content: /*
   * Copyright (C) 2020 ActiveJ LLC.
   *
   * Licensed 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.
   */
  
  package io.activej.fs.cluster;
  
  import java.util.List;
  import java.util.Set;
  
  import static io.activej.common.HashUtils.murmur3hash;
  import static java.util.Comparator.comparingInt;
  import static java.util.stream.Collectors.toList;
  
  /**
   * Strategy interface, which decides what file goes on which server from given ones.
   */
  @FunctionalInterface
  public interface ServerSelector {
  	/**
  	 * Implementation of rendezvous hash sharding algorithm
  	 */
  	ServerSelector RENDEZVOUS_HASH_SHARDER = (fileName, shards) -> {
  		class ShardWithHash {
  			private final Object shard;
  			private final int hash;
  
  			private ShardWithHash(Object shard, int hash) {
  				this.shard = shard;
  				this.hash = hash;
  			}
  		}
  		return shards.stream()
  			.map(shard -> new ShardWithHash(shard,
  				(int) murmur3hash(((long) fileName.hashCode() << 32) | (shard.hashCode() & 0xFFFFFFFFL))))
  			.sorted(comparingInt(h -> h.hash))
  			.map(h -> h.shard)
  			.collect(toList());
  	};
  
  	/**
  	 * Selects partitions where given file should belong.
  	 *
  	 * @param fileName name of the file
  	 * @param shards   set of partition ids to choose from
  	 * @return list of keys of servers ordered by priority where file with given name should be
  	 */
  	List<Object> selectFrom(String fileName, Set<Object> shards);
  }
  