class Concurrent::Collection::AtomicReferenceMapBackend

def get_or_default(key, else_value = nil)
  hash          = key_hash(key)
  current_table = table
  while current_table
    node = current_table.volatile_get_by_hash(hash)
    current_table =
      while node
        if (node_hash = node.hash) == MOVED
          break node.key
        elsif (node_hash & HASH_BITS) == hash && node.key?(key) && NULL != (value = node.value)
          return value
        end
        node = node.next
      end
  end
  else_value
end