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   * Licensed to the Apache Software Foundation (ASF) under one
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   * distributed with this work for additional information
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   * to you under the Apache License, Version 2.0 (the
   * "License"); you may not use this file except in compliance
   * with the License.  You may obtain a copy of the License at
   *
   *     http://www.apache.org/licenses/LICENSE-2.0
   *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
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  package org.apache.cassandra.db.view;
  
  import java.util.Optional;
  import java.util.function.Predicate;
  
  import com.google.common.collect.Iterables;
  import org.apache.cassandra.dht.Token;
  import org.apache.cassandra.locator.EndpointsForToken;
  import org.apache.cassandra.locator.NetworkTopologyStrategy;
  import org.apache.cassandra.locator.Replica;
  import org.apache.cassandra.schema.KeyspaceMetadata;
  import org.apache.cassandra.tcm.ClusterMetadata;
  import org.apache.cassandra.tcm.membership.Location;
  
  public final class ViewUtils
  {
      private ViewUtils()
      {
      }
  
      /**
       * Calculate the natural endpoint for the view.
       *
       * The view natural endpoint is the endpoint which has the same cardinality as this node in the replication factor.
       * The cardinality is the number at which this node would store a piece of data, given the change in replication
       * factor. If the keyspace's replication strategy is a NetworkTopologyStrategy, we filter the ring to contain only
       * nodes in the local datacenter when calculating cardinality.
       *
       * For example, if we have the following ring:
       *   {@code A, T1 -> B, T2 -> C, T3 -> A}
       *
       * For the token T1, at RF=1, A would be included, so A's cardinality for T1 is 1. For the token T1, at RF=2, B would
       * be included, so B's cardinality for token T1 is 2. For token T3, at RF = 2, A would be included, so A's cardinality
       * for T3 is 2.
       *
       * For a view whose base token is T1 and whose view token is T3, the pairings between the nodes would be:
       *  A writes to C (A's cardinality is 1 for T1, and C's cardinality is 1 for T3)
       *  B writes to A (B's cardinality is 2 for T1, and A's cardinality is 2 for T3)
       *  C writes to B (C's cardinality is 3 for T1, and B's cardinality is 3 for T3)
       *
       * @return Optional.empty() if this method is called using a base token which does not belong to this replica
       */
      public static Optional<Replica> getViewNaturalEndpoint(ClusterMetadata metadata, String keyspace, Token baseToken, Token viewToken)
      {
          Location local = metadata.locator.local();
          KeyspaceMetadata keyspaceMetadata = metadata.schema.getKeyspaces().getNullable(keyspace);
  
          EndpointsForToken naturalBaseReplicas = metadata.placements.get(keyspaceMetadata.params.replication).reads.forToken(baseToken).get();
          EndpointsForToken naturalViewReplicas = metadata.placements.get(keyspaceMetadata.params.replication).reads.forToken(viewToken).get();
  
          Optional<Replica> localReplica = Iterables.tryFind(naturalViewReplicas, Replica::isSelf).toJavaUtil();
          if (localReplica.isPresent())
              return localReplica;
  
          // We only select replicas from our own DC
          // TODO: this is poor encapsulation, leaking implementation details of replication strategy
          Predicate<Replica> isLocalDC = r -> !(keyspaceMetadata.replicationStrategy instanceof NetworkTopologyStrategy)
                  || metadata.locator.location(r.endpoint()).sameDatacenter(local);
  
          // We have to remove any endpoint which is shared between the base and the view, as it will select itself
          // and throw off the counts otherwise.
          EndpointsForToken baseReplicas = naturalBaseReplicas.filter(
                  r -> !naturalViewReplicas.endpoints().contains(r.endpoint()) && isLocalDC.test(r)
          );
          EndpointsForToken viewReplicas = naturalViewReplicas.filter(
                  r -> !naturalBaseReplicas.endpoints().contains(r.endpoint()) && isLocalDC.test(r)
          );
  
          // The replication strategy will be the same for the base and the view, as they must belong to the same keyspace.
          // Since the same replication strategy is used, the same placement should be used and we should get the same
          // number of replicas for all of the tokens in the ring.
          assert baseReplicas.size() == viewReplicas.size() :
          String.format("Replication strategy should have the same number of endpoints for the base (%d) and the view (%d)",
                        baseReplicas.size(), viewReplicas.size());
  
          int baseIdx = -1;
          for (int i=0; i<baseReplicas.size(); i++)
          {
              if (baseReplicas.get(i).isSelf())
              {
                  baseIdx = i;
                  break;
              }
          }
  
          if (baseIdx < 0)
              //This node is not a base replica of this key, so we return empty
              return Optional.empty();
  
          return Optional.of(viewReplicas.get(baseIdx));
      }
  }
  