# frozen_string_literal: true

module Karafka
  module Connection
    # Manager for tracking changes in the partitions assignment.
    #
    # We need tracking of those to clean up consumers that will no longer process given partitions
    # as they were taken away.
    #
    # @note Since this does not happen really often, we try to stick with same objects for the
    #   empty states most of the time, so we don't create many objects during the manager life
    #
    # @note Internally in the rebalance manager we have a notion of lost partitions. Partitions
    #   that are lost, are those that got revoked but did not get re-assigned back. We do not
    #   expose this concept outside and we normalize to have them revoked, as it is irrelevant
    #   from the rest of the code perspective as only those that are lost are truly revoked.
    #
    # @note For cooperative-sticky `#assigned_partitions` holds only the recently assigned
    #   partitions, not all the partitions that are owned
    class RebalanceManager
      # Empty array for internal usage not to create new objects
      EMPTY_ARRAY = [].freeze

      attr_reader :assigned_partitions, :revoked_partitions

      private_constant :EMPTY_ARRAY

      # @return [RebalanceManager]
      def initialize
        @assigned_partitions = {}
        @revoked_partitions = {}
        @changed = false
      end

      # Resets the rebalance manager state
      # This needs to be done before each polling loop as during the polling, the state may be
      # changed
      def clear
        @assigned_partitions.clear
        @revoked_partitions.clear
        @changed = false
      end

      # @return [Boolean] indicates a state change in the partitions assignment
      def changed?
        @changed
      end

      # Callback that kicks in inside of rdkafka, when new partitions are assigned.
      #
      # @private
      # @param _ [Rdkafka::Consumer]
      # @param partitions [Rdkafka::Consumer::TopicPartitionList]
      def on_partitions_assigned(_, partitions)
        @assigned_partitions = partitions.to_h.transform_values { |part| part.map(&:partition) }
        @changed = true
      end

      # Callback that kicks in inside of rdkafka, when partitions are revoked.
      #
      # @private
      # @param _ [Rdkafka::Consumer]
      # @param partitions [Rdkafka::Consumer::TopicPartitionList]
      def on_partitions_revoked(_, partitions)
        @revoked_partitions = partitions.to_h.transform_values { |part| part.map(&:partition) }
        @changed = true
      end

      # We consider as lost only partitions that were taken away and not re-assigned back to us
      def lost_partitions
        lost_partitions = {}

        revoked_partitions.each do |topic, partitions|
          lost_partitions[topic] = partitions - assigned_partitions.fetch(topic, EMPTY_ARRAY)
        end

        lost_partitions
      end
    end
  end
end