero?
              client.del result_id
              return []
            end

            # Get the stored result.
            #
            results = client.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.
            #
            client.del result_id
          else
            results = client.zrange identifiers.first, offset, (offset + amount)
          end

          results
        end
      end

    end

  end

end
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