code: func (n *BuilderNode) Responsible(key int) int {
  	partition := key % len(n.builder.ring.ReplicaToPartitionToNode[0])
  	for replica, partitionToNode := range n.builder.ring.ReplicaToPartitionToNode {
  		if int(partitionToNode[partition]) == n.index {
  			return replica
  		}
  	}
  	return -1
  }
docs: // Responsible returns the replica number this node is responsible for with
  // respect to the key given; will return -1 if this node is not responsible for
  // any replica for the key.