/*
    * Copyright The Apache Software Foundation
    *
    * Licensed to the Apache Software Foundation (ASF) under one
    * or more contributor license agreements.  See the NOTICE file
    * distributed with this work for additional information
    * regarding copyright ownership.  The ASF licenses this file
    * 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.
   */
  package org.apache.hadoop.hbase.util;
  
  
  import com.google.common.base.Supplier;
  
  import java.util.Comparator;
  import java.util.Set;
  import java.util.concurrent.ConcurrentHashMap;
  import java.util.concurrent.ConcurrentMap;
  import java.util.concurrent.ConcurrentSkipListSet;
  
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  import org.apache.hadoop.hbase.classification.InterfaceStability;
  
  /**
   * A simple concurrent map of sets. This is similar in concept to
   * {@link com.google.common.collect.Multiset}, with the following exceptions:
   * <ul>
   *   <li>The set is thread-safe and concurrent: no external locking or
   *   synchronization is required. This is important for the use case where
   *   this class is used to index cached blocks by filename for their
   *   efficient eviction from cache when the file is closed or compacted.</li>
   *   <li>The expectation is that all entries may only be removed for a key
   *   once no more additions of values are being made under that key.</li>
   * </ul>
   * @param <K> Key type
   * @param <V> Value type
   */
  @InterfaceAudience.Private
  @InterfaceStability.Evolving
  public class ConcurrentIndex<K, V> {
  
    /** Container for the sets, indexed by key */
    private final ConcurrentMap<K, Set<V>> container;
  
    /**
     * A factory that constructs new instances of the sets if no set is
     * associated with a given key.
     */
    private final Supplier<Set<V>> valueSetFactory;
  
    /**
     * Creates an instance with a specified factory object for sets to be
     * associated with a given key.
     * @param valueSetFactory The factory instance
     */
    public ConcurrentIndex(Supplier<Set<V>> valueSetFactory) {
      this.valueSetFactory = valueSetFactory;
      this.container = new ConcurrentHashMap<K, Set<V>>();
    }
  
    /**
     * Creates an instance using the DefaultValueSetFactory for sets,
     * which in turn creates instances of {@link ConcurrentSkipListSet}
     * @param valueComparator A {@link Comparator} for value types
     */
    public ConcurrentIndex(Comparator<V> valueComparator) {
      this(new DefaultValueSetFactory<V>(valueComparator));
    }
  
    /**
     * Associate a new unique value with a specified key. Under the covers, the
     * method employs optimistic concurrency: if no set is associated with a
     * given key, we create a new set; if another thread comes in, creates,
     * and associates a set with the same key in the mean-time, we simply add
     * the value to the already created set.
     * @param key The key
     * @param value An additional unique value we want to associate with a key
     */
    public void put(K key, V value) {
      Set<V> set = container.get(key);
      if (set != null) {
        set.add(value);
      } else {
        set = valueSetFactory.get();
        set.add(value);
        Set<V> existing = container.putIfAbsent(key, set);
        if (existing != null) {
          // If a set is already associated with a key, that means another
          // writer has already come in and created the set for the given key.
         // Pursuant to an optimistic concurrency policy, in this case we will
         // simply add the value to the existing set associated with the key.
         existing.add(value);
       }
     }
   }
 
   /**
    * Get all values associated with a specified key or null if no values are
    * associated. <b>Note:</b> if the caller wishes to add or removes values
    * to under the specified as they're iterating through the returned value,
    * they should make a defensive copy; otherwise, a
    * {@link java.util.ConcurrentModificationException} may be thrown.
    * @param key The key
    * @return All values associated with the specified key or null if no values
    *         are associated with the key.
    */
   public Set<V> values(K key) {
     return container.get(key);
   }
 
   /**
    * Removes the association between a specified key and value. If as a
    * result of removing a value a set becomes empty, we remove the given
    * set from the mapping as well.
    * @param key The specified key
    * @param value The value to disassociate with the key
    */
   public boolean remove(K key, V value) {
     Set<V> set = container.get(key);
     boolean success = false;
     if (set != null) {
       success = set.remove(value);
       if (set.isEmpty()) {
         container.remove(key);
       }
     }
     return success;
   }
 
   /**
    * Default factory class for the sets associated with given keys. Creates
    * a {@link ConcurrentSkipListSet} using the comparator passed into the
    * constructor.
    * @see ConcurrentSkipListSet
    * @see Supplier
    * @param <V> The value type. Should match value type of the
    *           ConcurrentIndex instances of this object are passed to.
    */
   private static class DefaultValueSetFactory<V> implements Supplier<Set<V>> {
     private final Comparator<V> comparator;
 
     /**
      * Creates an instance that passes a specified comparator to the
      * {@link ConcurrentSkipListSet}
      * @param comparator The specified comparator
      */
     public DefaultValueSetFactory(Comparator<V> comparator) {
       this.comparator = comparator;
     }
 
     /**
      * Creates a new {@link ConcurrentSkipListSet} instance using the
      * comparator specified when the class instance was constructed.
      * @return The instantiated {@link ConcurrentSkipListSet} object
      */
     @Override
     public Set<V> get() {
       return new ConcurrentSkipListSet<V>(comparator);
     }
   }
 }