class NamespaceEachBatch
  def initialize(namespace_id:, cursor: nil)
    @namespace_id = namespace_id
    @cursor = cursor || { current_id: namespace_id, depth: [namespace_id] }
  end

  def each_batch(of: 500)
    current_cursor = cursor.dup

    first_iteration = true
    loop do
      new_cursor, ids = load_batch(cursor: current_cursor, of: of, first_iteration: first_iteration)
      first_iteration = false
      current_cursor = new_cursor

      yield ids

      break if new_cursor[:depth].empty?
    end
  end

  private

  # yields array of namespace ids
  def load_batch(cursor:, of:, first_iteration: false)
    recursive_cte = Gitlab::SQL::RecursiveCTE.new(:result,
      union_args: { remove_order: false, remove_duplicates: false })

    ids = first_iteration ? namespace_id.to_s : ""

    recursive_cte << Namespace.select(
      Arel.sql(Integer(cursor.fetch(:current_id)).to_s).as('current_id'),
      Arel.sql("ARRAY[#{cursor.fetch(:depth).join(',')}]::int[]").as('depth'),
      Arel.sql("ARRAY[#{ids}]::int[]").as('ids'),
      Arel.sql("1::bigint AS count")
    ).from('(VALUES (1)) AS does_not_matter').limit(1)

    cte = Gitlab::SQL::CTE.new(:cte, Namespace.select('*').from('result'))

    union_query = Namespace.with(cte.to_arel).from_union(
      walk_down,
      next_elements,
      up_one_level,
      remove_duplicates: false,
      remove_order: false
    ).select('current_id', 'depth', 'ids', 'count').limit(1)

    recursive_cte << union_query

    scope = Namespace.with
      .recursive(recursive_cte.to_arel)
      .from(recursive_cte.alias_to(Namespace.arel_table))
      .limit(of)
    row = Namespace.from(scope.arel.as('namespaces')).order(count: :desc).limit(1).first

    [
      { current_id: row[:current_id], depth: row[:depth] },
      row[:ids]
    ]
  end

  attr_reader :namespace_id, :cursor

  def walk_down
    Namespace.select(
      Arel.sql('namespaces.id').as('current_id'),
      Arel.sql('cte.depth || namespaces.id').as('depth'),
      Arel.sql('cte.ids || namespaces.id').as('ids'),
      Arel.sql('cte.count + 1').as('count')
    ).from('cte, LATERAL (SELECT id FROM namespaces WHERE parent_id = cte.current_id ORDER BY id LIMIT 1) namespaces')
  end

  def next_elements
    Namespace.select(
      Arel.sql('namespaces.id').as('current_id'),
      Arel.sql('cte.depth[:array_length(cte.depth, 1) - 1] || namespaces.id').as('depth'),
      Arel.sql('cte.ids || namespaces.id').as('ids'),
      Arel.sql('cte.count + 1').as('count')
    ).from('cte, LATERAL (SELECT id FROM namespaces WHERE namespaces.parent_id = cte.depth[array_length(cte.depth, 1) - 1] AND namespaces.id > cte.depth[array_length(cte.depth, 1)] ORDER BY id LIMIT 1) namespaces')
  end

  def up_one_level
    Namespace.select(
      Arel.sql('cte.current_id').as('current_id'),
      Arel.sql('cte.depth[:array_length(cte.depth, 1) - 1]').as('depth'),
      Arel.sql('cte.ids').as('ids'),
      Arel.sql('cte.count + 1').as('count')
    ).from('cte')
      .where('cte.depth <> ARRAY[]::int[]')
      .limit(1)
  end
end

iterator = NamespaceEachBatch.new(namespace_id: 9970)
all_ids = []
iterator.each_batch do |ids|
  all_ids.concat(ids)
end

# 테스트
puts all_ids.count
puts all_ids.sort == Namespace.where('traversal_ids && ARRAY[9970]').pluck(:id).sort